Rework how the disallowed qualifier lints are generated

JonathanBrouwer · JonathanBrouwer · commit 43f3afd7bfd8 · 2025-06-10T19:28:58.000+02:00
Signed-off-by: Jonathan Brouwer &lt;jonathantbrouwer@gmail.com&gt;
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
@@ -2940,9 +2940,8 @@ pub(crate) struct DynAfterMut {
 #[diag(parse_fn_pointer_cannot_be_const)]
 pub(crate) struct FnPointerCannotBeConst {
     #[primary_span]
-    pub span: Span,
     #[label]
-    pub qualifier: Span,
+    pub span: Span,
     #[suggestion(code = "", applicability = "maybe-incorrect", style = "verbose")]
     pub suggestion: Span,
 }
@@ -2951,9 +2950,8 @@ pub(crate) struct FnPointerCannotBeConst {
 #[diag(parse_fn_pointer_cannot_be_async)]
 pub(crate) struct FnPointerCannotBeAsync {
     #[primary_span]
-    pub span: Span,
     #[label]
-    pub qualifier: Span,
+    pub span: Span,
     #[suggestion(code = "", applicability = "maybe-incorrect", style = "verbose")]
     pub suggestion: Span,
 }
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
@@ -23,7 +23,7 @@ use super::{
     AttrWrapper, ExpKeywordPair, ExpTokenPair, FollowedByType, ForceCollect, Parser, PathStyle,
     Recovered, Trailing, UsePreAttrPos,
 };
-use crate::errors::{self, MacroExpandsToAdtField};
+use crate::errors::{self, FnPointerCannotBeAsync, FnPointerCannotBeConst, MacroExpandsToAdtField};
 use crate::{exp, fluent_generated as fluent};
 
 impl<'a> Parser<'a> {
@@ -2402,7 +2402,7 @@ impl<'a> Parser<'a> {
         case: Case,
     ) -> PResult<'a, (Ident, FnSig, Generics, Option<P<FnContract>>, Option<P<Block>>)> {
         let fn_span = self.token.span;
-        let header = self.parse_fn_front_matter(vis, case)?; // `const ... fn`
+        let header = self.parse_fn_front_matter(vis, case, false)?; // `const ... fn`
         let ident = self.parse_ident()?; // `foo`
         let mut generics = self.parse_generics()?; // `<'a, T, ...>`
         let decl = match self.parse_fn_decl(
@@ -2658,16 +2658,35 @@ impl<'a> Parser<'a> {
     ///
     /// `vis` represents the visibility that was already parsed, if any. Use
     /// `Visibility::Inherited` when no visibility is known.
+    ///
+    /// If `is_fn_pointer_type`, we error on `const` and `async` qualifiers,
+    /// which are not allowed in function pointer types.
     pub(super) fn parse_fn_front_matter(
         &mut self,
         orig_vis: &Visibility,
         case: Case,
+        is_fn_pointer_type: bool,
     ) -> PResult<'a, FnHeader> {
         let sp_start = self.token.span;
         let constness = self.parse_constness(case);
+        if is_fn_pointer_type && let Const::Yes(const_span) = constness {
+            self.dcx().emit_err(FnPointerCannotBeConst {
+                span: const_span,
+                suggestion: const_span.until(self.token.span),
+            });
+        }
 
         let async_start_sp = self.token.span;
         let coroutine_kind = self.parse_coroutine_kind(case);
+        if is_fn_pointer_type
+            && let Some(ast::CoroutineKind::Async { span: async_span, .. }) = coroutine_kind
+        {
+            self.dcx().emit_err(FnPointerCannotBeAsync {
+                span: async_span,
+                suggestion: async_span.until(self.token.span),
+            });
+        }
+        // FIXME(gen_blocks): emit a similar error for `gen fn()`
 
         let unsafe_start_sp = self.token.span;
         let safety = self.parse_safety(case);
@@ -2703,6 +2722,11 @@ impl<'a> Parser<'a> {
                     enum WrongKw {
                         Duplicated(Span),
                         Misplaced(Span),
+                        /// `MisplacedDisallowedQualifier` is only used instead of `Misplaced`,
+                        /// when the misplaced keyword is disallowed by `is_fn_pointer_type`.
+                        /// In this case, we avoid generating the suggestion to swap around the keywords,
+                        /// as we already generated a suggestion to remove the keyword earlier.
+                        MisplacedDisallowedQualifier,
                     }
 
