From 529fd2da894ff290668d6c1524909ba218d5822b Mon Sep 17 00:00:00 2001
From: Takayuki Maeda <takoyaki0316@gmail.com>
Date: Tue, 8 Mar 2022 17:07:35 +0900
Subject: [PATCH] suggest adding `{ .. }` around a const function with
 arguments

---
 compiler/rustc_parse/src/parser/path.rs       |  9 ++++
 .../const-generics/const-generic-function.rs  | 21 ++++++++
 .../const-generic-function.stderr             | 54 +++++++++++++++++++
 3 files changed, 84 insertions(+)
 create mode 100644 src/test/ui/const-generics/const-generic-function.rs
 create mode 100644 src/test/ui/const-generics/const-generic-function.stderr

diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index 5e537d7b95c..0bccf4dc2ce 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -624,9 +624,18 @@ pub(super) fn parse_generic_arg(
             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.clone();
             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)));
+                        }
+                    }
+                    // self.parse_fn_call_expr();
                     // Try to recover from possible `const` arg without braces.
                     return self.recover_const_arg(start, err).map(Some);
                 }
diff --git a/src/test/ui/const-generics/const-generic-function.rs b/src/test/ui/const-generics/const-generic-function.rs
new file mode 100644
index 00000000000..c8d2683e53f
--- /dev/null
+++ b/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`
+}
diff --git a/src/test/ui/const-generics/const-generic-function.stderr b/src/test/ui/const-generics/const-generic-function.stderr
new file mode 100644
index 00000000000..5ad3f1006c1
--- /dev/null
+++ b/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`.