diff --git a/clap_complete/src/shells/bash.rs b/clap_complete/src/shells/bash.rs
index 5db6693da3a..22ad400e4f4 100644
--- a/clap_complete/src/shells/bash.rs
+++ b/clap_complete/src/shells/bash.rs
@@ -59,7 +59,7 @@ impl Generator for Bash {
 complete -F _{name} -o bashdefault -o default {name}
 ",
                 name = bin_name,
-                cmd = bin_name.replace("-", "__"),
+                cmd = bin_name.replace('-', "__"),
                 name_opts = all_options_for_path(app, bin_name),
                 name_opts_details = option_details_for_path(app, bin_name),
                 subcmds = all_subcommands(app),
@@ -88,7 +88,7 @@ fn all_subcommands(app: &App) -> String {
                 cmd+=\"__{fn_name}\"
                 ;;",
             name = sc,
-            fn_name = sc.replace("-", "__")
+            fn_name = sc.replace('-', "__")
         )
     }));
 
@@ -101,7 +101,7 @@ fn subcommand_details(app: &App) -> String {
     let mut subcmd_dets = vec![String::new()];
     let mut scs = utils::all_subcommands(app)
         .iter()
-        .map(|x| x.1.replace(" ", "__"))
+        .map(|x| x.1.replace(' ', "__"))
         .collect::<Vec<_>>();
 
     scs.sort();
@@ -122,7 +122,7 @@ fn subcommand_details(app: &App) -> String {
             COMPREPLY=( $(compgen -W \"${{opts}}\" -- \"${{cur}}\") )
             return 0
             ;;",
-            subcmd = sc.replace("-", "__"),
+            subcmd = sc.replace('-', "__"),
             sc_opts = all_options_for_path(app, &*sc),
             level = sc.split("__").map(|_| 1).sum::<u64>(),
             opts_details = option_details_for_path(app, &*sc)
diff --git a/clap_complete/src/shells/elvish.rs b/clap_complete/src/shells/elvish.rs
index 17cd98eeeca..bbcd7ee7fd9 100644
--- a/clap_complete/src/shells/elvish.rs
+++ b/clap_complete/src/shells/elvish.rs
@@ -54,7 +54,7 @@ set edit:completion:arg-completer[{bin_name}] = [@words]{{
 
 // Escape string inside single quotes
 fn escape_string(string: &str) -> String {
-    string.replace("'", "''")
+    string.replace('\'', "''")
 }
 
 fn get_tooltip<T: ToString>(help: Option<&str>, data: T) -> String {
diff --git a/clap_complete/src/shells/fish.rs b/clap_complete/src/shells/fish.rs
index 0cd165cea11..f5fd87df874 100644
--- a/clap_complete/src/shells/fish.rs
+++ b/clap_complete/src/shells/fish.rs
@@ -26,7 +26,7 @@ impl Generator for Fish {
 
 // Escape string inside single quotes
 fn escape_string(string: &str) -> String {
-    string.replace("\\", "\\\\").replace("'", "\\'")
+    string.replace('\\', "\\\\").replace('\'', "\\'")
 }
 
 fn gen_fish_inner(root_command: &str, parent_commands: &[&str], app: &App, buffer: &mut String) {
diff --git a/clap_complete/src/shells/powershell.rs b/clap_complete/src/shells/powershell.rs
index 4e3ad4253ae..7d9cadeb10e 100644
--- a/clap_complete/src/shells/powershell.rs
+++ b/clap_complete/src/shells/powershell.rs
@@ -58,7 +58,7 @@ Register-ArgumentCompleter -Native -CommandName '{bin_name}' -ScriptBlock {{
 
 // Escape string inside single quotes
 fn escape_string(string: &str) -> String {
-    string.replace("'", "''")
+    string.replace('\'', "''")
 }
 
 fn get_tooltip<T: ToString>(help: Option<&str>, data: T) -> String {
diff --git a/clap_complete/src/shells/zsh.rs b/clap_complete/src/shells/zsh.rs
index 84ee0e8ae64..9bc6e182106 100644
--- a/clap_complete/src/shells/zsh.rs
+++ b/clap_complete/src/shells/zsh.rs
@@ -92,7 +92,7 @@ _{bin_name_underscore}_commands() {{
     local commands; commands=({subcommands_and_args})
     _describe -t commands '{bin_name} commands' commands \"$@\"
 }}",
-        bin_name_underscore = name.replace(" ", "__"),
+        bin_name_underscore = name.replace(' ', "__"),
         bin_name = name,
         subcommands_and_args = subcommands_of(p)
     );
@@ -114,7 +114,7 @@ _{bin_name_underscore}_commands() {{
     local commands; commands=({subcommands_and_args})
     _describe -t commands '{bin_name} commands' commands \"$@\"
 }}",
-            bin_name_underscore = bin_name.replace(" ", "__"),
+            bin_name_underscore = bin_name.replace(' ', "__"),
             bin_name = bin_name,
             subcommands_and_args =
                 subcommands_of(parser_of(p, bin_name).expect(INTERNAL_ERROR_MSG))
@@ -259,7 +259,7 @@ fn get_subcommands_of(parent: &App) -> String {
     ;;
 esac",
         name = parent.get_name(),
-        name_hyphen = parent.get_bin_name().unwrap().replace(" ", "-"),
+        name_hyphen = parent.get_bin_name().unwrap().replace(' ', "-"),
         subcommands = all_subcommands.join("\n"),
         pos = parent.get_positionals().count() + 1
     )
@@ -328,7 +328,7 @@ fn get_args_of(parent: &App, p_global: Option<&App>) -> String {
     if parent.has_subcommands() {
         let subcommand_bin_name = format!(
             "\":: :_{name}_commands\" \\",
-            name = parent.get_bin_name().as_ref().unwrap().replace(" ", "__")
+            name = parent.get_bin_name().as_ref().unwrap().replace(' ', "__")
         );
         segments.push(subcommand_bin_name);
 
@@ -406,20 +406,20 @@ fn value_completion(arg: &Arg) -> Option<String> {
 /// Escape help string inside single quotes and brackets
 fn escape_help(string: &str) -> String {
     string
-        .replace("\\", "\\\\")
-        .replace("'", "'\\''")
-        .replace("[", "\\[")
-        .replace("]", "\\]")
+        .replace('\\', "\\\\")
+        .replace('\'', "'\\''")
+        .replace('[', "\\[")
+        .replace(']', "\\]")
 }
 
 /// Escape value string inside single quotes and parentheses
 fn escape_value(string: &str) -> String {
     string
-        .replace("\\", "\\\\")
-        .replace("'", "'\\''")
-        .replace("(", "\\(")
-        .replace(")", "\\)")
-        .replace(" ", "\\ ")
+        .replace('\\', "\\\\")
+        .replace('\'', "'\\''")
+        .replace('(', "\\(")
+        .replace(')', "\\)")
+        .replace(' ', "\\ ")
 }
 
 fn write_opts_of(p: &App, p_global: Option<&App>) -> String {
@@ -631,9 +631,9 @@ fn write_positionals_of(p: &App) -> String {
             help = arg
                 .get_help()
                 .map_or("".to_owned(), |v| " -- ".to_owned() + v)
-                .replace("[", "\\[")
-                .replace("]", "\\]")
-                .replace(":", "\\:"),
+                .replace('[', "\\[")
+                .replace(']', "\\]")
+                .replace(':', "\\:"),
             value_completion = value_completion(arg).unwrap_or_else(|| "".to_string())
         );
 
diff --git a/clap_complete_fig/src/fig.rs b/clap_complete_fig/src/fig.rs
index 74403f09343..924fce49741 100644
--- a/clap_complete_fig/src/fig.rs
+++ b/clap_complete_fig/src/fig.rs
@@ -38,7 +38,7 @@ impl Generator for Fig {
 
