Temporarily allow const impl and impl const at the same time to migrate

compiler/rustc_parse/src/parser/item.rs

@@ -562,9 +562,7 @@ impl<'a> Parser<'a> {
         attrs: &mut AttrVec,
         defaultness: Defaultness,
     ) -> PResult<'a, ItemKind> {
-        if self.eat_keyword(exp!(Const)) {
-            return self.recover_const_impl(self.prev_token.span, attrs, defaultness);
-        }
+        let mut constness = self.parse_constness(Case::Sensitive);
         let safety = self.parse_safety(Case::Sensitive);
         self.expect_keyword(exp!(Impl))?;
 
@@ -579,7 +577,11 @@ impl<'a> Parser<'a> {
             generics
         };
 
-        let constness = self.parse_constness(Case::Sensitive);
+        if let Const::No = constness {
+            // FIXME(const_trait_impl): disallow `impl const Trait`
+            constness = self.parse_constness(Case::Sensitive);
+        }
+
         if let Const::Yes(span) = constness {
             self.psess.gated_spans.gate(sym::const_trait_impl, span);
         }
@@ -1355,46 +1357,6 @@ impl<'a> Parser<'a> {
         }
     }
 
-    /// Recover on `const impl` with `const` already eaten.
-    fn recover_const_impl(
-        &mut self,
-        const_span: Span,
-        attrs: &mut AttrVec,
-        defaultness: Defaultness,
-    ) -> PResult<'a, ItemKind> {
-        let impl_span = self.token.span;
-        let err = self.expected_ident_found_err();
-
-        // Only try to recover if this is implementing a trait for a type
-        let mut item_kind = match self.parse_item_impl(attrs, defaultness) {
-            Ok(item_kind) => item_kind,
-            Err(recovery_error) => {
-                // Recovery failed, raise the "expected identifier" error
-                recovery_error.cancel();
-                return Err(err);
-            }
-        };
-
-        match &mut item_kind {
-            ItemKind::Impl(Impl { of_trait: Some(of_trait), .. }) => {
-                of_trait.constness = Const::Yes(const_span);
-
-                let before_trait = of_trait.trait_ref.path.span.shrink_to_lo();
-                let const_up_to_impl = const_span.with_hi(impl_span.lo());
-                err.with_multipart_suggestion(
-                    "you might have meant to write a const trait impl",
-                    vec![(const_up_to_impl, "".to_owned()), (before_trait, "const ".to_owned())],
-                    Applicability::MaybeIncorrect,
-                )
-                .emit();
-            }
-            ItemKind::Impl { .. } => return Err(err),
-            _ => unreachable!(),
-        }
-
-        Ok(item_kind)
-    }
-
     /// Parse a static item with the prefix `"static" "mut"?` already parsed and stored in
     /// `mutability`.
     ///

tests/ui/parser/issues/issue-81806.rs

@@ -1,5 +1,6 @@
-trait T { const
-impl //~ ERROR: expected identifier, found keyword `impl`
-}
+trait T {
+const //~ ERROR: const trait impls are experimental
+impl
+} //~ ERROR: expected type, found `}`
 
 fn main() {}

tests/ui/parser/issues/issue-81806.stderr

@@ -1,17 +1,25 @@
-error: expected identifier, found keyword `impl`
+error: expected type, found `}`
+  --> $DIR/issue-81806.rs:4:1
+   |
+LL | trait T {
+   |         - while parsing this item list starting here
+...
+LL | }
+   | ^
+   | |
+   | expected type
+   | the item list ends here
+
+error[E0658]: const trait impls are experimental
   --> $DIR/issue-81806.rs:2:1
    |
-LL | trait T { const
-   |         - while parsing this item list starting here
-LL | impl
-   | ^^^^ expected identifier, found keyword
-LL | }
-   | - the item list ends here
+LL | const
+   | ^^^^^
    |
-help: escape `impl` to use it as an identifier
-   |
-LL | r#impl
-   | ++
+   = note: see issue #143874 <https://github.com/rust-lang/rust/issues/143874> for more information
+   = help: add `#![feature(const_trait_impl)]` 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
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0658`.

tests/ui/traits/const-traits/const-impl-norecover.rs

@@ -2,7 +2,7 @@
 
 struct Foo;
 
-const impl Foo { //~ ERROR: expected identifier, found keyword
+const impl Foo { //~ ERROR: inherent impls cannot be const
     fn bar() {}
 }
 

tests/ui/traits/const-traits/const-impl-norecover.stderr

@@ -1,8 +1,12 @@
-error: expected identifier, found keyword `impl`
-  --> $DIR/const-impl-norecover.rs:5:7
+error: inherent impls cannot be const
+  --> $DIR/const-impl-norecover.rs:5:12
    |
 LL | const impl Foo {
-   |       ^^^^ expected identifier, found keyword
+   | -----      ^^^ inherent impl for this type
+   | |
+   | const because of this
+   |
+   = note: only trait implementations may be annotated with `const`
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/const-impl-recovery.rs

@@ -1,12 +1,14 @@
 #![feature(const_trait_impl)]
 
+//@ check-pass
+
 const trait Foo {}
 
-const impl Foo for i32 {} //~ ERROR: expected identifier, found keyword
+const impl Foo for i32 {}
 
 const trait Bar {}
 
-const impl<T: Foo> Bar for T {} //~ ERROR: expected identifier, found keyword
+const impl<T: Foo> Bar for T {}
 
 const fn still_implements<T: Bar>() {}
 

tests/ui/traits/const-traits/const-impl-recovery.stderr

@@ -1,26 +0,0 @@
-error: expected identifier, found keyword `impl`
-  --> $DIR/const-impl-recovery.rs:5:7
-   |
-LL | const impl Foo for i32 {}
-   |       ^^^^ expected identifier, found keyword
-   |
-help: you might have meant to write a const trait impl
-   |
-LL - const impl Foo for i32 {}
-LL + impl const Foo for i32 {}
-   |
-
-error: expected identifier, found keyword `impl`
-  --> $DIR/const-impl-recovery.rs:9:7
-   |
-LL | const impl<T: Foo> Bar for T {}
-   |       ^^^^ expected identifier, found keyword
-   |
-help: you might have meant to write a const trait impl
-   |
-LL - const impl<T: Foo> Bar for T {}
-LL + impl<T: Foo> const Bar for T {}
-   |
-
-error: aborting due to 2 previous errors
-