Make Default const and add some const Default impls

estebank · estebank · commit e2b12ba78950 · 2025-07-03T17:59:17.000Z
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&lt;T&gt;
- Option&lt;T&gt;
- std::iter::Empty&lt;T&gt;
- std::ptr::Alignment
- &amp;[T]
- &amp;mut [T]
- &amp;str
- &amp;mut str
- String
- Vec&lt;T&gt;
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
@@ -107,8 +107,10 @@
 #![feature(char_max_len)]
 #![feature(clone_to_uninit)]
 #![feature(coerce_unsized)]
+#![feature(const_default)]
 #![feature(const_eval_select)]
 #![feature(const_heap)]
+#![feature(const_trait_impl)]
 #![feature(core_intrinsics)]
 #![feature(deprecated_suggestion)]
 #![feature(deref_pure_trait)]
diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
@@ -2611,7 +2611,8 @@ 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 {
diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
@@ -3895,7 +3895,8 @@ 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.
diff --git a/library/core/src/default.rs b/library/core/src/default.rs
@@ -103,6 +103,9 @@ use crate::ascii::Char as AsciiChar;
 /// ```
 #[rustc_diagnostic_item = "Default"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[const_trait]
+#[rustc_trivial_field_reads]
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
 pub trait Default: Sized {
     /// Returns the "default value" for a type.
     ///
@@ -149,7 +152,8 @@ 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 {
diff --git a/library/core/src/iter/sources/empty.rs b/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)
     }
diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
@@ -858,7 +858,8 @@ impl<T: PointeeSized> Clone for PhantomData<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: PointeeSized> Default for PhantomData<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T: PointeeSized> const Default for PhantomData<T> {
     fn default() -> Self {
         Self
     }
diff --git a/library/core/src/option.rs b/library/core/src/option.rs
@@ -2111,7 +2111,8 @@ where
 impl<T> crate::clone::UseCloned for Option<T> where T: crate::clone::UseCloned {}
 
 #[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
diff --git a/library/core/src/ptr/alignment.rs b/library/core/src/ptr/alignment.rs
@@ -230,7 +230,8 @@ 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
     }
diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
@@ -5158,15 +5158,17 @@ 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 []
diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
@@ -3072,7 +3072,8 @@ 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 {
@@ -3081,7 +3082,8 @@ 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 {
diff --git a/tests/ui/const-generics/std/const-generics-alloc.rs b/tests/ui/const-generics/std/const-generics-alloc.rs
@@ -0,0 +1,9 @@
+//@ run-pass
+#![feature(const_trait_impl, const_default)]
+#![allow(dead_code)]
+// alloc::string
+const STRING: String = Default::default();
+// alloc::vec
+const VEC: Vec<()> = Default::default();
+
+fn main() {}
diff --git a/tests/ui/const-generics/std/const-generics-core.rs b/tests/ui/const-generics/std/const-generics-core.rs
@@ -0,0 +1,39 @@
+//@ run-pass
+#![feature(const_trait_impl, const_default, 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() {}
diff --git a/tests/ui/consts/rustc-impl-const-stability.rs b/tests/ui/consts/rustc-impl-const-stability.rs
@@ -2,7 +2,7 @@
 //@ known-bug: #110395
 
 #![crate_type = "lib"]
-#![feature(staged_api, const_trait_impl)]
+#![feature(staged_api, const_trait_impl, const_default)]
 #![stable(feature = "foo", since = "1.0.0")]
 
 #[stable(feature = "potato", since = "1.27.0")]
@@ -12,8 +12,8 @@ pub struct Data {
 
