Stabilize underscore_const_names.

src/test/ui/consts/const_short_circuit.rs

@@ -1,5 +1,3 @@
-#![feature(underscore_const_names)]
-
 const _: bool = false && false;
 const _: bool = true && false;
 const _: bool = {

src/test/ui/consts/const_short_circuit.stderr

@@ -1,23 +1,23 @@
 error: new features like let bindings are not permitted in constants which also use short circuiting operators
-  --> $DIR/const_short_circuit.rs:6:9
+  --> $DIR/const_short_circuit.rs:4:9
    |
 LL |     let mut x = true && false;
    |         ^^^^^
    |
 note: use of `&&` operator here does not actually short circuit due to the const evaluator presently not being able to do control flow. See https://github.com/rust-lang/rust/issues/49146 for more information.
-  --> $DIR/const_short_circuit.rs:6:22
+  --> $DIR/const_short_circuit.rs:4:22
    |
 LL |     let mut x = true && false;
    |                      ^^
 
 error: new features like let bindings are not permitted in constants which also use short circuiting operators
-  --> $DIR/const_short_circuit.rs:11:9
+  --> $DIR/const_short_circuit.rs:9:9
    |
 LL |     let x = true && false;
    |         ^
    |
 note: use of `&&` operator here does not actually short circuit due to the const evaluator presently not being able to do control flow. See https://github.com/rust-lang/rust/issues/49146 for more information.
-  --> $DIR/const_short_circuit.rs:11:18
+  --> $DIR/const_short_circuit.rs:9:18
    |
 LL |     let x = true && false;
    |                  ^^

src/test/ui/consts/underscore_const_names.rs

@@ -0,0 +1,31 @@
+// compile-pass
+
+#![deny(unused)]
+
+trait Trt {}
+pub struct Str {}
+impl Trt for Str {}
+
+macro_rules! check_impl {
+    ($struct:ident,$trait:ident) => {
+        const _ : () = {
+            use std::marker::PhantomData;
+            struct ImplementsTrait<T: $trait>(PhantomData<T>);
+            let _ = ImplementsTrait::<$struct>(PhantomData);
+            ()
+        };
+    }
+}
+
+const _ : () = ();
+
+const _ : i32 = 42;
+const _ : Str = Str{};
+
+check_impl!(Str, Trt);
+check_impl!(Str, Trt);
+
+fn main() {
+  check_impl!(Str, Trt);
+  check_impl!(Str, Trt);
+}