 // Escape string inside double quotes
 fn escape_string(string: &str) -> String {
-    string.replace("\\", "\\\\").replace("\"", "\\\"")
+    string.replace('\\', "\\\\").replace('\"', "\\\"")
 }
 
 fn gen_fig_inner(
diff --git a/clap_derive/src/attrs.rs b/clap_derive/src/attrs.rs
index 1f4eaacaeba..9cb31a8420c 100644
--- a/clap_derive/src/attrs.rs
+++ b/clap_derive/src/attrs.rs
@@ -879,8 +879,8 @@ impl Name {
                     Camel => s.to_lower_camel_case(),
                     ScreamingSnake => s.to_shouty_snake_case(),
                     Snake => s.to_snake_case(),
-                    Lower => s.to_snake_case().replace("_", ""),
-                    Upper => s.to_shouty_snake_case().replace("_", ""),
+                    Lower => s.to_snake_case().replace('_', ""),
+                    Upper => s.to_shouty_snake_case().replace('_', ""),
                     Verbatim => s,
                 };
                 quote_spanned!(ident.span()=> #s)
diff --git a/clap_derive/src/parse.rs b/clap_derive/src/parse.rs
index 64d80a77c48..5eb0eb50f27 100644
--- a/clap_derive/src/parse.rs
+++ b/clap_derive/src/parse.rs
@@ -252,8 +252,8 @@ fn raw_method_suggestion(ts: ParseBuffer) -> String {
     fn to_string<T: ToTokens>(val: &T) -> String {
         val.to_token_stream()
             .to_string()
-            .replace(" ", "")
-            .replace(",", ", ")
+            .replace(' ', "")
+            .replace(',', ", ")
     }
 
     if let Ok((name, exprs)) = do_parse() {
diff --git a/clap_derive/src/utils/doc_comments.rs b/clap_derive/src/utils/doc_comments.rs
index 7ac872cf364..f0a5034d700 100644
--- a/clap_derive/src/utils/doc_comments.rs
+++ b/clap_derive/src/utils/doc_comments.rs
@@ -78,7 +78,7 @@ fn split_paragraphs(lines: &[&str]) -> Vec<String> {
         let len = slice
             .iter()
             .position(|s| is_blank(s))
-            .unwrap_or_else(|| slice.len());
+            .unwrap_or(slice.len());
 
         last_line += start + len;
 
diff --git a/examples/tutorial_derive/04_04_custom.rs b/examples/tutorial_derive/04_04_custom.rs
index fc713adfe5d..c06719453b7 100644
--- a/examples/tutorial_derive/04_04_custom.rs
+++ b/examples/tutorial_derive/04_04_custom.rs
@@ -72,10 +72,13 @@ fn main() {
 
     // Check for usage of -c
     if let Some(config) = cli.config.as_deref() {
+        // todo: remove `#[allow(clippy::or_fun_call)]` lint when MSRV is bumped.
+        #[allow(clippy::or_fun_call)]
         let input = cli
             .input_file
             .as_deref()
-            .or_else(|| cli.spec_in.as_deref())
+            // 'or' is preferred to 'or_else' here since `Option::as_deref` is 'const'
+            .or(cli.spec_in.as_deref())
             .unwrap_or_else(|| {
                 let mut app = Cli::into_app();
                 app.error(
diff --git a/src/build/app/mod.rs b/src/build/app/mod.rs
index 0e68f034d36..9f6c30eb11e 100644
--- a/src/build/app/mod.rs
+++ b/src/build/app/mod.rs
@@ -160,6 +160,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     /// [argument]: Arg
+    #[must_use]
     pub fn arg<A: Into<Arg<'help>>>(mut self, a: A) -> Self {
         let mut arg = a.into();
         arg.help_heading.get_or_insert(self.current_help_heading);
@@ -181,6 +182,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     /// [arguments]: Arg
+    #[must_use]
     pub fn args<I, T>(mut self, args: I) -> Self
     where
         I: IntoIterator<Item = T>,
@@ -190,7 +192,7 @@ impl<'help> App<'help> {
         let (lower, _) = args.size_hint();
         self.args.reserve(lower);
 
