Make std::env::{set_var, remove_var} unsafe in edition 2024

compiler/rustc_feature/src/builtin_attrs.rs

@@ -578,6 +578,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "rustc_allowed_through_unstable_modules special cases accidental stabilizations of stable items \
         through unstable paths"
     ),
+    rustc_attr!(
+        rustc_deprecated_safe_2024, Normal, template!(Word), WarnFollowing,
+        EncodeCrossCrate::Yes,
+        "rustc_deprecated_safe_2024 is supposed to be used in libstd only",
+    ),
+
 
     // ==========================================================================
     // Internal attributes: Type system related:

compiler/rustc_lint_defs/src/builtin.rs

@@ -37,6 +37,7 @@ declare_lint_pass! {
         DEPRECATED,
         DEPRECATED_CFG_ATTR_CRATE_TYPE_NAME,
         DEPRECATED_IN_FUTURE,
+        DEPRECATED_SAFE,
         DEPRECATED_WHERE_CLAUSE_LOCATION,
         DUPLICATE_MACRO_ATTRIBUTES,
         ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
@@ -4844,3 +4845,51 @@ declare_lint! {
         reference: "issue #124559 <https://github.com/rust-lang/rust/issues/124559>",
     };
 }
+
+declare_lint! {
+    /// The `deprecated_safe` lint detects unsafe functions being used as safe
+    /// functions.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,edition2021,compile_fail
+    /// #![deny(deprecated_safe)]
+    /// // edition 2021
+    /// use std::env;
+    /// fn enable_backtrace() {
+    ///     env::set_var("RUST_BACKTRACE", "1");
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Rust [editions] allow the language to evolve without breaking backward
+    /// compatibility. This lint catches code that uses `unsafe` functions that
+    /// were declared as safe (non-`unsafe`) in earlier editions. If you switch
+    /// the compiler to a new edition without updating the code, then it
+    /// will fail to compile if you are using a function previously marked as
+    /// safe.
+    ///
+    /// You can audit the code to see if it suffices the preconditions of the
+    /// `unsafe` code, and if it does, you can wrap it in an `unsafe` block. If
+    /// you can't fulfill the preconditions, you probably need to switch to a
+    /// different way of doing what you want to achieve.
+    ///
+    /// This lint can automatically wrap the calls in `unsafe` blocks, but this
+    /// obviously cannot verify that the preconditions of the `unsafe`
+    /// functions are fulfilled, so that is still up to the user.
+    ///
+    /// The lint is currently "allow" by default, but that might change in the
+    /// future.
+    ///
+    /// [editions]: https://doc.rust-lang.org/edition-guide/
+    pub DEPRECATED_SAFE,
+    Allow,
+    "detects unsafe functions being used as safe functions",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::EditionError(Edition::Edition2024),
+        reference: "issue #27970 <https://github.com/rust-lang/rust/issues/27970>",
+    };
+}

compiler/rustc_mir_build/messages.ftl

@@ -28,6 +28,12 @@ mir_build_borrow_of_moved_value = borrow of moved value
     .value_borrowed_label = value borrowed here after move
     .suggestion = borrow this binding in the pattern to avoid moving the value
 
+mir_build_call_to_deprecated_safe_fn_requires_unsafe =
+    call to deprecated safe function `{$function}` is unsafe and requires unsafe block
+    .note = consult the function's documentation for information on how to avoid undefined behavior
+    .label = call to unsafe function
+    .suggestion = you can wrap the call in an `unsafe` block if you can guarantee the code is only ever called from single-threaded code
+
 mir_build_call_to_fn_with_requires_unsafe =
     call to function `{$function}` with `#[target_feature]` is unsafe and requires unsafe block
     .help = in order for the call to be safe, the context requires the following additional target {$missing_target_features_count ->

