Recover upon encountering mistyped Const in const param def

compiler/rustc_parse/src/parser/generics.rs

@@ -43,6 +43,15 @@ impl<'a> Parser<'a> {
     fn parse_ty_param(&mut self, preceding_attrs: AttrVec) -> PResult<'a, GenericParam> {
         let ident = self.parse_ident()?;
 
+        // We might have a typo'd `Const` that was parsed as a type parameter.
+        if self.may_recover()
+            && ident.name.as_str().to_ascii_lowercase() == kw::Const.as_str()
+            && self.check_ident()
+        // `Const` followed by IDENT
+        {
+            return Ok(self.recover_const_param_with_mistyped_const(preceding_attrs, ident)?);
+        }
+
         // Parse optional colon and param bounds.
         let mut colon_span = None;
         let bounds = if self.eat(&token::Colon) {
@@ -120,6 +129,41 @@ impl<'a> Parser<'a> {
         })
     }
 
+    pub(crate) fn recover_const_param_with_mistyped_const(
+        &mut self,
+        preceding_attrs: AttrVec,
+        mistyped_const_ident: Ident,
+    ) -> PResult<'a, GenericParam> {
+        let ident = self.parse_ident()?;
+        self.expect(&token::Colon)?;
+        let ty = self.parse_ty()?;
+
+        // Parse optional const generics default value.
+        let default = if self.eat(&token::Eq) { Some(self.parse_const_arg()?) } else { None };
+
+        let mut err = self.struct_span_err(
+            mistyped_const_ident.span,
+            format!("`const` keyword was mistyped as `{}`", mistyped_const_ident.as_str()),
+        );
+        err.span_suggestion_verbose(
+            mistyped_const_ident.span,
+            "use the `const` keyword",
+            kw::Const.as_str(),
+            Applicability::MachineApplicable,
+        );
+        err.emit();
+
+        Ok(GenericParam {
+            ident,
+            id: ast::DUMMY_NODE_ID,
+            attrs: preceding_attrs,
+            bounds: Vec::new(),
+            kind: GenericParamKind::Const { ty, kw_span: mistyped_const_ident.span, default },
+            is_placeholder: false,
+            colon_span: None,
+        })
+    }
+
     /// Parses a (possibly empty) list of lifetime and type parameters, possibly including
     /// a trailing comma and erroneous trailing attributes.
     pub(super) fn parse_generic_params(&mut self) -> PResult<'a, ThinVec<ast::GenericParam>> {

tests/ui/parser/typod-const-in-const-param-def.rs

@@ -0,0 +1,16 @@
+pub fn foo<Const N: u8>() {}
+//~^ ERROR `const` keyword was mistyped as `Const`
+
+pub fn bar<Const>() {}
+// OK
+
+pub fn baz<Const N: u8, T>() {}
+//~^ ERROR `const` keyword was mistyped as `Const`
+
+pub fn qux<T, Const N: u8>() {}
+//~^ ERROR `const` keyword was mistyped as `Const`
+
+pub fn quux<T, Const N: u8, U>() {}
+//~^ ERROR `const` keyword was mistyped as `Const`
+
+fn main() {}

tests/ui/parser/typod-const-in-const-param-def.stderr

@@ -0,0 +1,46 @@
+error: `const` keyword was mistyped as `Const`
+  --> $DIR/typod-const-in-const-param-def.rs:1:12
+   |
+LL | pub fn foo<Const N: u8>() {}
+   |            ^^^^^
+   |
+help: use the `const` keyword
+   |
+LL | pub fn foo<const N: u8>() {}
+   |            ~~~~~
+
+error: `const` keyword was mistyped as `Const`
+  --> $DIR/typod-const-in-const-param-def.rs:7:12
+   |
+LL | pub fn baz<Const N: u8, T>() {}
+   |            ^^^^^
+   |
+help: use the `const` keyword
+   |
+LL | pub fn baz<const N: u8, T>() {}
+   |            ~~~~~
+
+error: `const` keyword was mistyped as `Const`
+  --> $DIR/typod-const-in-const-param-def.rs:10:15
+   |
+LL | pub fn qux<T, Const N: u8>() {}
+   |               ^^^^^
+   |
+help: use the `const` keyword
+   |
+LL | pub fn qux<T, const N: u8>() {}
+   |               ~~~~~
+
+error: `const` keyword was mistyped as `Const`
+  --> $DIR/typod-const-in-const-param-def.rs:13:16
+   |
+LL | pub fn quux<T, Const N: u8, U>() {}
+   |                ^^^^^
+   |
+help: use the `const` keyword
+   |
+LL | pub fn quux<T, const N: u8, U>() {}
+   |                ~~~~~
+
+error: aborting due to 4 previous errors
+