-        for arg in args.into_iter() {
+        for arg in args {
             self = self.arg(arg);
         }
         self
@@ -220,6 +222,7 @@ impl<'help> App<'help> {
     /// let res = app.try_get_matches_from_mut(vec!["foo", "-B"]);
     /// assert!(res.is_ok());
     /// ```
+    #[must_use]
     pub fn mut_arg<T, F>(mut self, arg_id: T, f: F) -> Self
     where
         F: FnOnce(Arg<'help>) -> Arg<'help>,
@@ -274,6 +277,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn group<G: Into<ArgGroup<'help>>>(mut self, group: G) -> Self {
         self.groups.push(group.into());
         self
@@ -301,6 +305,7 @@ impl<'help> App<'help> {
     ///     ])
     /// # ;
     /// ```
+    #[must_use]
     pub fn groups<I, T>(mut self, groups: I) -> Self
     where
         I: IntoIterator<Item = T>,
@@ -333,6 +338,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn subcommand<S: Into<App<'help>>>(mut self, subcmd: S) -> Self {
         self.subcommands.push(subcmd.into());
         self
@@ -352,6 +358,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     /// [`IntoIterator`]: std::iter::IntoIterator
+    #[must_use]
     pub fn subcommands<I, T>(mut self, subcmds: I) -> Self
     where
         I: IntoIterator<Item = T>,
@@ -829,6 +836,7 @@ impl<'help> App<'help> {
     ///
     /// // continued logic goes here, such as `app.get_matches()` etc.
     /// ```
+    #[must_use]
     pub fn name<S: Into<String>>(mut self, name: S) -> Self {
         self.name = name.into();
         self
@@ -854,6 +862,7 @@ impl<'help> App<'help> {
     ///      .bin_name("my_binary")
     /// # ;
     /// ```
+    #[must_use]
     pub fn bin_name<S: Into<String>>(mut self, name: S) -> Self {
         self.bin_name = Some(name.into());
         self
@@ -874,6 +883,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     /// [`crate_authors!`]: ./macro.crate_authors!.html
+    #[must_use]
     pub fn author<S: Into<&'help str>>(mut self, author: S) -> Self {
         self.author = Some(author.into());
         self
@@ -896,6 +906,7 @@ impl<'help> App<'help> {
     ///     .about("Does really amazing things for great people")
     /// # ;
     /// ```
+    #[must_use]
     pub fn about<O: Into<Option<&'help str>>>(mut self, about: O) -> Self {
         self.about = about.into();
         self
@@ -920,6 +931,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     /// [`App::about`]: App::about()
+    #[must_use]
     pub fn long_about<O: Into<Option<&'help str>>>(mut self, long_about: O) -> Self {
         self.long_about = long_about.into();
         self
@@ -941,6 +953,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     ///
+    #[must_use]
     pub fn after_help<S: Into<&'help str>>(mut self, help: S) -> Self {
         self.after_help = Some(help.into());
         self
@@ -962,6 +975,7 @@ impl<'help> App<'help> {
     ///                      like, for real, be careful with this!")
     /// # ;
     /// ```
+    #[must_use]
     pub fn after_long_help<S: Into<&'help str>>(mut self, help: S) -> Self {
         self.after_long_help = Some(help.into());
         self
@@ -981,6 +995,7 @@ impl<'help> App<'help> {
     ///     .before_help("Some info I'd like to appear before the help info")
     /// # ;
     /// ```
+    #[must_use]
     pub fn before_help<S: Into<&'help str>>(mut self, help: S) -> Self {
         self.before_help = Some(help.into());
         self
@@ -1000,6 +1015,7 @@ impl<'help> App<'help> {
     ///     .before_long_help("Some verbose and long info I'd like to appear before the help info")
     /// # ;
     /// ```
+    #[must_use]
     pub fn before_long_help<S: Into<&'help str>>(mut self, help: S) -> Self {
         self.before_long_help = Some(help.into());
         self
@@ -1022,6 +1038,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     /// [`crate_version!`]: ./macro.crate_version!.html
+    #[must_use]
     pub fn version<S: Into<&'help str>>(mut self, ver: S) -> Self {
         self.version = Some(ver.into());
         self
@@ -1049,6 +1066,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     /// [`crate_version!`]: ./macro.crate_version!.html
+    #[must_use]
     pub fn long_version<S: Into<&'help str>>(mut self, ver: S) -> Self {
         self.long_version = Some(ver.into());
         self
@@ -1069,6 +1087,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     /// [`ArgMatches::usage`]: ArgMatches::usage()
+    #[must_use]
     pub fn override_usage<S: Into<&'help str>>(mut self, usage: S) -> Self {
         self.usage_str = Some(usage.into());
         self
@@ -1105,6 +1124,7 @@ impl<'help> App<'help> {
     ///            work             Do some work")
     /// # ;
     /// ```
+    #[must_use]
     pub fn override_help<S: Into<&'help str>>(mut self, help: S) -> Self {
         self.help_str = Some(help.into());
         self
@@ -1153,6 +1173,7 @@ impl<'help> App<'help> {
     /// [`App::after_long_help`]: App::after_long_help()
     /// [`App::before_help`]: App::before_help()
     /// [`App::before_long_help`]: App::before_long_help()
+    #[must_use]
     pub fn help_template<S: Into<&'help str>>(mut self, s: S) -> Self {
         self.template = Some(s.into());
         self
@@ -1181,6 +1202,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn setting<F>(mut self, setting: F) -> Self
     where
         F: Into<AppFlags>,
@@ -1210,6 +1232,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn unset_setting<F>(mut self, setting: F) -> Self
     where
         F: Into<AppFlags>,
@@ -1233,6 +1256,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn global_setting(mut self, setting: AppSettings) -> Self {
         self.settings.set(setting);
         self.g_settings.set(setting);
@@ -1253,6 +1277,7 @@ impl<'help> App<'help> {
     /// ```
     /// [global]: App::global_setting()
     #[inline]
+    #[must_use]
     pub fn unset_global_setting(mut self, setting: AppSettings) -> Self {
         self.settings.unset(setting);
         self.g_settings.unset(setting);
@@ -1276,6 +1301,7 @@ impl<'help> App<'help> {
     /// [`ColorChoice::Auto`]: crate::ColorChoice::Auto
     #[cfg(feature = "color")]
     #[inline]
+    #[must_use]
     pub fn color(self, color: ColorChoice) -> Self {
         #[allow(deprecated)]
         match color {
@@ -1297,6 +1323,7 @@ impl<'help> App<'help> {
     /// [`App::arg`]: App::arg()
     /// [`Arg::help_heading`]: crate::Arg::help_heading()
     #[inline]
+    #[must_use]
     pub fn help_heading<O>(mut self, heading: O) -> Self
     where
         O: Into<Option<&'help str>>,
@@ -1323,6 +1350,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn term_width(mut self, width: usize) -> Self {
         self.term_w = Some(width);
         self
@@ -1346,6 +1374,7 @@ impl<'help> App<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn max_term_width(mut self, w: usize) -> Self {
         self.max_w = Some(w);
         self
@@ -1456,6 +1485,7 @@ impl<'help> App<'help> {
     /// [`App::replace`]: App::replace()
     #[inline]
     #[cfg(feature = "unstable-replace")]
+    #[must_use]
     pub fn replace(mut self, name: &'help str, target: &'help [&'help str]) -> Self {
         self.replacers.insert(name, target);
         self
@@ -1488,6 +1518,7 @@ impl<'help> App<'help> {
     /// assert!(sync_matches.is_present("search"));
     /// ```
     /// [`Arg::short`]: Arg::short()
+    #[must_use]
     pub fn short_flag(mut self, short: char) -> Self {
         self.short_flag = Some(short);
         self
@@ -1524,6 +1555,7 @@ impl<'help> App<'help> {
     /// ```
     ///
     /// [`Arg::long`]: Arg::long()
+    #[must_use]
     pub fn long_flag(mut self, long: &'help str) -> Self {
         self.long_flag = Some(long.trim_start_matches(|c| c == '-'));
         self
@@ -1554,6 +1586,7 @@ impl<'help> App<'help> {
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
     /// [`App::visible_alias`]: App::visible_alias()
+    #[must_use]
     pub fn alias<S: Into<&'help str>>(mut self, name: S) -> Self {
         self.aliases.push((name.into(), false));
         self
@@ -1575,8 +1608,9 @@ impl<'help> App<'help> {
     ///             .get_matches_from(vec!["myprog", "-d"]);
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
+    #[must_use]
     pub fn short_flag_alias(mut self, name: char) -> Self {
-        assert!(!(name == '-'), "short alias name cannot be `-`");
+        assert!(name != '-', "short alias name cannot be `-`");
         self.short_flag_aliases.push((name, false));
         self
     }
@@ -1597,6 +1631,7 @@ impl<'help> App<'help> {
     ///             .get_matches_from(vec!["myprog", "--testing"]);
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
+    #[must_use]
     pub fn long_flag_alias(mut self, name: &'help str) -> Self {
         self.long_flag_aliases.push((name, false));
         self
@@ -1631,6 +1666,7 @@ impl<'help> App<'help> {
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
     /// [`App::visible_aliases`]: App::visible_aliases()
+    #[must_use]
     pub fn aliases(mut self, names: &[&'help str]) -> Self {
         self.aliases.extend(names.iter().map(|n| (*n, false)));
         self
@@ -1656,6 +1692,7 @@ impl<'help> App<'help> {
     ///     .get_matches_from(vec!["myprog", "-a"]);
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
+    #[must_use]
     pub fn short_flag_aliases(mut self, names: &[char]) -> Self {
         for s in names {
             assert!(s != &'-', "short alias name cannot be `-`");
@@ -1684,6 +1721,7 @@ impl<'help> App<'help> {
     ///             .get_matches_from(vec!["myprog", "--testing"]);
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
+    #[must_use]
     pub fn long_flag_aliases(mut self, names: &[&'help str]) -> Self {
         for s in names {
             self.long_flag_aliases.push((s, false));
@@ -1718,6 +1756,7 @@ impl<'help> App<'help> {
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
     /// [`App::alias`]: App::alias()
+    #[must_use]
     pub fn visible_alias<S: Into<&'help str>>(mut self, name: S) -> Self {
         self.aliases.push((name.into(), true));
         self
@@ -1742,6 +1781,7 @@ impl<'help> App<'help> {
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
     /// [`App::short_flag_alias`]: App::short_flag_alias()
+    #[must_use]
     pub fn visible_short_flag_alias(mut self, name: char) -> Self {
         assert!(name != '-', "short alias name cannot be `-`");
         self.short_flag_aliases.push((name, true));
@@ -1767,6 +1807,7 @@ impl<'help> App<'help> {
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
     /// [`App::long_flag_alias`]: App::long_flag_alias()
+    #[must_use]
     pub fn visible_long_flag_alias(mut self, name: &'help str) -> Self {
         self.long_flag_aliases.push((name, true));
         self
@@ -1799,6 +1840,7 @@ impl<'help> App<'help> {
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
     /// [`App::alias`]: App::alias()
+    #[must_use]
     pub fn visible_aliases(mut self, names: &[&'help str]) -> Self {
         self.aliases.extend(names.iter().map(|n| (*n, true)));
         self
@@ -1819,9 +1861,10 @@ impl<'help> App<'help> {
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
     /// [`App::short_flag_aliases`]: App::short_flag_aliases()
+    #[must_use]
     pub fn visible_short_flag_aliases(mut self, names: &[char]) -> Self {
         for s in names {
-            assert!(!(s == &'-'), "short alias name cannot be `-`");
+            assert!(s != &'-', "short alias name cannot be `-`");
             self.short_flag_aliases.push((*s, true));
         }
         self
@@ -1842,6 +1885,7 @@ impl<'help> App<'help> {
     /// assert_eq!(m.subcommand_name(), Some("test"));
     /// ```
     /// [`App::long_flag_aliases`]: App::long_flag_aliases()
+    #[must_use]
     pub fn visible_long_flag_aliases(mut self, names: &[&'help str]) -> Self {
         for s in names {
             self.long_flag_aliases.push((s, true));
@@ -1898,6 +1942,7 @@ impl<'help> App<'help> {
     ///     alpha   Some help and text
     /// ```
     #[inline]
+    #[must_use]
     pub fn display_order(mut self, ord: usize) -> Self {
         self.disp_ord = Some(ord);
         self
@@ -1963,6 +2008,7 @@ impl<'help> App<'help> {
     ///     help    Print this message or the help of the given subcommand(s)
     ///     sub1
     /// ```
+    #[must_use]
     pub fn subcommand_value_name<S>(mut self, value_name: S) -> Self
     where
         S: Into<&'help str>,
@@ -2031,6 +2077,7 @@ impl<'help> App<'help> {
     ///     help    Print this message or the help of the given subcommand(s)
     ///     sub1
     /// ```
+    #[must_use]
     pub fn subcommand_help_heading<T>(mut self, heading: T) -> Self
     where
         T: Into<&'help str>,
@@ -2393,48 +2440,56 @@ impl<'help> App<'help> {
 
     /// Deprecated, replaced with [`App::override_usage`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::override_usage`")]
+    #[must_use]
     pub fn usage<S: Into<&'help str>>(self, usage: S) -> Self {
         self.override_usage(usage)
     }
 
     /// Deprecated, replaced with [`App::override_help`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::override_help`")]
+    #[must_use]
     pub fn help<S: Into<&'help str>>(self, help: S) -> Self {
         self.override_help(help)
     }
 