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -9,7 +9,7 @@ use rustc_middle::thir::visit::Visitor;
 use rustc_middle::thir::*;
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt};
-use rustc_session::lint::builtin::{UNSAFE_OP_IN_UNSAFE_FN, UNUSED_UNSAFE};
+use rustc_session::lint::builtin::{DEPRECATED_SAFE, UNSAFE_OP_IN_UNSAFE_FN, UNUSED_UNSAFE};
 use rustc_session::lint::Level;
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::symbol::Symbol;
@@ -110,14 +110,34 @@ impl<'tcx> UnsafetyVisitor<'_, 'tcx> {
                 );
                 self.suggest_unsafe_block = false;
             }
-            SafetyContext::Safe => {
-                kind.emit_requires_unsafe_err(
+            SafetyContext::Safe => match kind {
+                // Allow calls to deprecated-safe unsafe functions if the
+                // caller is from an edition before 2024.
+                UnsafeOpKind::CallToUnsafeFunction(Some(id))
+                    if !span.at_least_rust_2024()
+                        && self.tcx.has_attr(id, sym::rustc_deprecated_safe_2024) =>
+                {
+                    self.tcx.emit_node_span_lint(
+                        DEPRECATED_SAFE,
+                        self.hir_context,
+                        span,
+                        CallToDeprecatedSafeFnRequiresUnsafe {
+                            span,
+                            function: with_no_trimmed_paths!(self.tcx.def_path_str(id)),
+                            sub: CallToDeprecatedSafeFnRequiresUnsafeSub {
+                                left: span.shrink_to_lo(),
+                                right: span.shrink_to_hi(),
+                            },
+                        },
+                    )
+                }
+                _ => kind.emit_requires_unsafe_err(
                     self.tcx,
                     span,
                     self.hir_context,
                     unsafe_op_in_unsafe_fn_allowed,
-                );
-            }
+                ),
+            },
         }
     }
 

compiler/rustc_mir_build/src/errors.rs

@@ -20,6 +20,25 @@ pub struct UnconditionalRecursion {
     pub call_sites: Vec<Span>,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(mir_build_call_to_deprecated_safe_fn_requires_unsafe)]
+pub struct CallToDeprecatedSafeFnRequiresUnsafe {
+    #[label]
+    pub span: Span,
+    pub function: String,
+    #[subdiagnostic]
+    pub sub: CallToDeprecatedSafeFnRequiresUnsafeSub,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(mir_build_suggestion, applicability = "machine-applicable")]
+pub struct CallToDeprecatedSafeFnRequiresUnsafeSub {
+    #[suggestion_part(code = "unsafe {{ ")]
+    pub left: Span,
+    #[suggestion_part(code = " }}")]
+    pub right: Span,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(mir_build_unsafe_op_in_unsafe_fn_call_to_unsafe_fn_requires_unsafe, code = E0133)]
 #[note]

compiler/rustc_span/src/symbol.rs

@@ -1575,6 +1575,7 @@ symbols! {
         rustc_def_path,
         rustc_default_body_unstable,
         rustc_deny_explicit_impl,
+        rustc_deprecated_safe_2024,
         rustc_diagnostic_item,
         rustc_diagnostic_macros,
         rustc_dirty,

library/std/src/env.rs

@@ -318,22 +318,25 @@ impl Error for VarError {
 ///
 /// # Safety
 ///
-/// Even though this function is currently not marked as `unsafe`, it needs to
-/// be because invoking it can cause undefined behaviour. The function will be
-/// marked `unsafe` in a future version of Rust. This is tracked in
-/// [rust#27970](https://github.com/rust-lang/rust/issues/27970).
-///
 /// This function is safe to call in a single-threaded program.
 ///
-/// In multi-threaded programs, you must ensure that are no other threads
-/// concurrently writing or *reading*(!) from the environment through functions
-/// other than the ones in this module. You are responsible for figuring out
-/// how to achieve this, but we strongly suggest not using `set_var` or
-/// `remove_var` in multi-threaded programs at all.
-///
-/// Most C libraries, including libc itself do not advertise which functions
-/// read from the environment. Even functions from the Rust standard library do
-/// that, e.g. for DNS lookups from [`std::net::ToSocketAddrs`].
+/// This function is also always safe to call on Windows, in single-threaded
+/// and multi-threaded programs.
+///
+/// In multi-threaded programs on other operating systems, we strongly suggest
+/// not using `set_var` or `remove_var` at all. The exact requirement is: you
+/// must ensure that there are no other threads concurrently writing or
+/// *reading*(!) the environment through functions or global variables other
+/// than the ones in this module. The problem is that these operating systems
+/// do not provide a thread-safe way to read the environment, and most C
+/// libraries, including libc itself, do not advertise which functions read
+/// from the environment. Even functions from the Rust standard library may
+/// read the environment without going through this module, e.g. for DNS
+/// lookups from [`std::net::ToSocketAddrs`]. No stable guarantee is made about
+/// which functions may read from the environment in future versions of a
+/// library. All this makes it not practically possible for you to guarantee
+/// that no other thread will read the environment, so the only safe option is
+/// to not use `set_var` or `remove_var` in multi-threaded programs at all.
 ///
 /// Discussion of this unsafety on Unix may be found in:
 ///
@@ -353,15 +356,26 @@ impl Error for VarError {
 /// use std::env;
 ///
 /// let key = "KEY";
-/// env::set_var(key, "VALUE");
+/// unsafe {
+///     env::set_var(key, "VALUE");
+/// }
 /// assert_eq!(env::var(key), Ok("VALUE".to_string()));
 /// ```
+#[cfg(not(bootstrap))]
+#[rustc_deprecated_safe_2024]
 #[stable(feature = "env", since = "1.0.0")]
-pub fn set_var<K: AsRef<OsStr>, V: AsRef<OsStr>>(key: K, value: V) {
+pub unsafe fn set_var<K: AsRef<OsStr>, V: AsRef<OsStr>>(key: K, value: V) {
     _set_var(key.as_ref(), value.as_ref())
 }
 
