Implement named Fn trait parameters

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -525,6 +525,10 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         half_open_range_patterns_in_slices,
         "half-open range patterns in slices are unstable"
     );
+    gate_all!(
+        named_fn_trait_parameters,
+        "named parameters in parenthesized generic argument lists are experimental"
+    );
 
     // `associated_const_equality` will be stabilized as part of `min_generic_const_args`.
     for &span in spans.get(&sym::associated_const_equality).into_iter().flatten() {

compiler/rustc_feature/src/unstable.rs

@@ -662,6 +662,8 @@ declare_features! (
     (unstable, naked_functions_rustic_abi, "1.88.0", Some(138997)),
     /// Allows using `#[target_feature(enable = "...")]` on `#[naked]` on functions.
     (unstable, naked_functions_target_feature, "1.86.0", Some(138568)),
+    /// Allows providing names to parameters of `impl Fn` etc
+    (unstable, named_fn_trait_parameters, "CURRENT_RUSTC_VERSION", Some(158499)),
     /// Allows specifying the as-needed link modifier
     (unstable, native_link_modifiers_as_needed, "1.53.0", Some(81490)),
     /// Allow negative trait implementations.

compiler/rustc_parse/src/errors.rs

@@ -2075,14 +2075,6 @@ pub(crate) struct ExpectedFnPathFoundFnKeyword {
     pub fn_token_span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag("`Trait(...)` syntax does not support named parameters")]
-pub(crate) struct FnPathFoundNamedParams {
-    #[primary_span]
-    #[suggestion("remove the parameter name", applicability = "machine-applicable", code = "")]
-    pub named_param_span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag("`Trait(...)` syntax does not support c_variadic parameters")]
 pub(crate) struct PathFoundCVariadicParams {

compiler/rustc_parse/src/parser/path.rs

@@ -17,8 +17,8 @@ use super::{Parser, Restrictions, TokenType};
 use crate::ast::{PatKind, TyKind};
 use crate::errors::{
     self, AttributeOnEmptyType, AttributeOnGenericArg, ConstGenericWithoutBraces,
-    ConstGenericWithoutBracesSugg, FnPathFoundNamedParams, PathFoundAttributeInParams,
-    PathFoundCVariadicParams, PathSingleColon, PathTripleColon,
+    ConstGenericWithoutBracesSugg, PathFoundAttributeInParams, PathFoundCVariadicParams,
+    PathSingleColon, PathTripleColon,
 };
 use crate::exp;
 use crate::parser::{
@@ -409,11 +409,11 @@ impl<'a> Parser<'a> {
                             req_body: false,
                         };
                         let param = p.parse_param_general(&mode, false, false);
-                        param.map(move |param| {
+                        param.map(|param| {
                             if !matches!(param.pat.kind, PatKind::Missing) {
-                                dcx.emit_err(FnPathFoundNamedParams {
-                                    named_param_span: param.pat.span,
-                                });
+                                self.psess
+                                    .gated_spans
+                                    .gate(sym::named_fn_trait_parameters, param.pat.span);
                             }
                             if matches!(param.ty.kind, TyKind::CVarArgs) {
                                 dcx.emit_err(PathFoundCVariadicParams { span: param.pat.span });

compiler/rustc_session/src/parse.rs

@@ -22,7 +22,7 @@ use crate::Session;
 use crate::lint::{Lint, LintId};
 