     /// Deprecated, replaced with [`App::mut_arg`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::mut_arg`")]
+    #[must_use]
     pub fn help_short(self, c: char) -> Self {
         self.mut_arg("help", |a| a.short(c))
     }
 
     /// Deprecated, replaced with [`App::mut_arg`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::mut_arg`")]
+    #[must_use]
     pub fn version_short(self, c: char) -> Self {
         self.mut_arg("version", |a| a.short(c))
     }
 
     /// Deprecated, replaced with [`App::mut_arg`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::mut_arg`")]
+    #[must_use]
     pub fn help_message(self, s: impl Into<&'help str>) -> Self {
         self.mut_arg("help", |a| a.help(s.into()))
     }
 
     /// Deprecated, replaced with [`App::mut_arg`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::mut_arg`")]
+    #[must_use]
     pub fn version_message(self, s: impl Into<&'help str>) -> Self {
         self.mut_arg("version", |a| a.help(s.into()))
     }
 
     /// Deprecated, replaced with [`App::help_template`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::help_template`")]
+    #[must_use]
     pub fn template<S: Into<&'help str>>(self, s: S) -> Self {
         self.help_template(s)
     }
 
     /// Deprecated, replaced with [`App::setting(a| b)`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::setting(a | b)`")]
+    #[must_use]
     pub fn settings(mut self, settings: &[AppSettings]) -> Self {
         for s in settings {
             self.settings.insert((*s).into());
@@ -2444,6 +2499,7 @@ impl<'help> App<'help> {
 
     /// Deprecated, replaced with [`App::unset_setting(a| b)`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::unset_setting(a | b)`")]
+    #[must_use]
     pub fn unset_settings(mut self, settings: &[AppSettings]) -> Self {
         for s in settings {
             self.settings.remove((*s).into());
@@ -2453,6 +2509,7 @@ impl<'help> App<'help> {
 
     /// Deprecated, replaced with [`App::global_setting(a| b)`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::global_setting(a | b)`")]
+    #[must_use]
     pub fn global_settings(mut self, settings: &[AppSettings]) -> Self {
         for s in settings {
             self.settings.insert((*s).into());
@@ -2463,12 +2520,14 @@ impl<'help> App<'help> {
 
     /// Deprecated, replaced with [`App::term_width`]
     #[deprecated(since = "3.0.0", note = "Replaced with `App::term_width`")]
+    #[must_use]
     pub fn set_term_width(self, width: usize) -> Self {
         self.term_width(width)
     }
 
     /// Deprecated in [Issue #3086](https://github.com/clap-rs/clap/issues/3086), see [`arg!`][crate::arg!].
     #[deprecated(since = "3.0.0", note = "Deprecated in Issue #3086, see `clap::arg!")]
+    #[must_use]
     pub fn arg_from_usage(self, usage: &'help str) -> Self {
         #![allow(deprecated)]
         self.arg(Arg::from_usage(usage))
@@ -2476,6 +2535,7 @@ impl<'help> App<'help> {
 
     /// Deprecated in [Issue #3086](https://github.com/clap-rs/clap/issues/3086), see [`arg!`][crate::arg!].
     #[deprecated(since = "3.0.0", note = "Deprecated in Issue #3086, see `clap::arg!")]
+    #[must_use]
     pub fn args_from_usage(mut self, usage: &'help str) -> Self {
         #![allow(deprecated)]
         for line in usage.lines() {
@@ -2925,7 +2985,7 @@ impl<'help> App<'help> {
         if let Some(bn) = self.bin_name.as_ref() {
             if bn.contains(' ') {
                 // In case we're dealing with subcommands i.e. git mv is translated to git-mv
-                format!("{} {}\n", bn.replace(" ", "-"), ver)
+                format!("{} {}\n", bn.replace(' ', "-"), ver)
             } else {
                 format!("{} {}\n", &self.name[..], ver)
             }
diff --git a/src/build/arg/mod.rs b/src/build/arg/mod.rs
index ec887f44049..053cb2dd4a5 100644
--- a/src/build/arg/mod.rs
+++ b/src/build/arg/mod.rs
@@ -127,6 +127,7 @@ impl<'help> Arg<'help> {
     /// Set the identifier used for referencing this argument in the clap API.
     ///
     /// See [`Arg::new`] for more details.
+    #[must_use]
     pub fn name<S: Into<&'help str>>(mut self, n: S) -> Self {
         let name = n.into();
         self.id = Id::from(&*name);
@@ -158,6 +159,7 @@ impl<'help> Arg<'help> {
     /// assert!(m.is_present("config"));
     /// ```
     #[inline]
+    #[must_use]
     pub fn short(mut self, s: char) -> Self {
         assert!(s != '-', "short option name cannot be `-`");
 
@@ -194,6 +196,7 @@ impl<'help> Arg<'help> {
     /// assert!(m.is_present("cfg"));
     /// ```
     #[inline]
+    #[must_use]
     pub fn long(mut self, l: &'help str) -> Self {
         self.long = Some(l.trim_start_matches(|c| c == '-'));
         self
@@ -219,6 +222,7 @@ impl<'help> Arg<'help> {
     /// assert!(m.is_present("test"));
     /// assert_eq!(m.value_of("test"), Some("cool"));
     /// ```
+    #[must_use]
     pub fn alias<S: Into<&'help str>>(mut self, name: S) -> Self {
         self.aliases.push((name.into(), false));
         self
@@ -244,6 +248,7 @@ impl<'help> Arg<'help> {
     /// assert!(m.is_present("test"));
     /// assert_eq!(m.value_of("test"), Some("cool"));
     /// ```
+    #[must_use]
     pub fn short_alias(mut self, name: char) -> Self {
         assert!(name != '-', "short alias name cannot be `-`");
 
@@ -271,6 +276,7 @@ impl<'help> Arg<'help> {
     ///             ]);
     /// assert!(m.is_present("test"));
     /// ```
+    #[must_use]
     pub fn aliases(mut self, names: &[&'help str]) -> Self {
         self.aliases.extend(names.iter().map(|&x| (x, false)));
         self
@@ -296,6 +302,7 @@ impl<'help> Arg<'help> {
     ///             ]);
     /// assert!(m.is_present("test"));
     /// ```
+    #[must_use]
     pub fn short_aliases(mut self, names: &[char]) -> Self {
         for s in names {
             assert!(s != &'-', "short alias name cannot be `-`");
@@ -324,6 +331,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(m.value_of("test"), Some("coffee"));
     /// ```
     /// [`App::alias`]: Arg::alias()
+    #[must_use]
     pub fn visible_alias<S: Into<&'help str>>(mut self, name: S) -> Self {
         self.aliases.push((name.into(), true));
         self
@@ -348,6 +356,7 @@ impl<'help> Arg<'help> {
     /// assert!(m.is_present("test"));
     /// assert_eq!(m.value_of("test"), Some("coffee"));
     /// ```
+    #[must_use]
     pub fn visible_short_alias(mut self, name: char) -> Self {
         assert!(name != '-', "short alias name cannot be `-`");
 