-fn _set_var(key: &OsStr, value: &OsStr) {
+#[cfg(bootstrap)]
+#[allow(missing_docs)]
+#[stable(feature = "env", since = "1.0.0")]
+pub fn set_var<K: AsRef<OsStr>, V: AsRef<OsStr>>(key: K, value: V) {
+    unsafe { _set_var(key.as_ref(), value.as_ref()) }
+}
+
+unsafe fn _set_var(key: &OsStr, value: &OsStr) {
     os_imp::setenv(key, value).unwrap_or_else(|e| {
         panic!("failed to set environment variable `{key:?}` to `{value:?}`: {e}")
     })
@@ -371,22 +385,25 @@ fn _set_var(key: &OsStr, value: &OsStr) {
 ///
 /// # Safety
 ///
-/// Even though this function is currently not marked as `unsafe`, it needs to
-/// be because invoking it can cause undefined behaviour. The function will be
-/// marked `unsafe` in a future version of Rust. This is tracked in
-/// [rust#27970](https://github.com/rust-lang/rust/issues/27970).
-///
 /// This function is safe to call in a single-threaded program.
 ///
-/// In multi-threaded programs, you must ensure that are no other threads
-/// concurrently writing or *reading*(!) from the environment through functions
-/// other than the ones in this module. You are responsible for figuring out
-/// how to achieve this, but we strongly suggest not using `set_var` or
-/// `remove_var` in multi-threaded programs at all.
-///
-/// Most C libraries, including libc itself do not advertise which functions
-/// read from the environment. Even functions from the Rust standard library do
-/// that, e.g. for DNS lookups from [`std::net::ToSocketAddrs`].
+/// This function is also always safe to call on Windows, in single-threaded
+/// and multi-threaded programs.
+///
+/// In multi-threaded programs on other operating systems, we strongly suggest
+/// not using `set_var` or `remove_var` at all. The exact requirement is: you
+/// must ensure that there are no other threads concurrently writing or
+/// *reading*(!) the environment through functions or global variables other
+/// than the ones in this module. The problem is that these operating systems
+/// do not provide a thread-safe way to read the environment, and most C
+/// libraries, including libc itself, do not advertise which functions read
+/// from the environment. Even functions from the Rust standard library may
+/// read the environment without going through this module, e.g. for DNS
+/// lookups from [`std::net::ToSocketAddrs`]. No stable guarantee is made about
+/// which functions may read from the environment in future versions of a
+/// library. All this makes it not practically possible for you to guarantee
+/// that no other thread will read the environment, so the only safe option is
+/// to not use `set_var` or `remove_var` in multi-threaded programs at all.
 ///
 /// Discussion of this unsafety on Unix may be found in:
 ///
@@ -403,22 +420,35 @@ fn _set_var(key: &OsStr, value: &OsStr) {
 ///
 /// # Examples
 ///
-/// ```
+/// ```no_run
 /// use std::env;
 ///
 /// let key = "KEY";
-/// env::set_var(key, "VALUE");
+/// unsafe {
+///     env::set_var(key, "VALUE");
+/// }
 /// assert_eq!(env::var(key), Ok("VALUE".to_string()));
 ///
-/// env::remove_var(key);
+/// unsafe {
+///     env::remove_var(key);
+/// }
 /// assert!(env::var(key).is_err());
 /// ```
+#[cfg(not(bootstrap))]
+#[rustc_deprecated_safe_2024]
 #[stable(feature = "env", since = "1.0.0")]
-pub fn remove_var<K: AsRef<OsStr>>(key: K) {
+pub unsafe fn remove_var<K: AsRef<OsStr>>(key: K) {
     _remove_var(key.as_ref())
 }
 
