Auto merge of #129753 - folkertdev:stabilize-const-extern-fn, r=RalfJung

stabilize `const_extern_fn`

closes #64926

tracking issue: #64926
reference PR: rust-lang/reference#1596

## Stabilizaton Report

### Summary

Using `const extern "Rust"` and `const extern "C"` was already stabilized (since version 1.62.0, see #95346). This PR stabilizes the other calling conventions: it is now possible to write  `const unsafe extern "calling-convention" fn` and `const extern "calling-convention" fn` for any supported calling convention:

```rust
const extern "C-unwind" fn foo1(val: u8) -> u8 { val + 1}
const extern "stdcall" fn foo2(val: u8) -> u8 { val + 1}
const unsafe extern "C-unwind" fn bar1(val: bool) -> bool { !val }
const unsafe extern "stdcall" fn bar2(val: bool) -> bool { !val }
```

This can be used to const-ify an `extern fn`, or conversely, to make a `const fn` callable from external code.

r? T-lang

cc `@RalfJung`

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -75,22 +75,9 @@ struct PostExpansionVisitor<'a> {
 
 impl<'a> PostExpansionVisitor<'a> {
     #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
-    fn check_abi(&self, abi: ast::StrLit, constness: ast::Const) {
+    fn check_abi(&self, abi: ast::StrLit) {
         let ast::StrLit { symbol_unescaped, span, .. } = abi;
 
-        if let ast::Const::Yes(_) = constness {
-            match symbol_unescaped {
-                // Stable
-                sym::Rust | sym::C => {}
-                abi => gate!(
-                    &self,
-                    const_extern_fn,
-                    span,
-                    format!("`{}` as a `const fn` ABI is unstable", abi)
-                ),
-            }
-        }
-
         match abi::is_enabled(self.features, span, symbol_unescaped.as_str()) {
             Ok(()) => (),
             Err(abi::AbiDisabled::Unstable { feature, explain }) => {
@@ -110,9 +97,9 @@ impl<'a> PostExpansionVisitor<'a> {
         }
     }
 
-    fn check_extern(&self, ext: ast::Extern, constness: ast::Const) {
+    fn check_extern(&self, ext: ast::Extern) {
         if let ast::Extern::Explicit(abi, _) = ext {
-            self.check_abi(abi, constness);
+            self.check_abi(abi);
         }
     }
 
@@ -239,7 +226,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         match &i.kind {
             ast::ItemKind::ForeignMod(foreign_module) => {
                 if let Some(abi) = foreign_module.abi {
-                    self.check_abi(abi, ast::Const::No);
+                    self.check_abi(abi);
                 }
             }
 
@@ -341,7 +328,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         match &ty.kind {
             ast::TyKind::BareFn(bare_fn_ty) => {
                 // Function pointers cannot be `const`
-                self.check_extern(bare_fn_ty.ext, ast::Const::No);
+                self.check_extern(bare_fn_ty.ext);
                 self.check_late_bound_lifetime_defs(&bare_fn_ty.generic_params);
             }
             ast::TyKind::Never => {
@@ -446,7 +433,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
     fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
         if let Some(header) = fn_kind.header() {
             // Stability of const fn methods are covered in `visit_assoc_item` below.
-            self.check_extern(header.ext, header.constness);
+            self.check_extern(header.ext);
         }
 
         if let FnKind::Closure(ast::ClosureBinder::For { generic_params, .. }, ..) = fn_kind {

compiler/rustc_feature/src/accepted.rs

@@ -115,6 +115,8 @@ declare_features! (
     (accepted, conservative_impl_trait, "1.26.0", Some(34511)),
     /// Allows calling constructor functions in `const fn`.
     (accepted, const_constructor, "1.40.0", Some(61456)),
+    /// Allows the definition of `const extern fn` and `const unsafe extern fn`.
+    (accepted, const_extern_fn, "CURRENT_RUSTC_VERSION", Some(64926)),
     /// Allows basic arithmetic on floating point types in a `const fn`.
     (accepted, const_fn_floating_point_arithmetic, "1.82.0", Some(57241)),
     /// Allows using and casting function pointers in a `const fn`.

compiler/rustc_feature/src/unstable.rs

@@ -401,8 +401,6 @@ declare_features! (
     (unstable, const_async_blocks, "1.53.0", Some(85368)),
     /// Allows `const || {}` closures in const contexts.
     (incomplete, const_closures, "1.68.0", Some(106003)),
-    /// Allows the definition of `const extern fn` and `const unsafe extern fn`.
-    (unstable, const_extern_fn, "1.40.0", Some(64926)),
     /// Allows `for _ in _` loops in const contexts.
     (unstable, const_for, "1.56.0", Some(87575)),
     /// Allows using `&mut` in constant functions.

