docs: Clarify value parser

Things that tripped up a user
- Derive reference was misunderstood to say that the only alternative to
  the built-in value parser behavior was to implement
  `ValueParserFactory`
  - We now delegate to `value_parser!`s docs any talk of integrating
    into it (it should have been a subbullet of "not present" anyways)
- `value_parser!` relies too much on the example to demonstrate behavior
  when the user will likely make the determination of whether its
  relevant before then
  - We are now more upfront what type mappings are supported
- Too many steps to find all information
  - For example, a user needs to look at `TypedValueParser`
    implementations, `ValueParserFactory` implementations, and `From<T>
    for ValueParser` implementations to understand what all can be used
  - We are now more upfront with a lot of this information at the entry
    points the user is most likely to look at

In addition, I did an audit of the docs to make sure they were updated
for us only supporting the new behavior and all of the new APIs.

See https://www.reddit.com/r/rust/comments/xjlie4/preannouncing_clap_40_a_rust_cli_argument_parser/ip9kzf1/

src/_derive/_tutorial.rs

@@ -154,9 +154,12 @@
 //!
 //! ## Validation
 //!
+//! An appropriate default parser/validator will be selected for the field's type.  See
+//! [`value_parser!][crate::value_parser!] for more details.
+//!
 //! ### Enumerated values
 //!
-//! If you have arguments of specific values you want to test for, you can derive
+//! For example, if you have arguments of specific values you want to test for, you can derive
 //! [`ValueEnum`][crate::ValueEnum].
 //!
 //! This allows you specify the valid values for that argument. If the user does not use one of
@@ -184,6 +187,8 @@
 //! ```
 #![doc = include_str!("../../examples/tutorial_derive/04_02_validate.md")]
 //!
+//! See [`Arg::value_parser`][crate::Arg::value_parser] for more details.
+//!
 //! ### Argument Relations
 //!
 //! You can declare dependencies or conflicts between [`Arg`][crate::Arg]s or even

src/_derive/mod.rs

@@ -186,7 +186,6 @@
 //! - `value_parser [= <expr>]`: [`Arg::value_parser`][crate::Arg::value_parser]
 //!   - When not present: will auto-select an implementation based on the field type using
 //!     [`value_parser!`][crate::value_parser!]
-//!   - To register a custom type's [`ValueParser`][crate::builder::ValueParser], implement [`ValueParserFactory`][crate::builder::ValueParserFactory]
 //! - `action [= <expr>]`: [`Arg::action`][crate::Arg::action]
 //!   - When not present: will auto-select an action based on the field type
 //! - `help = <expr>`: [`Arg::help`][crate::Arg::help]
@@ -254,19 +253,22 @@
 //!
 //! `clap` assumes some intent based on the type used:
 //!
-//! | Type                | Effect                               | Implies                                                                  |
-//! |---------------------|--------------------------------------|--------------------------------------------------------------------------|
-//! | `bool`              | flag                                 | `.action(ArgAction::SetTrue)                                             |
-//! | `Option<T>`         | optional argument                    | `.action(ArgAction::Set).required(false)`                                |
-//! | `Option<Option<T>>` | optional value for optional argument | `.action(ArgAction::Set).required(false).min_values(0).max_values(1)`    |
-//! | `T`                 | required argument                    | `.action(ArgAction::Set).required(!has_default)`                         |
-//! | `Vec<T>`            | `0..` occurrences of argument        | `.action(ArgAction::Append).required(false).multiple_occurrences(true)`  |
-//! | `Option<Vec<T>>`    | `0..` occurrences of argument        | `.action(ArgAction::Append).required(false).multiple_occurrences(true)`  |
+//! | Type                | Effect                               | Implies                                                     |
+//! |---------------------|--------------------------------------|-------------------------------------------------------------|
+//! | `bool`              | flag                                 | `.action(ArgAction::SetTrue)                                |
+//! | `Option<T>`         | optional argument                    | `.action(ArgAction::Set).required(false)`                   |
+//! | `Option<Option<T>>` | optional value for optional argument | `.action(ArgAction::Set).required(false).num_args(0..=1)`   |
+//! | `T`                 | required argument                    | `.action(ArgAction::Set).required(!has_default)`            |
+//! | `Vec<T>`            | `0..` occurrences of argument        | `.action(ArgAction::Append).required(false).num_args(1..)`  |
+//! | `Option<Vec<T>>`    | `0..` occurrences of argument        | `.action(ArgAction::Append).required(false).num_args(1..)`  |
+//!
+//! In addition, [`.value_parser(value_parser!(T))`][crate::value_parser!] is called for each
+//! field.
 //!
 //! Notes:
 //! - For custom type behavior, you can override the implied attributes/settings and/or set additional ones
 //! - `Option<Vec<T>>` will be `None` instead of `vec![]` if no arguments are provided.
-//!   - This gives the user some flexibility in designing their argument, like with `min_values(0)`
+//!   - This gives the user some flexibility in designing their argument, like with `num_args(0..)`
 //!
 //! ## Doc Comments
 //!