-fn _remove_var(key: &OsStr) {
+#[cfg(bootstrap)]
+#[allow(missing_docs)]
+#[stable(feature = "env", since = "1.0.0")]
+pub fn remove_var<K: AsRef<OsStr>>(key: K) {
+    unsafe { _remove_var(key.as_ref()) }
+}
+
+unsafe fn _remove_var(key: &OsStr) {
     os_imp::unsetenv(key)
         .unwrap_or_else(|e| panic!("failed to remove environment variable `{key:?}`: {e}"))
 }

library/std/src/sys/pal/hermit/os.rs

@@ -172,18 +172,14 @@ pub fn getenv(k: &OsStr) -> Option<OsString> {
     unsafe { ENV.as_ref().unwrap().lock().unwrap().get_mut(k).cloned() }
 }
 
-pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
-    unsafe {
-        let (k, v) = (k.to_owned(), v.to_owned());
-        ENV.as_ref().unwrap().lock().unwrap().insert(k, v);
-    }
+pub unsafe fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+    let (k, v) = (k.to_owned(), v.to_owned());
+    ENV.as_ref().unwrap().lock().unwrap().insert(k, v);
     Ok(())
 }
 
-pub fn unsetenv(k: &OsStr) -> io::Result<()> {
-    unsafe {
-        ENV.as_ref().unwrap().lock().unwrap().remove(k);
-    }
+pub unsafe fn unsetenv(k: &OsStr) -> io::Result<()> {
+    ENV.as_ref().unwrap().lock().unwrap().remove(k);
     Ok(())
 }
 

library/std/src/sys/pal/sgx/os.rs

@@ -157,13 +157,13 @@ pub fn getenv(k: &OsStr) -> Option<OsString> {
     get_env_store().and_then(|s| s.lock().unwrap().get(k).cloned())
 }
 
-pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     let (k, v) = (k.to_owned(), v.to_owned());
     create_env_store().lock().unwrap().insert(k, v);
     Ok(())
 }
 
-pub fn unsetenv(k: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(k: &OsStr) -> io::Result<()> {
     if let Some(env) = get_env_store() {
         env.lock().unwrap().remove(k);
     }

library/std/src/sys/pal/solid/os.rs

@@ -191,7 +191,7 @@ pub fn getenv(k: &OsStr) -> Option<OsString> {
     .flatten()
 }
 
-pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     run_with_cstr(k.as_bytes(), &|k| {
         run_with_cstr(v.as_bytes(), &|v| {
             let _guard = ENV_LOCK.write();
@@ -200,7 +200,7 @@ pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     })
 }
 
-pub fn unsetenv(n: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(n: &OsStr) -> io::Result<()> {
     run_with_cstr(n.as_bytes(), &|nbuf| {
         let _guard = ENV_LOCK.write();
         cvt_env(unsafe { libc::unsetenv(nbuf.as_ptr()) }).map(drop)

library/std/src/sys/pal/teeos/os.rs

@@ -109,11 +109,11 @@ pub fn getenv(_: &OsStr) -> Option<OsString> {
     None
 }
 
-pub fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
+pub unsafe fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
     Err(io::Error::new(io::ErrorKind::Unsupported, "cannot set env vars on this platform"))
 }
 
-pub fn unsetenv(_: &OsStr) -> io::Result<()> {
+pub unsafe fn unsetenv(_: &OsStr) -> io::Result<()> {
     Err(io::Error::new(io::ErrorKind::Unsupported, "cannot unset env vars on this platform"))
 }