Rollup merge of rust-lang#135489 - RalfJung:TryFromSliceError, r=tgross35

remove pointless allowed_through_unstable_modules on TryFromSliceError

This got added in rust-lang#132482 but the PR does not explain why. `@lukas-code` do you still remember? Also Cc `@Noratrieb` as reviewer of that PR.

If I understand the issue description correctly, all paths under which this type is exported are stable now: `core::array::TryFromSliceError` and `std::array::TryFromSliceError`. If that is the case, we shouldn't have the attribute; it's a terrible hack that should only be used when needed to maintain backward compatibility. Getting some historic information right is IMO *not* sufficient justification to risk accidentally exposing this type via more unstable paths today or in the future.

Author: matthiaskrgr

library/core/src/array/mod.rs

@@ -156,7 +156,6 @@ pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
 
 /// The error type returned when a conversion from a slice to an array fails.
 #[stable(feature = "try_from", since = "1.34.0")]
-#[rustc_allowed_through_unstable_modules]
 #[derive(Debug, Copy, Clone)]
 pub struct TryFromSliceError(());
 

src/doc/rustc-dev-guide/src/stability.md

@@ -34,7 +34,8 @@ Previously, due to a [rustc bug], stable items inside unstable modules were
 available to stable code in that location.
 As of <!-- date-check --> September 2024, items with [accidentally stabilized
 paths] are marked with the `#[rustc_allowed_through_unstable_modules]` attribute
-to prevent code dependent on those paths from breaking.
+to prevent code dependent on those paths from breaking. Do *not* add this attribute
+to any more items unless that is needed to avoid breaking changes.
 
 The `unstable` attribute may also have the `soft` value, which makes it a
 future-incompatible deny-by-default lint instead of a hard error. This is used