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`

Author: bors

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
-fncheck_abi(&self,abi: ast::StrLit,constness: ast::Const){
+fncheck_abi(&self,abi: ast::StrLit){
 let ast::StrLit{ symbol_unescaped, span, .. } = abi;
 
-iflet 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> {
 }
 }
 
-fncheck_extern(&self,ext: ast::Extern,constness: ast::Const){
+fncheck_extern(&self,ext: ast::Extern){
 iflet 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) => {
 ifletSome(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> {
 fnvisit_fn(&mutself,fn_kind:FnKind<'a>,span:Span, _:NodeId){
 ifletSome(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);
 }
 
 ifletFnKind::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);
 iflet 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 });
 }
 ifletSome(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{ .. }));
 
-ifalready_const(header)
- || has_const_generic_params
- || !could_be_const_with_abi(cx,&self.msrv, header.abi)
+ifalready_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
 }
 
-fncould_be_const_with_abi(cx:&LateContext<'_>,msrv:&Msrv,abi:Abi) -> bool{
+fncould_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"fnc(){}
 }
 
-mod with_extern {
-extern"C-unwind"fnc_unwind(){}
-extern"system"fnsystem(){}
-extern"system-unwind"fnsystem_unwind(){}
-}
-
 mod with_ty_alias {
 typeFoo = 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.
 fnalias_ty_is_projection(bar: <()asFooTrait>::Foo){}
 }
+
+mod extern_fn {
+extern"C-unwind"fnc_unwind(){}
+//~^ ERROR: this could be a `const fn`
+extern"system"fnsystem(){}
+//~^ ERROR: this could be a `const fn`
+extern"system-unwind"fnsystem_unwind(){}
+//~^ ERROR: this could be a `const fn`
+pubextern"stdcall"fnstd_call(){}
+//~^ ERROR: this could be a `const fn`
+pubextern"stdcall-unwind"fnstd_call_unwind(){}
+//~^ ERROR: this could be a `const fn`
+}