Recovery from Fn, FnOnce, FnMut trait with named params

xizheyin · xizheyin · commit 1e193e985830 · 2025-05-05T23:19:49.000+08:00
Signed-off-by: xizheyin &lt;xizheyin@smail.nju.edu.cn&gt;
diff --git a/compiler/rustc_parse/messages.ftl b/compiler/rustc_parse/messages.ftl
@@ -272,6 +272,9 @@ parse_float_literal_requires_integer_part = float literals must have an integer
 
 parse_float_literal_unsupported_base = {$base} float literal is not supported
 
+parse_fn_path_found_named_params = `Fn`, `FnMut`, `FnOnce`-trait syntax does not allow named arguments
+    .suggestion = remove name of the param
+
 parse_fn_pointer_cannot_be_async = an `fn` pointer type cannot be `async`
     .label = `async` because of this
     .suggestion = remove the `async` qualifier
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
@@ -1591,6 +1591,14 @@ pub(crate) struct ExpectedFnPathFoundFnKeyword {
     pub fn_token_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_fn_path_found_named_params)]
+pub(crate) struct FnPathFoundNamedParams {
+    #[primary_span]
+    #[suggestion(applicability = "machine-applicable", code = "")]
+    pub named_param_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_path_double_colon)]
 pub(crate) struct PathSingleColon {
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
@@ -14,9 +14,9 @@ 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,
+    FnPathFoundNamedParams, FnPointerCannotBeAsync, FnPointerCannotBeConst, FnPtrWithGenerics,
+    FnPtrWithGenericsSugg, HelpUseLatestEdition, InvalidDynKeyword, LifetimeAfterMut,
+    NeedPlusAfterTraitObjectLifetime, NestedCVariadicType, ReturnTypesUseThinArrow,
 };
 use crate::{exp, maybe_recover_from_interpolated_ty_qpath};
 
@@ -1203,6 +1203,8 @@ impl<'a> Parser<'a> {
 
             err.emit();
 
+            path
+        } else if let Some(path) = self.recover_path_for_fn_or_fn_onece_or_fn_mut() {
             path
         } else {
             self.parse_path(PathStyle::Type)?
@@ -1268,6 +1270,59 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Recover Fn(usize) from Fn(x: usize)
+    fn recover_path_for_fn_or_fn_onece_or_fn_mut(&mut self) -> Option<ast::Path> {
+        if !(self.token.is_ident_named(sym::FnOnce)
+            || self.token.is_ident_named(sym::FnMut)
+            || self.token.is_ident_named(sym::Fn))
+            || !self.look_ahead(1, |t| *t == TokenKind::OpenParen)
+        {
+            return None;
+        }
+
+        let fn_token_span = self.token.span;
+        let snapshot = self.create_snapshot_for_diagnostic();
+        self.bump();
+        let args_lo = self.token.span;
+
+        match self.parse_fn_decl(|_| false, AllowPlus::No, RecoverReturnSign::No) {
+            Ok(decl) => {
+                for param in &decl.inputs {
+                    if !matches!(param.pat.kind, crate::ast::PatKind::Missing) {
+                        self.dcx()
+                            .emit_err(FnPathFoundNamedParams { named_param_span: param.pat.span });
+
+                        return Some(ast::Path {
+                            span: fn_token_span.to(self.prev_token.span),
+                            segments: thin_vec![ast::PathSegment {
+                                ident: Ident::new(sym::Fn, fn_token_span),
+                                id: DUMMY_NODE_ID,
+                                args: Some(P(ast::GenericArgs::Parenthesized(
+                                    ast::ParenthesizedArgs {
+                                        span: args_lo.to(self.prev_token.span),
+                                        inputs: decl
+                                            .inputs
+                                            .iter()
+                                            .map(|param| { param.ty.clone() })
+                                            .collect(),
+                                        inputs_span: args_lo.until(decl.output.span()),
+                                        output: decl.output.clone(),
+                                    }
+                                ))),
+                            }],
+                            tokens: None,
+                        });
+                    }
+                }
+            }
+            Err(diag) => {
+                diag.cancel();
+            }
+        }
+        self.restore_snapshot(snapshot);
+        None
+    }
+
     /// Optionally parses `for<$generic_params>`.
     pub(super) fn parse_late_bound_lifetime_defs(
         &mut self,
diff --git a/tests/ui/fn/fn-trait-use-named-params-issue-140169.rs b/tests/ui/fn/fn-trait-use-named-params-issue-140169.rs
@@ -1,6 +1,6 @@
 fn g(_: fn(a: u8)) {}
-fn x(_: impl Fn(u8, vvvv: u8)) {} //~ ERROR expected one of `!`, `(`, `)`, `+`, `,`, `::`, or `<`, found `:`
-fn y(_: impl Fn(aaaa: u8, u8)) {} //~ ERROR expected one of `!`, `(`, `)`, `+`, `,`, `::`, or `<`, found `:`
-fn z(_: impl Fn(aaaa: u8, vvvv: u8)) {} //~ ERROR expected one of `!`, `(`, `)`, `+`, `,`, `::`, or `<`, found `:`
+fn x(_: impl Fn(u8, vvvv: u8)) {} //~ ERROR `Fn`, `FnMut`, `FnOnce`-trait syntax does not allow named arguments
+fn y(_: impl Fn(aaaa: u8, u8)) {} //~ ERROR `Fn`, `FnMut`, `FnOnce`-trait syntax does not allow named arguments
+fn z(_: impl Fn(aaaa: u8, vvvv: u8)) {} //~ ERROR `Fn`, `FnMut`, `FnOnce`-trait syntax does not allow named arguments
 