 #[stable(feature = "potato", since = "1.27.0")]
 #[rustc_const_unstable(feature = "data_foo", issue = "none")]
-impl const Default for Data {
-    fn default() -> Data {
-        Data { _data: 42 }
+impl const std::fmt::Debug for Data {
+    fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
+        Ok(())
     }
 }
diff --git a/tests/ui/consts/rustc-impl-const-stability.stderr b/tests/ui/consts/rustc-impl-const-stability.stderr
@@ -1,8 +1,8 @@
-error: const `impl` for trait `Default` which is not marked with `#[const_trait]`
+error: const `impl` for trait `Debug` which is not marked with `#[const_trait]`
   --> $DIR/rustc-impl-const-stability.rs:15:12
    |
-LL | impl const Default for Data {
-   |            ^^^^^^^ this trait is not `const`
+LL | impl const std::fmt::Debug for Data {
+   |            ^^^^^^^^^^^^^^^ this trait is not `const`
    |
    = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const`
    = note: adding a non-const method body in the future would be a breaking change
diff --git a/tests/ui/deriving/deriving-default-enum.rs b/tests/ui/deriving/deriving-default-enum.rs
@@ -1,4 +1,5 @@
 //@ run-pass
+#![allow(dead_code)]
 
 // nb: does not impl Default
 #[derive(Debug, PartialEq)]
@@ -8,7 +9,6 @@ struct NotDefault;
 enum Foo {
     #[default]
     Alpha,
-    #[allow(dead_code)]
     Beta(NotDefault),
 }
 
@@ -17,7 +17,6 @@ enum Foo {
 enum MyOption<T> {
     #[default]
     None,
-    #[allow(dead_code)]
     Some(T),
 }
 
diff --git a/tests/ui/lint/dead-code/unused-struct-derive-default.rs b/tests/ui/lint/dead-code/unused-struct-derive-default.rs
@@ -1,4 +1,4 @@
-#![deny(dead_code)]
+#![deny(dead_code)] //~ NOTE the lint level is defined here
 
 #[derive(Default)]
 struct T; //~ ERROR struct `T` is never constructed
@@ -7,10 +7,11 @@ struct T; //~ ERROR struct `T` is never constructed
 struct Used;
 
 #[derive(Default)]
-enum E {
+enum E { //~ NOTE variants in this enum
     #[default]
-    A,
-    B, //~ ERROR variant `B` is never constructed
+    A, //~ ERROR variants `A` and `B` are never constructed
+    //~^ NOTE `E` has a derived impl for the trait `Default`, but this is intentionally ignored during dead code analysis
+    B,
 }
 
 // external crate can call T2::default() to construct T2,
diff --git a/tests/ui/lint/dead-code/unused-struct-derive-default.stderr b/tests/ui/lint/dead-code/unused-struct-derive-default.stderr
@@ -10,14 +10,19 @@ note: the lint level is defined here
 LL | #![deny(dead_code)]
    |         ^^^^^^^^^
 
-error: variant `B` is never constructed
-  --> $DIR/unused-struct-derive-default.rs:13:5
+error: variants `A` and `B` are never constructed
+  --> $DIR/unused-struct-derive-default.rs:12:5
    |
 LL | enum E {
-   |      - variant in this enum
-...
+   |      - variants in this enum
+LL |     #[default]
+LL |     A,
+   |     ^
+LL |
 LL |     B,
    |     ^
+   |
+   = note: `E` has a derived impl for the trait `Default`, but this is intentionally ignored during dead code analysis
 
