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Document why Range*<&T> as RangeBounds<T> impls are not T: ?Sized, and give an alternative. #144167

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Jul 29, 2025
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Document why Range*<&T> as RangeBounds<T> impls are not T: ?Sized, and give an alternative. #144167

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30 changes: 30 additions & 0 deletions library/core/src/ops/range.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1141,6 +1141,12 @@ impl<'a, T: ?Sized + 'a> RangeBounds<T> for (Bound<&'a T>, Bound<&'a T>) {
}
}

// This impl intentionally does not have `T: ?Sized`;
// see https://github.com/rust-lang/rust/pull/61584 for discussion of why.
//
/// If you need to use this implementation where `T` is unsized,
/// consider using the `RangeBounds` impl for a 2-tuple of [`Bound<&T>`][Bound],
/// i.e. replace `start..` with `(Bound::Included(start), Bound::Unbounded)`.
#[stable(feature = "collections_range", since = "1.28.0")]
impl<T> RangeBounds<T> for RangeFrom<&T> {
fn start_bound(&self) -> Bound<&T> {
Expand All @@ -1151,6 +1157,12 @@ impl<T> RangeBounds<T> for RangeFrom<&T> {
}
}

// This impl intentionally does not have `T: ?Sized`;
// see https://github.com/rust-lang/rust/pull/61584 for discussion of why.
//
/// If you need to use this implementation where `T` is unsized,
/// consider using the `RangeBounds` impl for a 2-tuple of [`Bound<&T>`][Bound],
/// i.e. replace `..end` with `(Bound::Unbounded, Bound::Excluded(end))`.
#[stable(feature = "collections_range", since = "1.28.0")]
impl<T> RangeBounds<T> for RangeTo<&T> {
fn start_bound(&self) -> Bound<&T> {
Expand All @@ -1161,6 +1173,12 @@ impl<T> RangeBounds<T> for RangeTo<&T> {
}
}

// This impl intentionally does not have `T: ?Sized`;
// see https://github.com/rust-lang/rust/pull/61584 for discussion of why.
//
/// If you need to use this implementation where `T` is unsized,
/// consider using the `RangeBounds` impl for a 2-tuple of [`Bound<&T>`][Bound],
/// i.e. replace `start..end` with `(Bound::Included(start), Bound::Excluded(end))`.
#[stable(feature = "collections_range", since = "1.28.0")]
impl<T> RangeBounds<T> for Range<&T> {
fn start_bound(&self) -> Bound<&T> {
Expand All @@ -1171,6 +1189,12 @@ impl<T> RangeBounds<T> for Range<&T> {
}
}

// This impl intentionally does not have `T: ?Sized`;
// see https://github.com/rust-lang/rust/pull/61584 for discussion of why.
//
/// If you need to use this implementation where `T` is unsized,
/// consider using the `RangeBounds` impl for a 2-tuple of [`Bound<&T>`][Bound],
/// i.e. replace `start..=end` with `(Bound::Included(start), Bound::Included(end))`.
#[stable(feature = "collections_range", since = "1.28.0")]
impl<T> RangeBounds<T> for RangeInclusive<&T> {
fn start_bound(&self) -> Bound<&T> {
Expand All @@ -1181,6 +1205,12 @@ impl<T> RangeBounds<T> for RangeInclusive<&T> {
}
}

// This impl intentionally does not have `T: ?Sized`;
// see https://github.com/rust-lang/rust/pull/61584 for discussion of why.
//
/// If you need to use this implementation where `T` is unsized,
/// consider using the `RangeBounds` impl for a 2-tuple of [`Bound<&T>`][Bound],
/// i.e. replace `..=end` with `(Bound::Unbounded, Bound::Included(end))`.
#[stable(feature = "collections_range", since = "1.28.0")]
impl<T> RangeBounds<T> for RangeToInclusive<&T> {
fn start_bound(&self) -> Bound<&T> {
Expand Down
18 changes: 18 additions & 0 deletions library/core/src/range.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -167,6 +167,12 @@ impl<T> RangeBounds<T> for Range<T> {
}
}

// This impl intentionally does not have `T: ?Sized`;
// see https://github.com/rust-lang/rust/pull/61584 for discussion of why.
//
/// If you need to use this implementation where `T` is unsized,
/// consider using the `RangeBounds` impl for a 2-tuple of [`Bound<&T>`][Bound],
/// i.e. replace `start..end` with `(Bound::Included(start), Bound::Excluded(end))`.
#[unstable(feature = "new_range_api", issue = "125687")]
impl<T> RangeBounds<T> for Range<&T> {
fn start_bound(&self) -> Bound<&T> {
Expand Down Expand Up @@ -346,6 +352,12 @@ impl<T> RangeBounds<T> for RangeInclusive<T> {
}
}

// This impl intentionally does not have `T: ?Sized`;
// see https://github.com/rust-lang/rust/pull/61584 for discussion of why.
//
/// If you need to use this implementation where `T` is unsized,
/// consider using the `RangeBounds` impl for a 2-tuple of [`Bound<&T>`][Bound],
/// i.e. replace `start..=end` with `(Bound::Included(start), Bound::Included(end))`.
#[unstable(feature = "new_range_api", issue = "125687")]
impl<T> RangeBounds<T> for RangeInclusive<&T> {
fn start_bound(&self) -> Bound<&T> {
Expand Down Expand Up @@ -491,6 +503,12 @@ impl<T> RangeBounds<T> for RangeFrom<T> {
}
}

// This impl intentionally does not have `T: ?Sized`;
// see https://github.com/rust-lang/rust/pull/61584 for discussion of why.
//
/// If you need to use this implementation where `T` is unsized,
/// consider using the `RangeBounds` impl for a 2-tuple of [`Bound<&T>`][Bound],
/// i.e. replace `start..` with `(Bound::Included(start), Bound::Unbounded)`.
#[unstable(feature = "new_range_api", issue = "125687")]
impl<T> RangeBounds<T> for RangeFrom<&T> {
fn start_bound(&self) -> Bound<&T> {
Expand Down
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