 fn main(){}
diff --git a/tests/ui/fn/fn-trait-use-named-params-issue-140169.stderr b/tests/ui/fn/fn-trait-use-named-params-issue-140169.stderr
@@ -1,20 +1,20 @@
-error: expected one of `!`, `(`, `)`, `+`, `,`, `::`, or `<`, found `:`
-  --> $DIR/fn-trait-use-named-params-issue-140169.rs:2:25
+error: `Fn`, `FnMut`, `FnOnce`-trait syntax does not allow named arguments
+  --> $DIR/fn-trait-use-named-params-issue-140169.rs:2:21
    |
 LL | fn x(_: impl Fn(u8, vvvv: u8)) {}
-   |                         ^ expected one of 7 possible tokens
+   |                     ^^^^ help: remove name of the param
 
-error: expected one of `!`, `(`, `)`, `+`, `,`, `::`, or `<`, found `:`
-  --> $DIR/fn-trait-use-named-params-issue-140169.rs:3:21
+error: `Fn`, `FnMut`, `FnOnce`-trait syntax does not allow named arguments
+  --> $DIR/fn-trait-use-named-params-issue-140169.rs:3:17
    |
 LL | fn y(_: impl Fn(aaaa: u8, u8)) {}
-   |                     ^ expected one of 7 possible tokens
+   |                 ^^^^ help: remove name of the param
 
-error: expected one of `!`, `(`, `)`, `+`, `,`, `::`, or `<`, found `:`
-  --> $DIR/fn-trait-use-named-params-issue-140169.rs:4:21
+error: `Fn`, `FnMut`, `FnOnce`-trait syntax does not allow named arguments
+  --> $DIR/fn-trait-use-named-params-issue-140169.rs:4:17
    |
 LL | fn z(_: impl Fn(aaaa: u8, vvvv: u8)) {}
-   |                     ^ expected one of 7 possible tokens
+   |                 ^^^^ help: remove name of the param
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/impl-trait/impl-fn-parsing-ambiguities.stderr b/tests/ui/impl-trait/impl-fn-parsing-ambiguities.stderr
@@ -9,12 +9,36 @@ help: try adding parentheses
 LL | fn a() -> impl Fn(&u8) -> (impl Debug + '_) {
    |                           +               +
 
+error: ambiguous `+` in a type
+  --> $DIR/impl-fn-parsing-ambiguities.rs:4:27
+   |
+LL | fn a() -> impl Fn(&u8) -> impl Debug + '_ {
+   |                           ^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: try adding parentheses
+   |
+LL | fn a() -> impl Fn(&u8) -> (impl Debug + '_) {
+   |                           +               +
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-fn-parsing-ambiguities.rs:10:24
+   |
+LL | fn b() -> impl Fn() -> impl Debug + Send {
+   |                        ^^^^^^^^^^^^^^^^^
+   |
+help: try adding parentheses
+   |
+LL | fn b() -> impl Fn() -> (impl Debug + Send) {
+   |                        +                 +
+
 error: ambiguous `+` in a type
   --> $DIR/impl-fn-parsing-ambiguities.rs:10:24
    |
 LL | fn b() -> impl Fn() -> impl Debug + Send {
    |                        ^^^^^^^^^^^^^^^^^
    |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 help: try adding parentheses
    |
 LL | fn b() -> impl Fn() -> (impl Debug + Send) {
@@ -32,6 +56,6 @@ note: lifetime declared here
 LL | fn a() -> impl Fn(&u8) -> impl Debug + '_ {
    |                   ^
 
-error: aborting due to 3 previous errors
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0657`.
