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Make some Clone impls const #91804

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Mar 10, 2022
Merged

Make some Clone impls const #91804

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21 changes: 15 additions & 6 deletions library/core/src/clone.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -127,7 +127,11 @@ pub trait Clone: Sized {
/// allocations.
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn clone_from(&mut self, source: &Self) {
#[default_method_body_is_const]
fn clone_from(&mut self, source: &Self)
where
Self: ~const Drop,
{
*self = source.clone()
}
}
Expand Down Expand Up @@ -178,7 +182,8 @@ mod impls {
($($t:ty)*) => {
$(
#[stable(feature = "rust1", since = "1.0.0")]
impl Clone for $t {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl const Clone for $t {
#[inline]
fn clone(&self) -> Self {
*self
Expand All @@ -196,23 +201,26 @@ mod impls {
}

#[unstable(feature = "never_type", issue = "35121")]
impl Clone for ! {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl const Clone for ! {
#[inline]
fn clone(&self) -> Self {
*self
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Clone for *const T {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl<T: ?Sized> const Clone for *const T {
#[inline]
fn clone(&self) -> Self {
*self
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Clone for *mut T {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl<T: ?Sized> const Clone for *mut T {
#[inline]
fn clone(&self) -> Self {
*self
Expand All @@ -221,7 +229,8 @@ mod impls {

/// Shared references can be cloned, but mutable references *cannot*!
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Clone for &T {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl<T: ?Sized> const Clone for &T {
#[inline]
#[rustc_diagnostic_item = "noop_method_clone"]
fn clone(&self) -> Self {
Expand Down
3 changes: 2 additions & 1 deletion library/core/src/convert/mod.rs
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Expand Up @@ -683,7 +683,8 @@ impl AsMut<str> for str {
pub enum Infallible {}

#[stable(feature = "convert_infallible", since = "1.34.0")]
impl Clone for Infallible {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl const Clone for Infallible {
fn clone(&self) -> Infallible {
match *self {}
}
Expand Down
1 change: 1 addition & 0 deletions library/core/src/lib.rs
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Expand Up @@ -106,6 +106,7 @@
#![feature(const_caller_location)]
#![feature(const_cell_into_inner)]
#![feature(const_char_convert)]
#![feature(const_clone)]
#![feature(const_discriminant)]
#![feature(const_eval_select)]
#![feature(const_float_bits_conv)]
Expand Down
6 changes: 5 additions & 1 deletion library/core/src/option.rs
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Expand Up @@ -1668,7 +1668,11 @@ const fn expect_failed(msg: &str) -> ! {
/////////////////////////////////////////////////////////////////////////////

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone> Clone for Option<T> {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl<T> const Clone for Option<T>
where
T: ~const Clone + ~const Drop,
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Does this change the way rustdoc renders it?

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Before:
image
After:
image

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ah yea, there's a pending fix for the missing space, but the additional Drop bound is really confusing for users and we should probably figure out out story there (both for documentation and explanation) before we expose it

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Will all such cases need to be explained in the future? It seems a bit hard... I think it would be nice if we could switch displaying of ~const (and ~const Drop) bound...

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Yea, I'm unsure about ~const bounds in general, we should probably talk to the rustdoc team to figure out how it should display.

For ~const Drop I think we should just hide it from rustdoc. That bound is pretty much useless to see for anyone. We could potentially add a small symbol to the generic param that has a ~const Drop bound. Otherwise I fear that the useful bounds may get drowned out

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@lilasta lilasta Jan 30, 2022 •
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T: ~const Drop seems to mean "T doesn't require non-const Drop to destruct the value", not "T implements ~const Drop". For example, primitive types don't implement const Drop but are destructible at compile-time. Anyway, the point is that this is mostly needed in code that destructs values at compile-time and is implemented using generics.

The determination seems to be implemented here:
https://github.com/rust-lang/rust/blob/master/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs#L140

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T: ~const Drop seems to mean "T doesn't require non-const Drop to destruct the value"

I'm a bit confused about what you mean. Do you mean "T must be able to be Droped at compile-time"?

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Yes. I'm sorry for confusing you.

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@camelid camelid Feb 3, 2022 •
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What does the ~ mean? I thought ~const meant "maybe const" rather than "definitely const", but I may have misunderstood.

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It means "const if whatever the bound is on is in a const context". So if it's on a generic param of a const fn, calling that const fn in a const item or other const context requires the generic param to implement the trait constly, in regular runtime code it just requires the trait to be implemented with no requirement about constness at all

{
#[inline]
fn clone(&self) -> Self {
match self {
Expand Down
3 changes: 2 additions & 1 deletion library/core/src/ptr/non_null.rs
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Expand Up @@ -635,7 +635,8 @@ impl<T> NonNull<[T]> {
}

#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> Clone for NonNull<T> {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl<T: ?Sized> const Clone for NonNull<T> {
#[inline]
fn clone(&self) -> Self {
*self
Expand Down
3 changes: 2 additions & 1 deletion library/core/src/ptr/unique.rs
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Expand Up @@ -146,7 +146,8 @@ impl<T: ?Sized> Unique<T> {
}

#[unstable(feature = "ptr_internals", issue = "none")]
impl<T: ?Sized> Clone for Unique<T> {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl<T: ?Sized> const Clone for Unique<T> {
#[inline]
fn clone(&self) -> Self {
*self
Expand Down
7 changes: 6 additions & 1 deletion library/core/src/result.rs
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Expand Up @@ -1665,7 +1665,12 @@ fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
/////////////////////////////////////////////////////////////////////////////

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone, E: Clone> Clone for Result<T, E> {
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl<T, E> const Clone for Result<T, E>
where
T: ~const Clone + ~const Drop,
E: ~const Clone + ~const Drop,
{
#[inline]
fn clone(&self) -> Self {
match self {
Expand Down