Rollup merge of #79164 - varkor:unbraced-single-segment-const-arguments, r=petrochenkov

Permit standalone generic parameters as const generic arguments in macros

Fixes https://github.com/rust-lang/rust/issues/79127.

r? ```@petrochenkov```

2 files changed, 15 insertions, 3 deletions

diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index cd3b8db2303..350a372a684 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/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| {
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index 79e73749038..d64fd59b0a6 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/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,
         }
     }