Make Default const and add some const Default impls

Full list of `impl const Default` types:

- ()
- bool
- char
- 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>

library/alloc/src/lib.rs

@@ -107,6 +107,7 @@
 #![feature(coerce_unsized)]
 #![feature(const_eval_select)]
 #![feature(const_heap)]
+#![feature(const_trait_impl)]
 #![feature(core_intrinsics)]
 #![feature(deprecated_suggestion)]
 #![feature(deref_pure_trait)]

library/alloc/src/string.rs

@@ -2483,7 +2483,8 @@ impl_eq! { Cow<'a, str>, &'b str }
 impl_eq! { Cow<'a, str>, String }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl Default for String {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl const Default for String {
     /// Creates an empty `String`.
     #[inline]
     fn default() -> String {

library/alloc/src/vec/mod.rs

@@ -3771,7 +3771,8 @@ unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Default for Vec<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T> const Default for Vec<T> {
     /// Creates an empty `Vec<T>`.
     ///
     /// The vector will not allocate until elements are pushed onto it.

library/core/src/default.rs

@@ -103,6 +103,7 @@ use crate::ascii::Char as AsciiChar;
 /// ```
 #[cfg_attr(not(test), rustc_diagnostic_item = "Default")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[const_trait]
 #[rustc_trivial_field_reads]
 pub trait Default: Sized {
     /// Returns the "default value" for a type.
@@ -150,7 +151,8 @@ pub macro Default($item:item) {
 macro_rules! default_impl {
     ($t:ty, $v:expr, $doc:tt) => {
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl Default for $t {
+        #[rustc_const_unstable(feature = "const_default", issue = "none")]
+        impl const Default for $t {
             #[inline(always)]
             #[doc = $doc]
             fn default() -> $t {

library/core/src/iter/sources/empty.rs

@@ -81,7 +81,8 @@ impl<T> Clone for Empty<T> {
 // not #[derive] because that adds a Default bound on T,
 // which isn't necessary.
 #[stable(feature = "iter_empty", since = "1.2.0")]
-impl<T> Default for Empty<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T> const Default for Empty<T> {
     fn default() -> Empty<T> {
         Empty(marker::PhantomData)
     }

library/core/src/marker.rs

@@ -793,7 +793,8 @@ impl<T: ?Sized> Clone for PhantomData<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Default for PhantomData<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T: ?Sized> const Default for PhantomData<T> {
     fn default() -> Self {
         Self
     }

library/core/src/option.rs

@@ -2045,7 +2045,8 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Default for Option<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T> const Default for Option<T> {
     /// Returns [`None`][Option::None].
     ///
     /// # Examples

library/core/src/ptr/alignment.rs

@@ -223,7 +223,8 @@ impl hash::Hash for Alignment {
 
 /// Returns [`Alignment::MIN`], which is valid for any type.
 #[unstable(feature = "ptr_alignment_type", issue = "102070")]
-impl Default for Alignment {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl const Default for Alignment {
     fn default() -> Alignment {
         Alignment::MIN
     }

library/core/src/slice/mod.rs

@@ -4924,15 +4924,17 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Default for &[T] {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T> const Default for &[T] {
     /// Creates an empty slice.
     fn default() -> Self {
         &[]
     }
 }
 
 #[stable(feature = "mut_slice_default", since = "1.5.0")]
-impl<T> Default for &mut [T] {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T> const Default for &mut [T] {
     /// Creates a mutable empty slice.
     fn default() -> Self {
         &mut []

library/core/src/str/mod.rs

@@ -2794,7 +2794,8 @@ impl AsRef<[u8]> for str {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl Default for &str {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl const Default for &str {
     /// Creates an empty str
     #[inline]
     fn default() -> Self {
@@ -2803,7 +2804,8 @@ impl Default for &str {
 }
 
 #[stable(feature = "default_mut_str", since = "1.28.0")]
-impl Default for &mut str {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl const Default for &mut str {
     /// Creates an empty mutable str
     #[inline]
     fn default() -> Self {

tests/ui/const-generics/std/const-generics-alloc.rs

@@ -0,0 +1,9 @@
+//@ run-pass
+#![feature(const_trait_impl)]
+#![allow(dead_code)]
+// alloc::string
+const STRING: String = Default::default();
+// alloc::vec
+const VEC: Vec<()> = Default::default();
+
+fn main() {}

tests/ui/const-generics/std/const-generics-core.rs

@@ -0,0 +1,39 @@
+//@ run-pass
+#![feature(const_trait_impl, ptr_alignment_type, ascii_char, f16, f128)]
+#![allow(dead_code)]
+// core::default
+const UNIT: () = Default::default();
+const BOOL: bool = Default::default();
+const CHAR: char = Default::default();
+const ASCII_CHAR: std::ascii::Char = Default::default();
+const USIZE: usize = Default::default();
+const U8: u8 = Default::default();
+const U16: u16 = Default::default();
+const U32: u32 = Default::default();
+const U64: u64 = Default::default();
+const U128: u128 = Default::default();
+const I8: i8 = Default::default();
+const I16: i16 = Default::default();
+const I32: i32 = Default::default();
+const I64: i64 = Default::default();
+const I128: i128 = Default::default();
+const F16: f16 = Default::default();
+const F32: f32 = Default::default();
+const F64: f64 = Default::default();
+const F128: f128 = Default::default();
+// core::marker
+const PHANTOM: std::marker::PhantomData<()> = Default::default();
+// core::option
+const OPT: Option<i32> = Default::default();
+// core::iter::sources::empty
+const EMPTY: std::iter::Empty<()> = Default::default();
+// core::ptr::alignment
+const ALIGNMENT: std::ptr::Alignment = Default::default();
+// core::slice
+const SLICE: &[()] = Default::default();
+const MUT_SLICE: &mut [()] = Default::default();
+//core::str
+const STR: &str = Default::default();
+const MUT_STR: &mut str = Default::default();
+
+fn main() {}