src/_tutorial.rs

@@ -143,6 +143,8 @@
 //!
 //! ## Validation
 //!
+//! By default, arguments are assumed to be `String`s and only UTF-8 validation is performed.
+//!
 //! ### Enumerated values
 //!
 //! If you have arguments of specific values you want to test for, you can use the
@@ -183,6 +185,8 @@
 //! ```
 #![doc = include_str!("../examples/tutorial_builder/04_02_validate.md")]
 //!
+//! See [`Arg::value_parser`][crate::Arg::value_parser] for more details.
+//!
 //! ### Argument Relations
 //!
 //! You can declare dependencies or conflicts between [`Arg`][crate::Arg]s or even

src/builder/arg.rs

@@ -867,21 +867,22 @@ impl Arg {
         self
     }
 
-    /// Specify the type of the argument.
+    /// Specify the typed behavior of the argument.
     ///
     /// This allows parsing and validating a value before storing it into
-    /// [`ArgMatches`][crate::ArgMatches].
+    /// [`ArgMatches`][crate::ArgMatches] as the given type.
+    ///
+    /// Possible value parsers include:
+    /// - [`value_parser!(T)`][crate::value_parser!] for auto-selecting a value parser for a given type
+    ///   - Or [range expressions like `0..=1`][std::ops::RangeBounds] as a shorthand for [`RangedI64ValueParser`][crate::builder::RangedI64ValueParser]
+    /// - `Fn(&str) -> Result<T, E>`
+    /// - `[&str]` and [`PossibleValuesParser`][crate::builder::PossibleValuesParser] for static enumerated values
+    /// - [`BoolishValueParser`][crate::builder::BoolishValueParser], and [`FalseyValueParser`][crate::builder::FalseyValueParser] for alternative `bool` implementations
+    /// - [`NonEmptyStringValueParser`][crate::builder::NonEmptyStringValueParser] for basic validation for strings
+    /// - or any other [`TypedValueParser`][crate::builder::TypedValueParser] implementation
     ///
     /// The default value is [`ValueParser::string`][crate::builder::ValueParser::string].
     ///
-    /// See also
-    /// - [`value_parser!`][crate::value_parser!] for auto-selecting a value parser for a given type
-    ///   - [`BoolishValueParser`][crate::builder::BoolishValueParser], and [`FalseyValueParser`][crate::builder::FalseyValueParser] for alternative `bool` implementations
-    ///   - [`NonEmptyStringValueParser`][crate::builder::NonEmptyStringValueParser] for basic validation for strings
-    /// - [`RangedI64ValueParser`][crate::builder::RangedI64ValueParser] and [`RangedU64ValueParser`][crate::builder::RangedU64ValueParser] for numeric ranges
-    /// - [`EnumValueParser`][crate::builder::EnumValueParser] and  [`PossibleValuesParser`][crate::builder::PossibleValuesParser] for static enumerated values
-    /// - or any other [`TypedValueParser`][crate::builder::TypedValueParser] implementation
-    ///
     /// ```rust
     /// # use clap::ArgAction;
     /// let mut cmd = clap::Command::new("raw")

src/builder/value_parser.rs

@@ -73,9 +73,8 @@ enum ValueParserInner {
 impl ValueParser {
     /// Custom parser for argument values
     ///
-    /// To create a custom parser, see [`TypedValueParser`]
-    ///
-    /// Pre-existing implementations include:
+    /// Pre-existing [`TypedValueParser`] implementations include:
+    /// - `Fn(&str) -> Result<T, E>`
     /// - [`EnumValueParser`] and  [`PossibleValuesParser`] for static enumerated values
     /// - [`BoolishValueParser`] and [`FalseyValueParser`] for alternative `bool` implementations
     /// - [`RangedI64ValueParser`] and [`RangedU64ValueParser`]
@@ -989,7 +988,8 @@ impl<E: crate::ValueEnum + Clone + Send + Sync + 'static> Default for EnumValueP
 /// Verify the value is from an enumerated set of [`PossibleValue`][crate::builder::PossibleValue].
 ///
 /// See also:
-/// - [`EnumValueParser`]
+/// - [`EnumValueParser`] for directly supporting `enum`s
+/// - [`TypedValueParser::map`] for adapting values to a more specialized type
 ///
 /// # Example
 ///
@@ -2083,7 +2083,12 @@ pub mod via_prelude {
 
 /// Select a [`ValueParser`] implementation from the intended type
 ///
-/// To register a custom type with this macro, implement [`ValueParserFactory`].
+/// Supported types
+/// - [`ValueParserFactory` types][ValueParserFactory], including
+///   - [Native types][ValueParser]: `bool`, `String`, `OsString`, `PathBuf`
+///   - [Ranged numeric types][RangedI64ValueParser]: `u8`, `i8`, `u16`, `i16, `u32`, `i32`, `u64`, `i64`
+/// - [`ValueEnum` types][crate::ValueEnum]
+/// - [`FromStr` types][std::str::FromStr], including usize, isize
 ///
 /// # Example
 ///
@@ -2104,7 +2109,7 @@ pub mod via_prelude {
 /// assert_eq!(port, Path::new("file.txt"));
 /// ```
 ///
-/// Supported types:
+/// Example mappings:
 /// ```rust
 /// // Built-in types
 /// let parser = clap::value_parser!(String);