 /// Collected spans during parsing for places where a certain feature was
-/// used and should be feature gated accordingly in `check_crate`.
+/// used and should be feature gated accordingly in `check_crate` in `rustc_ast_passes`.
 #[derive(Default)]
 pub struct GatedSpans {
     pub spans: Lock<FxHashMap<Symbol, Vec<Span>>>,

compiler/rustc_span/src/symbol.rs

@@ -1378,6 +1378,7 @@ symbols! {
         naked_functions_rustic_abi,
         naked_functions_target_feature,
         name,
+        named_fn_trait_parameters,
         names,
         native_link_modifiers,
         native_link_modifiers_as_needed,

tests/ui/feature-gates/feature-gate-named-fn-trait-parameters.rs

@@ -0,0 +1,7 @@
+fn parse_my_data(
+    data: &str,
+    log: impl Fn(msg: String),
+    //~^ ERROR named parameters in parenthesized generic argument lists are experimental [E0658]
+) { }
+
+fn main() {}

tests/ui/feature-gates/feature-gate-named-fn-trait-parameters.stderr

@@ -0,0 +1,13 @@
+error[E0658]: named parameters in parenthesized generic argument lists are experimental
+  --> $DIR/feature-gate-named-fn-trait-parameters.rs:3:18
+   |
+LL |     log: impl Fn(msg: String),
+   |                  ^^^
+   |
+   = note: see issue #158499 <https://github.com/rust-lang/rust/issues/158499> for more information
+   = help: add `#![feature(named_fn_trait_parameters)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/fn/fn-trait-use-named-params-issue-140169.rs

@@ -1,9 +1,9 @@
 fn f1(_: fn(a: u8)) {}
-fn f2(_: impl Fn(u8, vvvv: u8)) {} //~ ERROR `Trait(...)` syntax does not support named parameters
-fn f3(_: impl Fn(aaaa: u8, u8)) {} //~ ERROR `Trait(...)` syntax does not support named parameters
+fn f2(_: impl Fn(u8, vvvv: u8)) {} //~ ERROR named parameters in parenthesized generic argument lists are experimental
+fn f3(_: impl Fn(aaaa: u8, u8)) {} //~ ERROR named parameters in parenthesized generic argument lists are experimental
 fn f4(_: impl Fn(aaaa: u8, vvvv: u8)) {}
-//~^ ERROR `Trait(...)` syntax does not support named parameters
-//~| ERROR `Trait(...)` syntax does not support named parameters
+//~^ ERROR named parameters in parenthesized generic argument lists are experimental
+//~| ERROR named parameters in parenthesized generic argument lists are experimental
 fn f5(_: impl Fn(u8, ...)) {}
 //~^ ERROR `Trait(...)` syntax does not support c_variadic parameters
 fn f6(_: impl Fn(u8, #[allow(unused_attributes)] u8)) {}