                     // We may be able to recover
@@ -2716,7 +2740,18 @@ impl<'a> Parser<'a> {
                             Const::Yes(sp) => Some(WrongKw::Duplicated(sp)),
                             Const::No => {
                                 recover_constness = Const::Yes(self.token.span);
-                                Some(WrongKw::Misplaced(async_start_sp))
+                                if is_fn_pointer_type {
+                                    self.dcx().emit_err(FnPointerCannotBeConst {
+                                        span: self.token.span,
+                                        suggestion: self
+                                            .token
+                                            .span
+                                            .with_lo(self.prev_token.span.hi()),
+                                    });
+                                    Some(WrongKw::MisplacedDisallowedQualifier)
+                                } else {
+                                    Some(WrongKw::Misplaced(async_start_sp))
+                                }
                             }
                         }
                     } else if self.check_keyword(exp!(Async)) {
@@ -2742,7 +2777,18 @@ impl<'a> Parser<'a> {
                                     closure_id: DUMMY_NODE_ID,
                                     return_impl_trait_id: DUMMY_NODE_ID,
                                 });
-                                Some(WrongKw::Misplaced(unsafe_start_sp))
+                                if is_fn_pointer_type {
+                                    self.dcx().emit_err(FnPointerCannotBeAsync {
+                                        span: self.token.span,
+                                        suggestion: self
+                                            .token
+                                            .span
+                                            .with_lo(self.prev_token.span.hi()),
+                                    });
+                                    Some(WrongKw::MisplacedDisallowedQualifier)
+                                } else {
+                                    Some(WrongKw::Misplaced(unsafe_start_sp))
+                                }
                             }
                         }
                     } else if self.check_keyword(exp!(Unsafe)) {
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
@@ -15,9 +15,9 @@ use thin_vec::{ThinVec, thin_vec};
 use super::{Parser, PathStyle, SeqSep, TokenType, Trailing};
 use crate::errors::{
     self, DynAfterMut, ExpectedFnPathFoundFnKeyword, ExpectedMutOrConstInRawPointerType,
-    FnPointerCannotBeAsync, FnPointerCannotBeConst, FnPtrWithGenerics, FnPtrWithGenericsSugg,
-    HelpUseLatestEdition, InvalidDynKeyword, LifetimeAfterMut, NeedPlusAfterTraitObjectLifetime,
-    NestedCVariadicType, ReturnTypesUseThinArrow,
+    FnPtrWithGenerics, FnPtrWithGenericsSugg, HelpUseLatestEdition, InvalidDynKeyword,
+    LifetimeAfterMut, NeedPlusAfterTraitObjectLifetime, NestedCVariadicType,
+    ReturnTypesUseThinArrow,
 };
 use crate::{exp, maybe_recover_from_interpolated_ty_qpath};
 