@@ -373,6 +382,7 @@ impl<'help> Arg<'help> {
     /// assert!(m.is_present("test"));
     /// ```
     /// [`App::aliases`]: Arg::aliases()
+    #[must_use]
     pub fn visible_aliases(mut self, names: &[&'help str]) -> Self {
         self.aliases.extend(names.iter().map(|n| (*n, true)));
         self
@@ -395,6 +405,7 @@ impl<'help> Arg<'help> {
     ///         ]);
     /// assert!(m.is_present("test"));
     /// ```
+    #[must_use]
     pub fn visible_short_aliases(mut self, names: &[char]) -> Self {
         for n in names {
             assert!(n != &'-', "short alias name cannot be `-`");
@@ -454,6 +465,7 @@ impl<'help> Arg<'help> {
     /// [`panic!`]: https://doc.rust-lang.org/std/macro.panic!.html
     /// [`App`]: crate::App
     #[inline]
+    #[must_use]
     pub fn index(mut self, idx: usize) -> Self {
         self.index = Some(idx);
         self
@@ -535,6 +547,7 @@ impl<'help> Arg<'help> {
     /// [index]: Arg::index()
     /// [`UnknownArgument`]: crate::ErrorKind::UnknownArgument
     #[inline]
+    #[must_use]
     pub fn last(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::Last)
@@ -597,6 +610,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(res.unwrap_err().kind, ErrorKind::MissingRequiredArgument);
     /// ```
     #[inline]
+    #[must_use]
     pub fn required(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::Required)
@@ -663,6 +677,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::requires(name)`]: Arg::requires()
     /// [Conflicting]: Arg::conflicts_with()
     /// [override]: Arg::overrides_with()
+    #[must_use]
     pub fn requires<T: Key>(mut self, arg_id: T) -> Self {
         self.requires.push((None, arg_id.into()));
         self
@@ -701,6 +716,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(res.unwrap_err().kind, ErrorKind::ArgumentConflict);
     /// ```
     #[inline]
+    #[must_use]
     pub fn exclusive(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::Exclusive)
@@ -743,6 +759,7 @@ impl<'help> Arg<'help> {
     /// [`Subcommand`]: crate::Subcommand
     /// [`ArgMatches::is_present("flag")`]: ArgMatches::is_present()
     #[inline]
+    #[must_use]
     pub fn global(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::Global)
@@ -797,6 +814,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(files, ["file1", "file2", "file3"]);
     /// ```
     #[inline]
+    #[must_use]
     pub fn multiple_occurrences(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::MultipleOccurrences)
@@ -855,6 +873,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::multiple_occurrences(true)`]: Arg::multiple_occurrences()
     #[inline]
+    #[must_use]
     pub fn max_occurrences(mut self, qty: usize) -> Self {
         self.max_occurs = Some(qty);
         if qty > 1 {
@@ -893,6 +912,7 @@ impl<'help> Arg<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn setting<F>(mut self, setting: F) -> Self
     where
         F: Into<ArgFlags>,
@@ -922,6 +942,7 @@ impl<'help> Arg<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn unset_setting<F>(mut self, setting: F) -> Self
     where
         F: Into<ArgFlags>,
@@ -965,6 +986,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::value_delimiter(char)`]: Arg::value_delimiter()
     /// [multiple values]: Arg::multiple_values
     #[inline]
+    #[must_use]
     pub fn takes_value(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::TakesValue)
@@ -1139,6 +1161,7 @@ impl<'help> Arg<'help> {
     /// [maximum]: Arg::max_values()
     /// [specific]: Arg::number_of_values()
     #[inline]
+    #[must_use]
     pub fn multiple_values(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::MultipleValues)
@@ -1187,6 +1210,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::multiple_occurrences(true)`]: Arg::multiple_occurrences()
     #[inline]
+    #[must_use]
     pub fn number_of_values(mut self, qty: usize) -> Self {
         self.num_vals = Some(qty);
         self.takes_value(true).multiple_values(true)
@@ -1250,6 +1274,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::multiple_occurrences(true)`]: Arg::multiple_occurrences()
     #[inline]
+    #[must_use]
     pub fn max_values(mut self, qty: usize) -> Self {
         self.max_vals = Some(qty);
         self.takes_value(true).multiple_values(true)
@@ -1314,6 +1339,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::multiple_occurrences(true)`]: Arg::multiple_occurrences()
     #[inline]
+    #[must_use]
     pub fn min_values(mut self, qty: usize) -> Self {
         self.min_vals = Some(qty);
         self.takes_value(true).multiple_values(true)
@@ -1366,6 +1392,7 @@ impl<'help> Arg<'help> {
     /// [positional]: Arg::index()
     /// [`Arg::takes_value(true)`]: Arg::takes_value()
     #[inline]
+    #[must_use]
     pub fn value_name(self, name: &'help str) -> Self {
         self.value_names(&[name])
     }
@@ -1423,6 +1450,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::number_of_values`]: Arg::number_of_values()
     /// [`Arg::takes_value(true)`]: Arg::takes_value()
     /// [`Arg::multiple_values(true)`]: Arg::multiple_values()
+    #[must_use]
     pub fn value_names(mut self, names: &[&'help str]) -> Self {
         self.val_names = names.to_vec();
         self.takes_value(true)
@@ -1457,6 +1485,7 @@ impl<'help> Arg<'help> {
     ///             .value_hint(ValueHint::CommandWithArguments)
     ///     );
     /// ```
+    #[must_use]
     pub fn value_hint(mut self, value_hint: ValueHint) -> Self {
         self.value_hint = value_hint;
         self.takes_value(true)
@@ -1500,6 +1529,7 @@ impl<'help> Arg<'help> {
     /// [`Result`]: std::result::Result
     /// [`Err(String)`]: std::result::Result::Err
     /// [`Arc`]: std::sync::Arc
+    #[must_use]
     pub fn validator<F, O, E>(mut self, mut f: F) -> Self
     where
         F: FnMut(&str) -> Result<O, E> + Send + 'help,
@@ -1542,6 +1572,7 @@ impl<'help> Arg<'help> {
     /// [`Result`]: std::result::Result
     /// [`Err(String)`]: std::result::Result::Err
     /// [`Rc`]: std::rc::Rc
+    #[must_use]
     pub fn validator_os<F, O, E>(mut self, mut f: F) -> Self
     where
         F: FnMut(&OsStr) -> Result<O, E> + Send + 'help,
@@ -1607,6 +1638,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(res.err().unwrap().kind, ErrorKind::ValueValidation)
     /// ```
     #[cfg(feature = "regex")]
+    #[must_use]
     pub fn validator_regex(
         self,
         regex: impl Into<RegexRef<'help>>,
@@ -1692,6 +1724,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [options]: Arg::takes_value()
     /// [positional arguments]: Arg::index()
+    #[must_use]
     pub fn possible_value<T>(mut self, value: T) -> Self
     where
         T: Into<PossibleValue<'help>>,
@@ -1767,6 +1800,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [options]: Arg::takes_value()
     /// [positional arguments]: Arg::index()
+    #[must_use]
     pub fn possible_values<I, T>(mut self, values: I) -> Self
     where
         I: IntoIterator<Item = T>,
@@ -1826,6 +1860,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(&*matched_vals, &["TeSt123", "teST123", "tESt321"]);
     /// ```
     #[inline]
+    #[must_use]
     pub fn ignore_case(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::IgnoreCase)
@@ -1885,6 +1920,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::number_of_values(1)`]: Arg::number_of_values()
     #[inline]
+    #[must_use]
     pub fn allow_hyphen_values(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::AllowHyphenValues)
@@ -1925,6 +1961,7 @@ impl<'help> Arg<'help> {
     /// [`ArgMatches::value_of_lossy`]: crate::ArgMatches::value_of_lossy()
     /// [`ArgMatches::values_of_lossy`]: crate::ArgMatches::values_of_lossy()
     #[inline]
+    #[must_use]
     pub fn allow_invalid_utf8(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::AllowInvalidUtf8)
@@ -1979,6 +2016,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(res.unwrap_err().kind, ErrorKind::EmptyValue);
     /// ```
     #[inline]
+    #[must_use]
     pub fn forbid_empty_values(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::ForbidEmptyValues)
@@ -2030,6 +2068,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(res.unwrap_err().kind, ErrorKind::NoEquals);
     /// ```
     #[inline]
+    #[must_use]
     pub fn require_equals(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::RequireEquals)
@@ -2088,6 +2127,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::value_delimiter`]: Arg::value_delimiter()
     #[inline]
+    #[must_use]
     pub fn use_delimiter(mut self, yes: bool) -> Self {
         if yes {
             if self.val_delim.is_none() {
@@ -2125,6 +2165,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::use_delimiter(true)`]: Arg::use_delimiter()
     /// [`Arg::takes_value(true)`]: Arg::takes_value()
     #[inline]
+    #[must_use]
     pub fn value_delimiter(mut self, d: char) -> Self {
         self.val_delim = Some(d);
         self.takes_value(true).use_delimiter(true)
@@ -2207,6 +2248,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(delims.values_of("opt").unwrap().collect::<Vec<_>>(), ["val1", "val2", "val3"]);
     /// ```
     #[inline]
+    #[must_use]
     pub fn require_delimiter(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::RequireDelimiter)
@@ -2265,6 +2307,7 @@ impl<'help> Arg<'help> {
     /// [`number_of_values`]: Arg::number_of_values()
     /// [`max_values`]: Arg::max_values()
     #[inline]
+    #[must_use]
     pub fn value_terminator(mut self, term: &'help str) -> Self {
         self.terminator = Some(term);
         self.takes_value(true)
@@ -2293,6 +2336,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::allow_hyphen_values(true)`]: Arg::allow_hyphen_values()
     /// [`Arg::last(true)`]: Arg::last()
     #[inline]
+    #[must_use]
     pub fn raw(self, yes: bool) -> Self {
         self.takes_value(yes)
             .multiple_values(yes)
@@ -2364,6 +2408,7 @@ impl<'help> Arg<'help> {
     /// [`ArgMatches::is_present`]: crate::ArgMatches::is_present()
     /// [`Arg::default_value_if`]: Arg::default_value_if()
     #[inline]
+    #[must_use]
     pub fn default_value(self, val: &'help str) -> Self {
         self.default_values_os(&[OsStr::new(val)])
     }
@@ -2375,6 +2420,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::default_value`]: Arg::default_value()
     /// [`OsStr`]: std::ffi::OsStr
     #[inline]
+    #[must_use]
     pub fn default_value_os(self, val: &'help OsStr) -> Self {
         self.default_values_os(&[val])
     }
@@ -2385,6 +2431,7 @@ impl<'help> Arg<'help> {
     ///
     /// [`Arg::default_value`]: Arg::default_value()
     #[inline]
+    #[must_use]
     pub fn default_values(self, vals: &[&'help str]) -> Self {
         let vals_vec: Vec<_> = vals.iter().map(|val| OsStr::new(*val)).collect();
         self.default_values_os(&vals_vec[..])
@@ -2397,6 +2444,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::default_values`]: Arg::default_values()
     /// [`OsStr`]: std::ffi::OsStr
     #[inline]
+    #[must_use]
     pub fn default_values_os(mut self, vals: &[&'help OsStr]) -> Self {
         self.default_vals = vals.to_vec();
         self.takes_value(true)
@@ -2474,6 +2522,7 @@ impl<'help> Arg<'help> {
