Auto merge of #87408 - kornelski:try_reserve_error, r=yaahc

Hide allocator details from TryReserveError

I think there's [no need for TryReserveError to carry detailed information](https://github.com/rust-lang/rust/issues/48043#issuecomment-825139280), but I wouldn't want that issue to delay stabilization of the `try_reserve` feature.

So I'm proposing to stabilize `try_reserve` with a `TryReserveError` as an opaque structure, and if needed, expose error details later.

This PR moves the `enum` to an unstable inner `TryReserveErrorKind` that lives under a separate feature flag. `TryReserveErrorKind` could possibly be left as an implementation detail forever, and the `TryReserveError` get methods such as `allocation_size() -> Option<usize>` or `layout() -> Option<Layout>` instead, or the details could be dropped completely to make try-reserve errors just a unit struct, and thus smaller and cheaper.

1 files changed, 2 insertions, 1 deletions

diff --git a/library/alloc/src/collections/vec_deque/mod.rs b/library/alloc/src/collections/vec_deque/mod.rs
index 6bbddeb69b2..bea5cf11be5 100644
--- a/library/alloc/src/collections/vec_deque/mod.rs
+++ b/library/alloc/src/collections/vec_deque/mod.rs
@@ -19,6 +19,7 @@ use core::slice;
 
 use crate::alloc::{Allocator, Global};
 use crate::collections::TryReserveError;
+use crate::collections::TryReserveErrorKind;
 use crate::raw_vec::RawVec;
 use crate::vec::Vec;
 
@@ -773,7 +774,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
         let new_cap = used_cap
             .checked_add(additional)
             .and_then(|needed_cap| needed_cap.checked_next_power_of_two())
-            .ok_or(TryReserveError::CapacityOverflow)?;
+            .ok_or(TryReserveErrorKind::CapacityOverflow)?;
 
         if new_cap > old_cap {
             self.buf.try_reserve_exact(used_cap, new_cap - used_cap)?;