tests/ui/fn/fn-trait-use-named-params-issue-140169.stderr

@@ -1,38 +1,55 @@
-error: `Trait(...)` syntax does not support named parameters
+error: `Trait(...)` syntax does not support c_variadic parameters
+  --> $DIR/fn-trait-use-named-params-issue-140169.rs:7:22
+   |
+LL | fn f5(_: impl Fn(u8, ...)) {}
+   |                      ^^^ help: remove the `...`
+
+error: `Trait(...)` syntax does not support attributes in parameters
+  --> $DIR/fn-trait-use-named-params-issue-140169.rs:9:22
+   |
+LL | fn f6(_: impl Fn(u8, #[allow(unused_attributes)] u8)) {}
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the attributes
+
+error[E0658]: named parameters in parenthesized generic argument lists are experimental
   --> $DIR/fn-trait-use-named-params-issue-140169.rs:2:22
    |
 LL | fn f2(_: impl Fn(u8, vvvv: u8)) {}
-   |                      ^^^^ help: remove the parameter name
+   |                      ^^^^
+   |
+   = note: see issue #158499 <https://github.com/rust-lang/rust/issues/158499> for more information
+   = help: add `#![feature(named_fn_trait_parameters)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: `Trait(...)` syntax does not support named parameters
+error[E0658]: named parameters in parenthesized generic argument lists are experimental
   --> $DIR/fn-trait-use-named-params-issue-140169.rs:3:18
    |
 LL | fn f3(_: impl Fn(aaaa: u8, u8)) {}
-   |                  ^^^^ help: remove the parameter name
+   |                  ^^^^
+   |
+   = note: see issue #158499 <https://github.com/rust-lang/rust/issues/158499> for more information
+   = help: add `#![feature(named_fn_trait_parameters)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: `Trait(...)` syntax does not support named parameters
+error[E0658]: named parameters in parenthesized generic argument lists are experimental
   --> $DIR/fn-trait-use-named-params-issue-140169.rs:4:18
    |
 LL | fn f4(_: impl Fn(aaaa: u8, vvvv: u8)) {}
-   |                  ^^^^ help: remove the parameter name
+   |                  ^^^^
+   |
+   = note: see issue #158499 <https://github.com/rust-lang/rust/issues/158499> for more information
+   = help: add `#![feature(named_fn_trait_parameters)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: `Trait(...)` syntax does not support named parameters
+error[E0658]: named parameters in parenthesized generic argument lists are experimental
   --> $DIR/fn-trait-use-named-params-issue-140169.rs:4:28
    |
 LL | fn f4(_: impl Fn(aaaa: u8, vvvv: u8)) {}
-   |                            ^^^^ help: remove the parameter name
-
-error: `Trait(...)` syntax does not support c_variadic parameters
-  --> $DIR/fn-trait-use-named-params-issue-140169.rs:7:22
+   |                            ^^^^
    |
-LL | fn f5(_: impl Fn(u8, ...)) {}
-   |                      ^^^ help: remove the `...`
-
-error: `Trait(...)` syntax does not support attributes in parameters
-  --> $DIR/fn-trait-use-named-params-issue-140169.rs:9:22
-   |
-LL | fn f6(_: impl Fn(u8, #[allow(unused_attributes)] u8)) {}
-   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the attributes
+   = note: see issue #158499 <https://github.com/rust-lang/rust/issues/158499> for more information
+   = help: add `#![feature(named_fn_trait_parameters)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 6 previous errors
 
+For more information about this error, try `rustc --explain E0658`.

tests/ui/parser/diagnostics-parenthesized-type-arguments-ice-issue-122345.rs

@@ -2,7 +2,7 @@
 
 fn main() {
     unsafe {
-        dealloc(ptr2, Layout::(x: !)(1, 1)); //~ ERROR `Trait(...)` syntax does not support named parameters
+        dealloc(ptr2, Layout::(x: !)(1, 1)); //~ ERROR named parameters in parenthesized generic argument lists are experimental
         //~^ ERROR cannot find function `dealloc` in this scope [E0425]
         //~| ERROR cannot find value `ptr2` in this scope [E0425]
         //~| ERROR the `!` type is experimental [E0658]

tests/ui/parser/diagnostics-parenthesized-type-arguments-ice-issue-122345.stderr

@@ -1,9 +1,3 @@
-error: `Trait(...)` syntax does not support named parameters
-  --> $DIR/diagnostics-parenthesized-type-arguments-ice-issue-122345.rs:5:32
-   |
-LL |         dealloc(ptr2, Layout::(x: !)(1, 1));
-   |                                ^ help: remove the parameter name
-
 error[E0425]: cannot find function `dealloc` in this scope
   --> $DIR/diagnostics-parenthesized-type-arguments-ice-issue-122345.rs:5:9
    |
@@ -21,6 +15,16 @@ error[E0425]: cannot find value `ptr2` in this scope
 LL |         dealloc(ptr2, Layout::(x: !)(1, 1));
    |                 ^^^^ not found in this scope
 
+error[E0658]: named parameters in parenthesized generic argument lists are experimental
+  --> $DIR/diagnostics-parenthesized-type-arguments-ice-issue-122345.rs:5:32
+   |
+LL |         dealloc(ptr2, Layout::(x: !)(1, 1));
+   |                                ^
+   |
+   = note: see issue #158499 <https://github.com/rust-lang/rust/issues/158499> for more information
+   = help: add `#![feature(named_fn_trait_parameters)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
 error[E0658]: the `!` type is experimental
   --> $DIR/diagnostics-parenthesized-type-arguments-ice-issue-122345.rs:5:35
    |