Suggest adding { .. } around a const function call with arguments

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -179,6 +179,13 @@ impl<'a> Parser<'a> {
         &self.sess.span_diagnostic
     }
 
+    pub(super) fn diagnostic_snapshot(&self) -> Self {
+        let mut snapshot = self.clone();
+        // initialize unclosed_delims to avoid duplicate errors.
+        snapshot.unclosed_delims = vec![];
+        snapshot
+    }
+
     pub(super) fn span_to_snippet(&self, span: Span) -> Result<String, SpanSnippetError> {
         self.sess.source_map().span_to_snippet(span)
     }

compiler/rustc_parse/src/parser/path.rs

@@ -624,9 +624,17 @@ impl<'a> Parser<'a> {
             GenericArg::Const(self.parse_const_arg()?)
         } else if self.check_type() {
             // Parse type argument.
+            let is_const_fn = self.look_ahead(1, |t| t.kind == token::OpenDelim(token::Paren));
+            let mut snapshot = self.diagnostic_snapshot();
             match self.parse_ty() {
                 Ok(ty) => GenericArg::Type(ty),
                 Err(err) => {
+                    if is_const_fn {
+                        if let Ok(expr) = snapshot.parse_expr_res(Restrictions::CONST_EXPR, None) {
+                            *self = snapshot;
+                            return Ok(Some(self.dummy_const_arg_needs_braces(err, expr.span)));
+                        }
+                    }
                     // Try to recover from possible `const` arg without braces.
                     return self.recover_const_arg(start, err).map(Some);
                 }

src/test/ui/const-generics/const-generic-function.rs

@@ -0,0 +1,21 @@
+fn foo<const N: i32>() -> i32 {
+    N
+}
+
+const fn bar(n: i32, m: i32) -> i32 {
+    n
+}
+
+const fn baz() -> i32 {
+    1
+}
+
+const FOO: i32 = 3;
+
+fn main() {
+    foo::<baz()>(); //~ ERROR expected type, found function `baz`
+    //~| ERROR unresolved item provided when a constant was expected
+    foo::<bar(bar(1, 1), bar(1, 1))>(); //~ ERROR expected type, found `1`
+    foo::<bar(1, 1)>(); //~ ERROR expected type, found `1`
+    foo::<bar(FOO, 2)>(); //~ ERROR expected type, found `2`
+}

src/test/ui/const-generics/const-generic-function.stderr

@@ -0,0 +1,54 @@
+error: expected type, found `1`
+  --> $DIR/const-generic-function.rs:18:19
+   |
+LL |     foo::<bar(bar(1, 1), bar(1, 1))>();
+   |                   ^ expected type
+   |
+help: expressions must be enclosed in braces to be used as const generic arguments
+   |
+LL |     foo::<{ bar(bar(1, 1), bar(1, 1)) }>();
+   |           +                           +
+
+error: expected type, found `1`
+  --> $DIR/const-generic-function.rs:19:15
+   |
+LL |     foo::<bar(1, 1)>();
+   |               ^ expected type
+   |
+help: expressions must be enclosed in braces to be used as const generic arguments
+   |
+LL |     foo::<{ bar(1, 1) }>();
+   |           +           +
+
+error: expected type, found `2`
+  --> $DIR/const-generic-function.rs:20:20
+   |
+LL |     foo::<bar(FOO, 2)>();
+   |                    ^ expected type
+   |
+help: expressions must be enclosed in braces to be used as const generic arguments
+   |
+LL |     foo::<{ bar(FOO, 2) }>();
+   |           +             +
+
+error[E0573]: expected type, found function `baz`
+  --> $DIR/const-generic-function.rs:16:11
+   |
+LL |     foo::<baz()>();
+   |           ^^^^^ not a type
+
+error[E0747]: unresolved item provided when a constant was expected
+  --> $DIR/const-generic-function.rs:16:11
+   |
+LL |     foo::<baz()>();
+   |           ^^^^^
+   |
+help: if this generic argument was intended as a const parameter, surround it with braces
+   |
+LL |     foo::<{ baz() }>();
+   |           +       +
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0573, E0747.
+For more information about an error, try `rustc --explain E0573`.