     /// [`ArgMatches::is_present`]: ArgMatches::is_present()
     /// [`Arg::default_value`]: Arg::default_value()
     #[inline]
+    #[must_use]
     pub fn default_missing_value(self, val: &'help str) -> Self {
         self.default_missing_values_os(&[OsStr::new(val)])
     }
@@ -2485,6 +2534,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::default_missing_value`]: Arg::default_missing_value()
     /// [`OsStr`]: std::ffi::OsStr
     #[inline]
+    #[must_use]
     pub fn default_missing_value_os(self, val: &'help OsStr) -> Self {
         self.default_missing_values_os(&[val])
     }
@@ -2495,6 +2545,7 @@ impl<'help> Arg<'help> {
     ///
     /// [`Arg::default_missing_value`]: Arg::default_missing_value()
     #[inline]
+    #[must_use]
     pub fn default_missing_values(self, vals: &[&'help str]) -> Self {
         let vals_vec: Vec<_> = vals.iter().map(|val| OsStr::new(*val)).collect();
         self.default_missing_values_os(&vals_vec[..])
@@ -2507,6 +2558,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::default_missing_values`]: Arg::default_missing_values()
     /// [`OsStr`]: std::ffi::OsStr
     #[inline]
+    #[must_use]
     pub fn default_missing_values_os(mut self, vals: &[&'help OsStr]) -> Self {
         self.default_missing_vals = vals.to_vec();
         self.takes_value(true)
@@ -2652,6 +2704,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::use_delimiter(true)`]: Arg::use_delimiter()
     #[cfg(feature = "env")]
     #[inline]
+    #[must_use]
     pub fn env(self, name: &'help str) -> Self {
         self.env_os(OsStr::new(name))
     }
@@ -2661,6 +2714,7 @@ impl<'help> Arg<'help> {
     /// See [`Arg::env`].
     #[cfg(feature = "env")]
     #[inline]
+    #[must_use]
     pub fn env_os(mut self, name: &'help OsStr) -> Self {
         self.env = Some((name, env::var_os(name)));
         self
@@ -2712,6 +2766,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::long_help`]: Arg::long_help()
     #[inline]
+    #[must_use]
     pub fn help(mut self, h: impl Into<Option<&'help str>>) -> Self {
         self.help = h.into();
         self
@@ -2773,6 +2828,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::help`]: Arg::help()
     #[inline]
+    #[must_use]
     pub fn long_help(mut self, h: impl Into<Option<&'help str>>) -> Self {
         self.long_help = h.into();
         self
@@ -2832,6 +2888,7 @@ impl<'help> Arg<'help> {
     /// [positional arguments]: Arg::index()
     /// [index]: Arg::index()
     #[inline]
+    #[must_use]
     pub fn display_order(mut self, ord: usize) -> Self {
         self.disp_ord = Some(ord);
         self
@@ -2841,6 +2898,7 @@ impl<'help> Arg<'help> {
     ///
     /// [current]: crate::App::help_heading
     #[inline]
+    #[must_use]
     pub fn help_heading<O>(mut self, heading: O) -> Self
     where
         O: Into<Option<&'help str>>,
@@ -2893,6 +2951,7 @@ impl<'help> Arg<'help> {
     ///         on a line after the option
     /// ```
     #[inline]
+    #[must_use]
     pub fn next_line_help(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::NextLineHelp)
@@ -2934,6 +2993,7 @@ impl<'help> Arg<'help> {
     /// -V, --version    Print version information
     /// ```
     #[inline]
+    #[must_use]
     pub fn hide(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::Hidden)
@@ -2966,6 +3026,7 @@ impl<'help> Arg<'help> {
     /// If we were to run the above program with `--help` the `[values: fast, slow]` portion of
     /// the help text would be omitted.
     #[inline]
+    #[must_use]
     pub fn hide_possible_values(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::HidePossibleValues)
@@ -2996,6 +3057,7 @@ impl<'help> Arg<'help> {
     /// If we were to run the above program with `--help` the `[default: localhost]` portion of
     /// the help text would be omitted.
     #[inline]
+    #[must_use]
     pub fn hide_default_value(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::HideDefaultValue)
@@ -3024,6 +3086,7 @@ impl<'help> Arg<'help> {
     /// text would be omitted.
     #[cfg(feature = "env")]
     #[inline]
+    #[must_use]
     pub fn hide_env(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::HideEnv)
@@ -3053,6 +3116,7 @@ impl<'help> Arg<'help> {
     /// `[default: CONNECT=super_secret]` portion of the help text would be omitted.
     #[cfg(feature = "env")]
     #[inline]
+    #[must_use]
     pub fn hide_env_values(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::HideEnvValues)
@@ -3131,6 +3195,7 @@ impl<'help> Arg<'help> {
     /// -V, --version    Print version information
     /// ```
     #[inline]
+    #[must_use]
     pub fn hide_short_help(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::HiddenShortHelp)
@@ -3203,6 +3268,7 @@ impl<'help> Arg<'help> {
     /// -V, --version    Print version information
     /// ```
     #[inline]
+    #[must_use]
     pub fn hide_long_help(self, yes: bool) -> Self {
         if yes {
             self.setting(ArgSettings::HiddenLongHelp)
@@ -3245,6 +3311,7 @@ impl<'help> Arg<'help> {
     /// ```
     ///
     /// [`ArgGroup`]: crate::ArgGroup
+    #[must_use]
     pub fn group<T: Key>(mut self, group_id: T) -> Self {
         self.groups.push(group_id.into());
         self
@@ -3282,6 +3349,7 @@ impl<'help> Arg<'help> {
     /// ```
     ///
     /// [`ArgGroup`]: crate::ArgGroup
+    #[must_use]
     pub fn groups<T: Key>(mut self, group_ids: &[T]) -> Self {
         self.groups.extend(group_ids.iter().map(Id::from));
         self
@@ -3396,6 +3464,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::takes_value(true)`]: Arg::takes_value()
     /// [`Arg::default_value`]: Arg::default_value()
+    #[must_use]
     pub fn default_value_if<T: Key>(
         self,
         arg_id: T,
@@ -3410,6 +3479,7 @@ impl<'help> Arg<'help> {
     ///
     /// [`Arg::default_value_if`]: Arg::default_value_if()
     /// [`OsStr`]: std::ffi::OsStr
+    #[must_use]
     pub fn default_value_if_os<T: Key>(
         mut self,
         arg_id: T,
@@ -3497,6 +3567,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::takes_value(true)`]: Arg::takes_value()
     /// [`Arg::default_value_if`]: Arg::default_value_if()
+    #[must_use]
     pub fn default_value_ifs<T: Key>(
         mut self,
         ifs: &[(T, Option<&'help str>, Option<&'help str>)],
@@ -3512,6 +3583,7 @@ impl<'help> Arg<'help> {
     ///
     /// [`Arg::default_value_ifs`]: Arg::default_value_ifs()
     /// [`OsStr`]: std::ffi::OsStr
+    #[must_use]
     pub fn default_value_ifs_os<T: Key>(
         mut self,
         ifs: &[(T, Option<&'help OsStr>, Option<&'help OsStr>)],
@@ -3574,6 +3646,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(res.unwrap_err().kind, ErrorKind::MissingRequiredArgument);
     /// ```
     /// [required]: Arg::required()
+    #[must_use]
     pub fn required_unless_present<T: Key>(mut self, arg_id: T) -> Self {
         self.r_unless.push(arg_id.into());
         self
@@ -3644,6 +3717,7 @@ impl<'help> Arg<'help> {
     /// [required]: Arg::required()
     /// [`Arg::required_unless_present_any`]: Arg::required_unless_present_any()
     /// [`Arg::required_unless_present_all(names)`]: Arg::required_unless_present_all()
+    #[must_use]
     pub fn required_unless_present_all<T, I>(mut self, names: I) -> Self
     where
         I: IntoIterator<Item = T>,
@@ -3720,6 +3794,7 @@ impl<'help> Arg<'help> {
     /// [required]: Arg::required()
     /// [`Arg::required_unless_present_any(names)`]: Arg::required_unless_present_any()
     /// [`Arg::required_unless_present_all`]: Arg::required_unless_present_all()
+    #[must_use]
     pub fn required_unless_present_any<T, I>(mut self, names: I) -> Self
     where
         I: IntoIterator<Item = T>,
@@ -3810,6 +3885,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::requires(name)`]: Arg::requires()
     /// [Conflicting]: Arg::conflicts_with()
     /// [required]: Arg::required()
+    #[must_use]
     pub fn required_if_eq<T: Key>(mut self, arg_id: T, val: &'help str) -> Self {
         self.r_ifs.push((arg_id.into(), val));
         self
@@ -3888,6 +3964,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::requires(name)`]: Arg::requires()
     /// [Conflicting]: Arg::conflicts_with()
     /// [required]: Arg::required()
+    #[must_use]
     pub fn required_if_eq_any<T: Key>(mut self, ifs: &[(T, &'help str)]) -> Self {
         self.r_ifs
             .extend(ifs.iter().map(|(id, val)| (Id::from_ref(id), *val)));
@@ -3965,6 +4042,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(res.unwrap_err().kind, ErrorKind::MissingRequiredArgument);
     /// ```
     /// [required]: Arg::required()
+    #[must_use]
     pub fn required_if_eq_all<T: Key>(mut self, ifs: &[(T, &'help str)]) -> Self {
         self.r_ifs_all
             .extend(ifs.iter().map(|(id, val)| (Id::from_ref(id), *val)));
@@ -4028,6 +4106,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::requires(name)`]: Arg::requires()
     /// [Conflicting]: Arg::conflicts_with()
     /// [override]: Arg::overrides_with()
+    #[must_use]
     pub fn requires_if<T: Key>(mut self, val: &'help str, arg_id: T) -> Self {
         self.requires.push((Some(val), arg_id.into()));
         self
@@ -4079,6 +4158,7 @@ impl<'help> Arg<'help> {
     /// [`Arg::requires(name)`]: Arg::requires()
     /// [Conflicting]: Arg::conflicts_with()
     /// [override]: Arg::overrides_with()
+    #[must_use]
     pub fn requires_ifs<T: Key>(mut self, ifs: &[(&'help str, T)]) -> Self {
         self.requires
             .extend(ifs.iter().map(|(val, arg)| (Some(*val), Id::from(arg))));
@@ -4149,6 +4229,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [Conflicting]: Arg::conflicts_with()
     /// [override]: Arg::overrides_with()
+    #[must_use]
     pub fn requires_all<T: Key>(mut self, names: &[T]) -> Self {
         self.requires.extend(names.iter().map(|s| (None, s.into())));
         self
@@ -4199,6 +4280,7 @@ impl<'help> Arg<'help> {
     ///
     /// [`Arg::conflicts_with_all(names)`]: Arg::conflicts_with_all()
     /// [`Arg::exclusive(true)`]: Arg::exclusive()
+    #[must_use]
     pub fn conflicts_with<T: Key>(mut self, arg_id: T) -> Self {
         self.blacklist.push(arg_id.into());
         self
@@ -4251,6 +4333,7 @@ impl<'help> Arg<'help> {
     /// ```
     /// [`Arg::conflicts_with`]: Arg::conflicts_with()
     /// [`Arg::exclusive(true)`]: Arg::exclusive()
+    #[must_use]
     pub fn conflicts_with_all(mut self, names: &[&str]) -> Self {
         self.blacklist.extend(names.iter().map(Id::from));
         self
@@ -4366,6 +4449,7 @@ impl<'help> Arg<'help> {
     /// assert_eq!(m.occurrences_of("opt"), 2);
     /// assert_eq!(m.values_of("opt").unwrap().collect::<Vec<_>>(), &["first", "over", "other", "val"]);
     /// ```
+    #[must_use]
     pub fn overrides_with<T: Key>(mut self, arg_id: T) -> Self {
         self.overrides.push(arg_id.into());
         self
@@ -4399,6 +4483,7 @@ impl<'help> Arg<'help> {
     /// assert!(!m.is_present("debug"));
     /// assert!(!m.is_present("flag"));
     /// ```
+    #[must_use]
     pub fn overrides_with_all<T: Key>(mut self, names: &[T]) -> Self {
         self.overrides.extend(names.iter().map(Id::from));
         self
@@ -4686,6 +4771,7 @@ impl<'help> Arg<'help> {
 
