Make `Default` const and add some `const Default` impls

Full list of `impl const Default` types:

- ()
- bool
- char
- Cell
- std::ascii::Char
- usize
- u8
- u16
- u32
- u64
- u128
- i8
- i16
- i32
- i64
- i128
- f16
- f32
- f64
- f128
- std::marker::PhantomData<T>
- Option<T>
- std::iter::Empty<T>
- std::ptr::Alignment
- &[T]
- &mut [T]
- &str
- &mut str
- String
- Vec<T>

2 files changed, 35 insertions, 33 deletions

diff --git a/tests/ui/specialization/const_trait_impl.rs b/tests/ui/specialization/const_trait_impl.rs
index 2df92dfad3b..e917263d193 100644
--- a/tests/ui/specialization/const_trait_impl.rs
+++ b/tests/ui/specialization/const_trait_impl.rs
@@ -2,6 +2,8 @@
 
 #![feature(const_trait_impl, min_specialization, rustc_attrs)]
 
+use std::fmt::Debug;
+
 #[rustc_specialization_trait]
 #[const_trait]
 pub unsafe trait Sup {
@@ -31,19 +33,19 @@ pub trait A {
     fn a() -> u32;
 }
 
-impl<T: [const] Default> const A for T {
+impl<T: [const] Debug> const A for T {
     default fn a() -> u32 {
         2
     }
 }
 
-impl<T: [const] Default + [const] Sup> const A for T {
+impl<T: [const] Debug + [const] Sup> const A for T {
     default fn a() -> u32 {
         3
     }
 }
 
-impl<T: [const] Default + [const] Sub> const A for T {
+impl<T: [const] Debug + [const] Sub> const A for T {
     fn a() -> u32 {
         T::foo()
     }
diff --git a/tests/ui/specialization/const_trait_impl.stderr b/tests/ui/specialization/const_trait_impl.stderr
index ea3ec16ac1e..b9c768812c8 100644
--- a/tests/ui/specialization/const_trait_impl.stderr
+++ b/tests/ui/specialization/const_trait_impl.stderr
@@ -1,58 +1,58 @@
 error: `[const]` can only be applied to `#[const_trait]` traits
-  --> $DIR/const_trait_impl.rs:34:9
+  --> $DIR/const_trait_impl.rs:36:9
    |
-LL | impl<T: [const] Default> const A for T {
-   |         ^^^^^^^ can't be applied to `Default`
+LL | impl<T: [const] Debug> const A for T {
+   |         ^^^^^^^ can't be applied to `Debug`
    |
-note: `Default` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
-  --> $SRC_DIR/core/src/default.rs:LL:COL
+note: `Debug` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
+  --> $SRC_DIR/core/src/fmt/mod.rs:LL:COL
 
 error: `[const]` can only be applied to `#[const_trait]` traits
-  --> $DIR/const_trait_impl.rs:40:9
+  --> $DIR/const_trait_impl.rs:42:9
    |
-LL | impl<T: [const] Default + [const] Sup> const A for T {
-   |         ^^^^^^^ can't be applied to `Default`
+LL | impl<T: [const] Debug + [const] Sup> const A for T {
+   |         ^^^^^^^ can't be applied to `Debug`
    |
-note: `Default` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
-  --> $SRC_DIR/core/src/default.rs:LL:COL
+note: `Debug` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
+  --> $SRC_DIR/core/src/fmt/mod.rs:LL:COL
 
 error: `[const]` can only be applied to `#[const_trait]` traits
-  --> $DIR/const_trait_impl.rs:46:9
+  --> $DIR/const_trait_impl.rs:48:9
    |
-LL | impl<T: [const] Default + [const] Sub> const A for T {
-   |         ^^^^^^^ can't be applied to `Default`
+LL | impl<T: [const] Debug + [const] Sub> const A for T {
+   |         ^^^^^^^ can't be applied to `Debug`
    |
-note: `Default` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
-  --> $SRC_DIR/core/src/default.rs:LL:COL
+note: `Debug` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
+  --> $SRC_DIR/core/src/fmt/mod.rs:LL:COL
 
 error: `[const]` can only be applied to `#[const_trait]` traits
-  --> $DIR/const_trait_impl.rs:40:9
+  --> $DIR/const_trait_impl.rs:42:9
    |
-LL | impl<T: [const] Default + [const] Sup> const A for T {
-   |         ^^^^^^^ can't be applied to `Default`
+LL | impl<T: [const] Debug + [const] Sup> const A for T {
+   |         ^^^^^^^ can't be applied to `Debug`
    |
-note: `Default` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
-  --> $SRC_DIR/core/src/default.rs:LL:COL
+note: `Debug` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
+  --> $SRC_DIR/core/src/fmt/mod.rs:LL:COL
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error: `[const]` can only be applied to `#[const_trait]` traits
-  --> $DIR/const_trait_impl.rs:34:9
+  --> $DIR/const_trait_impl.rs:36:9
    |
-LL | impl<T: [const] Default> const A for T {
-   |         ^^^^^^^ can't be applied to `Default`
+LL | impl<T: [const] Debug> const A for T {
+   |         ^^^^^^^ can't be applied to `Debug`
    |
-note: `Default` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
-  --> $SRC_DIR/core/src/default.rs:LL:COL
+note: `Debug` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
+  --> $SRC_DIR/core/src/fmt/mod.rs:LL:COL
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error: `[const]` can only be applied to `#[const_trait]` traits
-  --> $DIR/const_trait_impl.rs:46:9
+  --> $DIR/const_trait_impl.rs:48:9
    |
-LL | impl<T: [const] Default + [const] Sub> const A for T {
-   |         ^^^^^^^ can't be applied to `Default`
+LL | impl<T: [const] Debug + [const] Sub> const A for T {
+   |         ^^^^^^^ can't be applied to `Debug`
    |
-note: `Default` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
-  --> $SRC_DIR/core/src/default.rs:LL:COL
+note: `Debug` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
+  --> $SRC_DIR/core/src/fmt/mod.rs:LL:COL
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error: aborting due to 6 previous errors