Auto merge of #106210 - fee1-dead-contrib:const-closure-trait-method, r=compiler-errors

Allow trait method paths to satisfy const Fn bounds

r? `@oli-obk`

src/test/ui/rfc-2632-const-trait-impl/const-closure-trait-method-fail.rs

@@ -0,0 +1,19 @@
+#![feature(const_trait_impl)]
+
+#[const_trait]
+trait Tr {
+    fn a(self) -> i32;
+}
+
+impl Tr for () {
+    fn a(self) -> i32 { 42 }
+}
+
+const fn need_const_closure<T: ~const FnOnce(()) -> i32>(x: T) -> i32 {
+    x(())
+}
+
+const _: () = assert!(need_const_closure(Tr::a) == 42);
+//~^ ERROR: the trait bound
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-closure-trait-method-fail.stderr

@@ -0,0 +1,23 @@
+error[E0277]: the trait bound `(): ~const Tr` is not satisfied in `fn(()) -> i32 {<() as Tr>::a}`
+  --> $DIR/const-closure-trait-method-fail.rs:16:42
+   |
+LL | const _: () = assert!(need_const_closure(Tr::a) == 42);
+   |                       ------------------ ^^^^^ within `fn(()) -> i32 {<() as Tr>::a}`, the trait `~const Tr` is not implemented for `()`
+   |                       |
+   |                       required by a bound introduced by this call
+   |
+note: the trait `Tr` is implemented for `()`, but that implementation is not `const`
+  --> $DIR/const-closure-trait-method-fail.rs:16:42
+   |
+LL | const _: () = assert!(need_const_closure(Tr::a) == 42);
+   |                                          ^^^^^
+   = note: required because it appears within the type `fn(()) -> i32 {<() as Tr>::a}`
+note: required by a bound in `need_const_closure`
+  --> $DIR/const-closure-trait-method-fail.rs:12:32
+   |
+LL | const fn need_const_closure<T: ~const FnOnce(()) -> i32>(x: T) -> i32 {
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `need_const_closure`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/rfc-2632-const-trait-impl/const-closure-trait-method.rs

@@ -0,0 +1,19 @@
+// check-pass
+#![feature(const_trait_impl)]
+
+#[const_trait]
+trait Tr {
+    fn a(self) -> i32;
+}
+
+impl const Tr for () {
+    fn a(self) -> i32 { 42 }
+}
+
+const fn need_const_closure<T: ~const FnOnce(()) -> i32>(x: T) -> i32 {
+    x(())
+}
+
+const _: () = assert!(need_const_closure(Tr::a) == 42);
+
+fn main() {}