     /// Deprecated, replaced with [`Arg::required_unless_present`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::required_unless_present`")]
+    #[must_use]
     pub fn required_unless<T: Key>(self, arg_id: T) -> Self {
         self.required_unless_present(arg_id)
     }
@@ -4695,6 +4781,7 @@ impl<'help> Arg<'help> {
         since = "3.0.0",
         note = "Replaced with `Arg::required_unless_present_all`"
     )]
+    #[must_use]
     pub fn required_unless_all<T, I>(self, names: I) -> Self
     where
         I: IntoIterator<Item = T>,
@@ -4708,6 +4795,7 @@ impl<'help> Arg<'help> {
         since = "3.0.0",
         note = "Replaced with `Arg::required_unless_present_any`"
     )]
+    #[must_use]
     pub fn required_unless_one<T, I>(self, names: I) -> Self
     where
         I: IntoIterator<Item = T>,
@@ -4718,12 +4806,14 @@ impl<'help> Arg<'help> {
 
     /// Deprecated, replaced with [`Arg::required_if_eq`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::required_if_eq`")]
+    #[must_use]
     pub fn required_if<T: Key>(self, arg_id: T, val: &'help str) -> Self {
         self.required_if_eq(arg_id, val)
     }
 
     /// Deprecated, replaced with [`Arg::required_if_eq_any`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::required_if_eq_any`")]
+    #[must_use]
     pub fn required_ifs<T: Key>(self, ifs: &[(T, &'help str)]) -> Self {
         self.required_if_eq_any(ifs)
     }
@@ -4731,6 +4821,7 @@ impl<'help> Arg<'help> {
     /// Deprecated, replaced with [`Arg::hide`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::hide`")]
     #[inline]
+    #[must_use]
     pub fn hidden(self, yes: bool) -> Self {
         self.hide(yes)
     }
@@ -4738,12 +4829,14 @@ impl<'help> Arg<'help> {
     /// Deprecated, replaced with [`Arg::ignore_case`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::ignore_case`")]
     #[inline]
+    #[must_use]
     pub fn case_insensitive(self, yes: bool) -> Self {
         self.ignore_case(yes)
     }
 
     /// Deprecated, replaced with [`Arg::forbid_empty_values`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::forbid_empty_values`")]
+    #[must_use]
     pub fn empty_values(self, yes: bool) -> Self {
         self.forbid_empty_values(!yes)
     }
@@ -4754,6 +4847,7 @@ impl<'help> Arg<'help> {
         since = "3.0.0",
         note = "Split into `Arg::multiple_occurrences` (most likely what you want)  and `Arg::multiple_values`"
     )]
+    #[must_use]
     pub fn multiple(self, yes: bool) -> Self {
         self.multiple_occurrences(yes).multiple_values(yes)
     }
@@ -4761,6 +4855,7 @@ impl<'help> Arg<'help> {
     /// Deprecated, replaced with [`Arg::hide_short_help`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::hide_short_help`")]
     #[inline]
+    #[must_use]
     pub fn hidden_short_help(self, yes: bool) -> Self {
         self.hide_short_help(yes)
     }
@@ -4768,18 +4863,21 @@ impl<'help> Arg<'help> {
     /// Deprecated, replaced with [`Arg::hide_long_help`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::hide_long_help`")]
     #[inline]
+    #[must_use]
     pub fn hidden_long_help(self, yes: bool) -> Self {
         self.hide_long_help(yes)
     }
 
