Permit standalone generic parameters as const generic arguments in macros

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1808,9 +1808,13 @@ impl<'a> Parser<'a> {
         return Ok(false); // Don't continue.
     }
 
-    /// Handle a generic const argument that had not been enclosed in braces, and suggest enclosing
-    /// it braces. In this situation, unlike in `handle_ambiguous_unbraced_const_arg`, this is
-    /// almost certainly a const argument, so we always offer a suggestion.
+    /// Attempt to parse a generic const argument that has not been enclosed in braces.
+    /// There are a limited number of expressions that are permitted without being encoded
+    /// in braces:
+    /// - Literals.
+    /// - Single-segment paths (i.e. standalone generic const parameters).
+    /// All other expressions that can be parsed will emit an error suggesting the expression be
+    /// wrapped in braces.
     pub fn handle_unambiguous_unbraced_const_arg(&mut self) -> PResult<'a, P<Expr>> {
         let start = self.token.span;
         let expr = self.parse_expr_res(Restrictions::CONST_EXPR, None).map_err(|mut err| {

compiler/rustc_parse/src/parser/path.rs

@@ -489,6 +489,7 @@ impl<'a> Parser<'a> {
     /// - An expression surrounded in `{}`.
     /// - A literal.
     /// - A numeric literal prefixed by `-`.
+    /// - A single-segment path.
     pub(super) fn expr_is_valid_const_arg(&self, expr: &P<rustc_ast::Expr>) -> bool {
         match &expr.kind {
             ast::ExprKind::Block(_, _) | ast::ExprKind::Lit(_) => true,
@@ -496,6 +497,13 @@ impl<'a> Parser<'a> {
                 ast::ExprKind::Lit(_) => true,
                 _ => false,
             },
+            // We can only resolve single-segment paths at the moment, because multi-segment paths
+            // require type-checking: see `visit_generic_arg` in `src/librustc_resolve/late.rs`.
+            ast::ExprKind::Path(None, path)
+                if path.segments.len() == 1 && path.segments[0].args.is_none() =>
+            {
+                true
+            }
             _ => false,
         }
     }

src/test/ui/const-generics/macro_rules-braces.full.stderr

@@ -1,14 +1,3 @@
-error: expressions must be enclosed in braces to be used as const generic arguments
-  --> $DIR/macro_rules-braces.rs:34:17
-   |
-LL |     let _: baz!(N);
-   |                 ^
-   |
-help: enclose the `const` expression in braces
-   |
-LL |     let _: baz!({ N });
-   |                 ^   ^
-
 error: expressions must be enclosed in braces to be used as const generic arguments
   --> $DIR/macro_rules-braces.rs:54:17
    |
@@ -68,5 +57,5 @@ LL |     let _: biz!({ N });
    = note: this may fail depending on what value the parameter takes
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 

src/test/ui/const-generics/macro_rules-braces.min.stderr

@@ -1,14 +1,3 @@
-error: expressions must be enclosed in braces to be used as const generic arguments
-  --> $DIR/macro_rules-braces.rs:34:17
-   |
-LL |     let _: baz!(N);
-   |                 ^
-   |
-help: enclose the `const` expression in braces
-   |
-LL |     let _: baz!({ N });
-   |                 ^   ^
-
 error: expressions must be enclosed in braces to be used as const generic arguments
   --> $DIR/macro_rules-braces.rs:54:17
    |
@@ -52,5 +41,5 @@ LL |     let _: biz!({ N });
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
 
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 

src/test/ui/const-generics/macro_rules-braces.rs

@@ -31,7 +31,7 @@ fn test<const N: usize>() {
     let _: foo!({{ N }}); //[min]~ ERROR generic parameters may not
     let _: bar!(N);
     let _: bar!({ N }); //[min]~ ERROR generic parameters may not
-    let _: baz!(N); //~ ERROR expressions must be enclosed in braces
+    let _: baz!(N);
     let _: baz!({ N });
     let _: baz!({{ N }}); //[min]~ ERROR generic parameters may not
     let _: biz!(N);