Rollup merge of #64028 - Centril:stabilize-alloc-new-2, r=alexcrichton

Stabilize `Vec::new` and `String::new` as `const fn`s

Closes https://github.com/rust-lang/rust/issues/64022.

r? @oli-obk

src/test/ui/collections-const-new.rs

@@ -1,15 +1,11 @@
-// run-pass
+// check-pass
 
-#![allow(dead_code)]
 // Test several functions can be used for constants
 // 1. Vec::new()
 // 2. String::new()
 
-#![feature(const_vec_new)]
-#![feature(const_string_new)]
-
 const MY_VEC: Vec<usize> = Vec::new();
 
 const MY_STRING: String = String::new();
 
-pub fn main() {}
+fn main() {}

src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.rs

@@ -14,8 +14,9 @@ trait Bar<T, U: Foo<T>> {
 impl Foo<u32> for () {
     const X: u32 = 42;
 }
+
 impl Foo<Vec<u32>> for String {
-    const X: Vec<u32> = Vec::new(); //~ ERROR not yet stable as a const fn
+    const X: Vec<u32> = Vec::new();
 }
 
 impl Bar<u32, ()> for () {}

src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.stderr

@@ -4,13 +4,5 @@ error[E0493]: destructors cannot be evaluated at compile-time
 LL |     const F: u32 = (U::X, 42).1;
    |                    ^^^^^^^^^^ constants cannot evaluate destructors
 
-error: `std::vec::Vec::<T>::new` is not yet stable as a const fn
-  --> $DIR/feature-gate-unleash_the_miri_inside_of_you.rs:18:25
-   |
-LL |     const X: Vec<u32> = Vec::new();
-   |                         ^^^^^^^^^^
-   |
-   = help: add `#![feature(const_vec_new)]` to the crate attributes to enable
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error