@@ -669,62 +669,13 @@ impl<'a> Parser<'a> {
             tokens: None,
         };
         let span_start = self.token.span;
-        let ast::FnHeader { ext, safety, constness, coroutine_kind } =
-            self.parse_fn_front_matter(&inherited_vis, Case::Sensitive)?;
-        let fn_start_lo = self.prev_token.span.lo();
+        let ast::FnHeader { ext, safety, .. } =
+            self.parse_fn_front_matter(&inherited_vis, Case::Sensitive, true)?;
         if self.may_recover() && self.token == TokenKind::Lt {
             self.recover_fn_ptr_with_generics(lo, &mut params, param_insertion_point)?;
         }
         let decl = self.parse_fn_decl(|_| false, AllowPlus::No, recover_return_sign)?;
-        let whole_span = lo.to(self.prev_token.span);
-
-        // Order/parsing of "front matter" follows:
-        // `<constness> <coroutine_kind> <safety> <extern> fn()`
-        //  ^           ^                ^        ^        ^
-        //  |           |                |        |        fn_start_lo
-        //  |           |                |        ext_sp.lo
-        //  |           |                safety_sp.lo
-        //  |           coroutine_sp.lo
-        //  const_sp.lo
-        if let ast::Const::Yes(const_span) = constness {
-            let next_token_lo = if let Some(
-                ast::CoroutineKind::Async { span, .. }
-                | ast::CoroutineKind::Gen { span, .. }
-                | ast::CoroutineKind::AsyncGen { span, .. },
-            ) = coroutine_kind
-            {
-                span.lo()
-            } else if let ast::Safety::Unsafe(span) | ast::Safety::Safe(span) = safety {
-                span.lo()
-            } else if let ast::Extern::Implicit(span) | ast::Extern::Explicit(_, span) = ext {
-                span.lo()
-            } else {
-                fn_start_lo
-            };
-            let sugg_span = const_span.with_hi(next_token_lo);
-            self.dcx().emit_err(FnPointerCannotBeConst {
-                span: whole_span,
-                qualifier: const_span,
-                suggestion: sugg_span,
-            });
-        }
-        if let Some(ast::CoroutineKind::Async { span: async_span, .. }) = coroutine_kind {
-            let next_token_lo = if let ast::Safety::Unsafe(span) | ast::Safety::Safe(span) = safety
-            {
-                span.lo()
-            } else if let ast::Extern::Implicit(span) | ast::Extern::Explicit(_, span) = ext {
-                span.lo()
-            } else {
-                fn_start_lo
-            };
-            let sugg_span = async_span.with_hi(next_token_lo);
-            self.dcx().emit_err(FnPointerCannotBeAsync {
-                span: whole_span,
-                qualifier: async_span,
-                suggestion: sugg_span,
-            });
-        }
-        // FIXME(gen_blocks): emit a similar error for `gen fn()`
+
         let decl_span = span_start.to(self.prev_token.span);
         Ok(TyKind::BareFn(P(BareFnTy { ext, safety, generic_params: params, decl, decl_span })))
     }
diff --git a/tests/ui/parser/bad-fn-ptr-qualifier.fixed b/tests/ui/parser/bad-fn-ptr-qualifier.fixed
@@ -23,4 +23,15 @@ pub type FTT6 = for<'a> unsafe extern "C" fn();
 //~^ ERROR an `fn` pointer type cannot be `const`
 //~| ERROR an `fn` pointer type cannot be `async`
 
+// Tests with qualifiers in the wrong order
+pub type W1 = unsafe fn();
+//~^ ERROR an `fn` pointer type cannot be `const`
+//~| ERROR expected one of `extern` or `fn`, found keyword `const`
+pub type W2 = unsafe fn();
+//~^ ERROR an `fn` pointer type cannot be `async`
+//~| ERROR expected one of `extern` or `fn`, found keyword `async`
+pub type W3 = for<'a> unsafe fn();
+//~^ ERROR an `fn` pointer type cannot be `const`
+//~| ERROR expected one of `extern` or `fn`, found keyword `const`
+
 fn main() {}
diff --git a/tests/ui/parser/bad-fn-ptr-qualifier.stderr b/tests/ui/parser/bad-fn-ptr-qualifier.stderr
diff --git a/tests/ui/parser/recover/recover-const-async-fn-ptr.stderr b/tests/ui/parser/recover/recover-const-async-fn-ptr.stderr