 error: aborting due to 2 previous errors
 
diff --git 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
diff --git a/tests/ui/traits/const-traits/const_derives/derive-const-gate.rs b/tests/ui/traits/const-traits/const_derives/derive-const-gate.rs
diff --git a/tests/ui/traits/const-traits/const_derives/derive-const-gate.stderr b/tests/ui/traits/const-traits/const_derives/derive-const-gate.stderr
diff --git a/tests/ui/traits/const-traits/const_derives/derive-const-non-const-type.stderr b/tests/ui/traits/const-traits/const_derives/derive-const-non-const-type.stderr
diff --git a/tests/ui/traits/const-traits/const_derives/derive-const-use.rs b/tests/ui/traits/const-traits/const_derives/derive-const-use.rs
diff --git a/tests/ui/traits/const-traits/const_derives/derive-const-use.stderr b/tests/ui/traits/const-traits/const_derives/derive-const-use.stderr
diff --git a/tests/ui/traits/const-traits/std-impl-gate.gated.stderr b/tests/ui/traits/const-traits/std-impl-gate.gated.stderr
diff --git a/tests/ui/traits/const-traits/std-impl-gate.rs b/tests/ui/traits/const-traits/std-impl-gate.rs
diff --git a/tests/ui/traits/const-traits/std-impl-gate.stock.stderr b/tests/ui/traits/const-traits/std-impl-gate.stock.stderr

Original file line number	Diff line number	Diff line change
`@@ -3895,7 +3895,8 @@ unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {`
`3895`	`3895`	`}`
`3896`	`3896`
`3897`	`3897`	`#[stable(feature = "rust1", since = "1.0.0")]`
`3898`		`-impl<T> Default for Vec<T> {`
	`3898`	`+#[rustc_const_unstable(feature = "const_default", issue = "none")]`
	`3899`	`+impl<T> const Default for Vec<T> {`
`3899`	`3900`	/// Creates an empty `Vec<T>`.
`3900`	`3901`	`///`
`3901`	`3902`	`/// The vector will not allocate until elements are pushed onto it.`
Original file line number	Diff line number	Diff line change
`@@ -858,7 +858,8 @@ impl<T: PointeeSized> Clone for PhantomData<T> {`
`858`	`858`	`}`
`859`	`859`
`860`	`860`	`#[stable(feature = "rust1", since = "1.0.0")]`
`861`		`-impl<T: PointeeSized> Default for PhantomData<T> {`
	`861`	`+#[rustc_const_unstable(feature = "const_default", issue = "none")]`
	`862`	`+impl<T: PointeeSized> const Default for PhantomData<T> {`
`862`	`863`	`fn default() -> Self {`
`863`	`864`	`Self`
`864`	`865`	`}`
Original file line number	Diff line number	Diff line change
`@@ -230,7 +230,8 @@ impl hash::Hash for Alignment {`
`230`	`230`
`231`	`231`	/// Returns [`Alignment::MIN`], which is valid for any type.
`232`	`232`	`#[unstable(feature = "ptr_alignment_type", issue = "102070")]`
`233`		`-impl Default for Alignment {`
	`233`	`+#[rustc_const_unstable(feature = "const_default", issue = "none")]`
	`234`	`+impl const Default for Alignment {`
`234`	`235`	`fn default() -> Alignment {`
`235`	`236`	`Alignment::MIN`
`236`	`237`	`}`
Original file line number	Diff line number	Diff line change
`@@ -5158,15 +5158,17 @@ where`
`5158`	`5158`	`}`
`5159`	`5159`
`5160`	`5160`	`#[stable(feature = "rust1", since = "1.0.0")]`
`5161`		`-impl<T> Default for &[T] {`
	`5161`	`+#[rustc_const_unstable(feature = "const_default", issue = "none")]`
	`5162`	`+impl<T> const Default for &[T] {`
`5162`	`5163`	`/// Creates an empty slice.`
`5163`	`5164`	`fn default() -> Self {`
`5164`	`5165`	`&[]`
`5165`	`5166`	`}`
`5166`	`5167`	`}`
`5167`	`5168`
`5168`	`5169`	`#[stable(feature = "mut_slice_default", since = "1.5.0")]`
`5169`		`-impl<T> Default for &mut [T] {`
	`5170`	`+#[rustc_const_unstable(feature = "const_default", issue = "none")]`
	`5171`	`+impl<T> const Default for &mut [T] {`
`5170`	`5172`	`/// Creates a mutable empty slice.`
`5171`	`5173`	`fn default() -> Self {`
`5172`	`5174`	`&mut []`