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

@@ -489,13 +489,21 @@ 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,
             ast::ExprKind::Unary(ast::UnOp::Neg, expr) => match &expr.kind {
                 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

@@ -1,16 +1,16 @@
 error: expressions must be enclosed in braces to be used as const generic arguments
-  --> $DIR/macro_rules-braces.rs:34:17
+  --> $DIR/macro_rules-braces.rs:49:17
    |
-LL |     let _: baz!(N);
-   |                 ^
+LL |     let _: baz!(m::P);
+   |                 ^^^^
    |
 help: enclose the `const` expression in braces
    |
-LL |     let _: baz!({ N });
-   |                 ^   ^
+LL |     let _: baz!({ m::P });
+   |                 ^      ^
 
 error: expressions must be enclosed in braces to be used as const generic arguments
-  --> $DIR/macro_rules-braces.rs:54:17
+  --> $DIR/macro_rules-braces.rs:69:17
    |
 LL |     let _: baz!(10 + 7);
    |                 ^^^^^^
@@ -21,7 +21,7 @@ LL |     let _: baz!({ 10 + 7 });
    |                 ^        ^
 
 error: constant expression depends on a generic parameter
-  --> $DIR/macro_rules-braces.rs:10:13
+  --> $DIR/macro_rules-braces.rs:16:13
    |
 LL |             [u8; $x]
    |             ^^^^^^^^
@@ -33,7 +33,7 @@ LL |     let _: foo!({{ N }});
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: constant expression depends on a generic parameter
-  --> $DIR/macro_rules-braces.rs:15:13
+  --> $DIR/macro_rules-braces.rs:21:13
    |
 LL |             [u8; { $x }]
    |             ^^^^^^^^^^^^
@@ -45,7 +45,7 @@ LL |     let _: bar!({ N });
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: constant expression depends on a generic parameter
-  --> $DIR/macro_rules-braces.rs:20:13
+  --> $DIR/macro_rules-braces.rs:26:13
    |
 LL |             Foo<$x>
    |             ^^^^^^^
@@ -57,7 +57,7 @@ LL |     let _: baz!({{ N }});
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: constant expression depends on a generic parameter
-  --> $DIR/macro_rules-braces.rs:25:13
+  --> $DIR/macro_rules-braces.rs:31:13
    |
 LL |             Foo<{ $x }>
    |             ^^^^^^^^^^^

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

@@ -1,16 +1,16 @@
 error: expressions must be enclosed in braces to be used as const generic arguments
-  --> $DIR/macro_rules-braces.rs:34:17
+  --> $DIR/macro_rules-braces.rs:49:17
    |
-LL |     let _: baz!(N);
-   |                 ^
+LL |     let _: baz!(m::P);
+   |                 ^^^^
    |
 help: enclose the `const` expression in braces
    |
-LL |     let _: baz!({ N });
-   |                 ^   ^
+LL |     let _: baz!({ m::P });
+   |                 ^      ^
 
 error: expressions must be enclosed in braces to be used as const generic arguments
-  --> $DIR/macro_rules-braces.rs:54:17
+  --> $DIR/macro_rules-braces.rs:69:17
    |
 LL |     let _: baz!(10 + 7);
    |                 ^^^^^^
@@ -21,31 +21,31 @@ LL |     let _: baz!({ 10 + 7 });
    |                 ^        ^
 
 error: generic parameters may not be used in const operations
-  --> $DIR/macro_rules-braces.rs:31:20
+  --> $DIR/macro_rules-braces.rs:37:20
    |
 LL |     let _: foo!({{ N }});
    |                    ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
 
 error: generic parameters may not be used in const operations
-  --> $DIR/macro_rules-braces.rs:33:19
+  --> $DIR/macro_rules-braces.rs:41:19
    |
 LL |     let _: bar!({ N });
    |                   ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
 
 error: generic parameters may not be used in const operations
-  --> $DIR/macro_rules-braces.rs:36:20
+  --> $DIR/macro_rules-braces.rs:46:20
    |
 LL |     let _: baz!({{ N }});
    |                    ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
 
 error: generic parameters may not be used in const operations
-  --> $DIR/macro_rules-braces.rs:38:19
+  --> $DIR/macro_rules-braces.rs:51:19
    |
 LL |     let _: biz!({ N });
    |                   ^ cannot perform const operation using `N`

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

@@ -3,6 +3,12 @@
 #![cfg_attr(full, feature(const_generics))]
 #![cfg_attr(min, feature(min_const_generics))]
 
+mod m {
+    pub const P: usize = 0;
+}
+
+const Q: usize = 0;
+
 fn test<const N: usize>() {
     struct Foo<const M: usize>;
     macro_rules! foo {
@@ -29,13 +35,22 @@ fn test<const N: usize>() {
     let _: foo!(N);
     let _: foo!({ N });
     let _: foo!({{ N }}); //[min]~ ERROR generic parameters may not
+    let _: foo!(Q);
+    let _: foo!(m::P);
     let _: bar!(N);
     let _: bar!({ N }); //[min]~ ERROR generic parameters may not
-    let _: baz!(N); //~ ERROR expressions must be enclosed in braces
+    let _: bar!(Q);
+    let _: bar!(m::P);
+    let _: baz!(N);
     let _: baz!({ N });
     let _: baz!({{ N }}); //[min]~ ERROR generic parameters may not
+    let _: baz!(Q);
+    let _: baz!({ m::P });
+    let _: baz!(m::P); //~ ERROR expressions must be enclosed in braces
     let _: biz!(N);
     let _: biz!({ N }); //[min]~ ERROR generic parameters may not
+    let _: biz!(Q);
+    let _: biz!(m::P);
     let _: foo!(3);
     let _: foo!({ 3 });
     let _: foo!({{ 3 }});