     /// Deprecated, replaced with [`Arg::setting`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::setting`")]
+    #[must_use]
     pub fn set(self, s: ArgSettings) -> Self {
         self.setting(s)
     }
 
     /// Deprecated, replaced with [`Arg::unset_setting`]
     #[deprecated(since = "3.0.0", note = "Replaced with `Arg::unset_setting`")]
+    #[must_use]
     pub fn unset(self, s: ArgSettings) -> Self {
         self.unset_setting(s)
     }
diff --git a/src/build/arg/possible_value.rs b/src/build/arg/possible_value.rs
index f9433315aa9..0bd795c3f3a 100644
--- a/src/build/arg/possible_value.rs
+++ b/src/build/arg/possible_value.rs
@@ -71,6 +71,7 @@ impl<'help> PossibleValue<'help> {
     /// # ;
     /// ```
     #[inline]
+    #[must_use]
     pub fn help(mut self, help: &'help str) -> Self {
         self.help = Some(help);
         self
@@ -91,6 +92,7 @@ impl<'help> PossibleValue<'help> {
     /// ```
     /// [`Arg::hide_possible_values(true)`]: crate::Arg::hide_possible_values()
     #[inline]
+    #[must_use]
     pub fn hide(mut self, yes: bool) -> Self {
         self.hide = yes;
         self
@@ -106,6 +108,7 @@ impl<'help> PossibleValue<'help> {
     ///     .alias("not-fast")
     /// # ;
     /// ```
+    #[must_use]
     pub fn alias(mut self, name: &'help str) -> Self {
         self.aliases.push(name);
         self
@@ -121,6 +124,7 @@ impl<'help> PossibleValue<'help> {
     ///     .aliases(["not-fast", "snake-like"])
     /// # ;
     /// ```
+    #[must_use]
     pub fn aliases<I>(mut self, names: I) -> Self
     where
         I: IntoIterator<Item = &'help str>,
diff --git a/src/build/arg_group.rs b/src/build/arg_group.rs
index 3b9b015b446..49d95a869fe 100644
--- a/src/build/arg_group.rs
+++ b/src/build/arg_group.rs
@@ -121,6 +121,7 @@ impl<'help> ArgGroup<'help> {
     /// ArgGroup::default().name("config")
     /// # ;
     /// ```
+    #[must_use]
     pub fn name<S: Into<&'help str>>(mut self, n: S) -> Self {
         self.name = n.into();
         self.id = Id::from(&self.name);
@@ -148,6 +149,7 @@ impl<'help> ArgGroup<'help> {
     /// assert!(m.is_present("flag"));
     /// ```
     /// [argument]: crate::Arg
+    #[must_use]
     pub fn arg<T: Key>(mut self, arg_id: T) -> Self {
         self.args.push(arg_id.into());
         self
@@ -173,6 +175,7 @@ impl<'help> ArgGroup<'help> {
     /// assert!(m.is_present("flag"));
     /// ```
     /// [arguments]: crate::Arg
+    #[must_use]
     pub fn args<T: Key>(mut self, ns: &[T]) -> Self {
         for n in ns {
             self = self.arg(n);
@@ -222,6 +225,7 @@ impl<'help> ArgGroup<'help> {
     ///
     /// [`Arg`]: crate::Arg
     #[inline]
+    #[must_use]
     pub fn multiple(mut self, yes: bool) -> Self {
         self.multiple = yes;
         self
@@ -266,6 +270,7 @@ impl<'help> ArgGroup<'help> {
     /// [`ArgGroup::multiple`]: ArgGroup::multiple()
     /// [`App`]: crate::App
     #[inline]
+    #[must_use]
     pub fn required(mut self, yes: bool) -> Self {
         self.required = yes;
         self
@@ -305,6 +310,7 @@ impl<'help> ArgGroup<'help> {
     /// ```
     /// [required group]: ArgGroup::required()
     /// [argument requirement rules]: crate::Arg::requires()
+    #[must_use]
     pub fn requires<T: Key>(mut self, id: T) -> Self {
         self.requires.push(id.into());
         self
@@ -346,6 +352,7 @@ impl<'help> ArgGroup<'help> {
     /// ```
     /// [required group]: ArgGroup::required()
     /// [argument requirement rules]: crate::Arg::requires_all()
+    #[must_use]
     pub fn requires_all(mut self, ns: &[&'help str]) -> Self {
         for n in ns {
             self = self.requires(n);
@@ -385,6 +392,7 @@ impl<'help> ArgGroup<'help> {
     /// assert_eq!(err.kind, ErrorKind::ArgumentConflict);
     /// ```
     /// [argument exclusion rules]: crate::Arg::conflicts_with()
+    #[must_use]
     pub fn conflicts_with<T: Key>(mut self, id: T) -> Self {
         self.conflicts.push(id.into());
         self
@@ -425,6 +433,7 @@ impl<'help> ArgGroup<'help> {
     /// ```
     ///
     /// [argument exclusion rules]: crate::Arg::conflicts_with_all()
+    #[must_use]
     pub fn conflicts_with_all(mut self, ns: &[&'help str]) -> Self {
         for n in ns {
             self = self.conflicts_with(n);
diff --git a/src/lib.rs b/src/lib.rs
index 02b4d6829e3..b9b4de27f21 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,7 +13,8 @@
     missing_copy_implementations,
     trivial_casts,
     unused_allocation,
-    trivial_numeric_casts
+    trivial_numeric_casts,
+    clippy::single_char_pattern
 )]
 #![forbid(unsafe_code)]
 // HACK https://github.com/rust-lang/rust-clippy/issues/7290
diff --git a/src/output/fmt.rs b/src/output/fmt.rs
index e4317f799d1..a416935bb8a 100644
--- a/src/output/fmt.rs
+++ b/src/output/fmt.rs
@@ -8,6 +8,7 @@ use std::{
 #[derive(Clone, Debug)]
 pub(crate) struct Colorizer {
     use_stderr: bool,
+    #[allow(unused)]
     color_when: ColorChoice,
     pieces: Vec<(String, Style)>,
 }
diff --git a/src/output/help.rs b/src/output/help.rs
index ed6bf218eb2..7544b0c18df 100644
--- a/src/output/help.rs
+++ b/src/output/help.rs
@@ -868,7 +868,7 @@ impl<'help, 'app, 'parser, 'writer> Help<'help, 'app, 'parser, 'writer> {
         let bin_name = if let Some(bn) = self.parser.app.bin_name.as_ref() {
             if bn.contains(' ') {
                 // In case we're dealing with subcommands i.e. git mv is translated to git-mv
-                bn.replace(" ", "-")
+                bn.replace(' ', "-")
             } else {
                 text_wrapper(&self.parser.app.name.replace("{n}", "\n"), self.term_w)
             }
diff --git a/src/parse/errors.rs b/src/parse/errors.rs
index 0e902337d09..0e889ad774f 100644
--- a/src/parse/errors.rs
+++ b/src/parse/errors.rs
@@ -456,6 +456,7 @@ impl Error {
     }
 
     /// Format the existing message with the App's context
+    #[must_use]
     pub fn format(mut self, app: &mut App) -> Self {
         app._build();
         let usage = app.render_usage();
diff --git a/tests/builder/double_require.rs b/tests/builder/double_require.rs
index 166c0a7fe30..71a99947f4a 100644
--- a/tests/builder/double_require.rs
+++ b/tests/builder/double_require.rs
@@ -68,7 +68,7 @@ fn help_text() {
     assert!(res.is_err());
     let err = res.unwrap_err();
     assert_eq!(err.kind, ErrorKind::DisplayHelp);
-    println!("{}", err.to_string());
+    println!("{}", err);
     assert_eq!(err.to_string(), HELP);
 }
 
diff --git a/tests/builder/utils.rs b/tests/builder/utils.rs
index 56017c1a0e0..094531362e6 100644
--- a/tests/builder/utils.rs
+++ b/tests/builder/utils.rs
@@ -14,8 +14,8 @@ where
 {
     let re = Regex::new("\x1b[^m]*m").unwrap();
     // Strip out any mismatching \r character on windows that might sneak in on either side
-    let ls = l.as_ref().replace("\r", "");
-    let rs = r.as_ref().replace("\r", "");
+    let ls = l.as_ref().replace('\r', "");
+    let rs = r.as_ref().replace('\r', "");
     let left_ = re.replace_all(&*ls, "");
     let right = re.replace_all(&*rs, "");
     let b = left_ == right;