compiler/rustc_parse/src/parser/ty.rs

@@ -607,9 +607,6 @@ impl<'a> Parser<'a> {
         let decl = self.parse_fn_decl(|_| false, AllowPlus::No, recover_return_sign)?;
         let whole_span = lo.to(self.prev_token.span);
         if let ast::Const::Yes(span) = constness {
-            // If we ever start to allow `const fn()`, then update
-            // feature gating for `#![feature(const_extern_fn)]` to
-            // cover it.
             self.dcx().emit_err(FnPointerCannotBeConst { span: whole_span, qualifier: span });
         }
         if let Some(ast::CoroutineKind::Async { span, .. }) = coroutine_kind {

src/tools/clippy/clippy_config/src/msrvs.rs

@@ -17,6 +17,7 @@ macro_rules! msrv_aliases {
 
 // names may refer to stabilized feature flags or library items
 msrv_aliases! {
+    1,83,0 { CONST_EXTERN_FN }
     1,83,0 { CONST_FLOAT_BITS_CONV }
     1,81,0 { LINT_REASONS_STABILIZATION }
     1,80,0 { BOX_INTO_ITER}
@@ -27,7 +28,7 @@ msrv_aliases! {
     1,68,0 { PATH_MAIN_SEPARATOR_STR }
     1,65,0 { LET_ELSE, POINTER_CAST_CONSTNESS }
     1,63,0 { CLONE_INTO }
-    1,62,0 { BOOL_THEN_SOME, DEFAULT_ENUM_ATTRIBUTE, CONST_EXTERN_FN }
+    1,62,0 { BOOL_THEN_SOME, DEFAULT_ENUM_ATTRIBUTE, CONST_EXTERN_C_FN }
     1,59,0 { THREAD_LOCAL_CONST_INIT }
     1,58,0 { FORMAT_ARGS_CAPTURE, PATTERN_TRAIT_CHAR_ARRAY, CONST_RAW_PTR_DEREF }
     1,56,0 { CONST_FN_UNION }

src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs

@@ -119,9 +119,7 @@ impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
                     .iter()
                     .any(|param| matches!(param.kind, GenericParamKind::Const { .. }));
 
-                if already_const(header)
-                    || has_const_generic_params
-                    || !could_be_const_with_abi(cx, &self.msrv, header.abi)
+                if already_const(header) || has_const_generic_params || !could_be_const_with_abi(&self.msrv, header.abi)
                 {
                     return;
                 }
@@ -183,13 +181,13 @@ fn already_const(header: hir::FnHeader) -> bool {
     header.constness == Constness::Const
 }
 
-fn could_be_const_with_abi(cx: &LateContext<'_>, msrv: &Msrv, abi: Abi) -> bool {
+fn could_be_const_with_abi(msrv: &Msrv, abi: Abi) -> bool {
     match abi {
         Abi::Rust => true,
         // `const extern "C"` was stabilized after 1.62.0
-        Abi::C { unwind: false } => msrv.meets(msrvs::CONST_EXTERN_FN),
+        Abi::C { unwind: false } => msrv.meets(msrvs::CONST_EXTERN_C_FN),
         // Rest ABIs are still unstable and need the `const_extern_fn` feature enabled.
-        _ => cx.tcx.features().const_extern_fn,
+        _ => msrv.meets(msrvs::CONST_EXTERN_FN),
     }
 }
 

src/tools/clippy/tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -186,12 +186,6 @@ mod msrv {
     extern "C" fn c() {}
 }
 
-mod with_extern {
-    extern "C-unwind" fn c_unwind() {}
-    extern "system" fn system() {}
-    extern "system-unwind" fn system_unwind() {}
-}
-
 mod with_ty_alias {
     type Foo = impl std::fmt::Debug;
 

src/tools/clippy/tests/ui/missing_const_for_fn/could_be_const.fixed

@@ -1,5 +1,6 @@
 #![warn(clippy::missing_const_for_fn)]
 #![allow(incomplete_features, clippy::let_and_return, clippy::missing_transmute_annotations)]
+#![allow(unsupported_calling_conventions)]
 #![feature(const_mut_refs)]
 #![feature(const_trait_impl)]
 
@@ -204,3 +205,16 @@ mod with_ty_alias {
     // in this test.
     const fn alias_ty_is_projection(bar: <() as FooTrait>::Foo) {}
 }
+
+mod extern_fn {
+    const extern "C-unwind" fn c_unwind() {}
+    //~^ ERROR: this could be a `const fn`
+    const extern "system" fn system() {}
+    //~^ ERROR: this could be a `const fn`
+    const extern "system-unwind" fn system_unwind() {}
+    //~^ ERROR: this could be a `const fn`
+    pub const extern "stdcall" fn std_call() {}
+    //~^ ERROR: this could be a `const fn`
+    pub const extern "stdcall-unwind" fn std_call_unwind() {}
+    //~^ ERROR: this could be a `const fn`
+}

src/tools/clippy/tests/ui/missing_const_for_fn/could_be_const.rs

@@ -1,5 +1,6 @@
 #![warn(clippy::missing_const_for_fn)]
 #![allow(incomplete_features, clippy::let_and_return, clippy::missing_transmute_annotations)]
+#![allow(unsupported_calling_conventions)]
 #![feature(const_mut_refs)]
 #![feature(const_trait_impl)]
 
@@ -204,3 +205,16 @@ mod with_ty_alias {
     // in this test.
     fn alias_ty_is_projection(bar: <() as FooTrait>::Foo) {}
 }
+
+mod extern_fn {
+    extern "C-unwind" fn c_unwind() {}
+    //~^ ERROR: this could be a `const fn`
+    extern "system" fn system() {}
+    //~^ ERROR: this could be a `const fn`
+    extern "system-unwind" fn system_unwind() {}
+    //~^ ERROR: this could be a `const fn`
+    pub extern "stdcall" fn std_call() {}
+    //~^ ERROR: this could be a `const fn`
+    pub extern "stdcall-unwind" fn std_call_unwind() {}
+    //~^ ERROR: this could be a `const fn`
+}