Remove potentially misleading realloc parenthetical

[CAD97]

This parenthetical is problematic, because it suggests that the following is sound:

let layout = Layout::new::<[u8; 32]>();
let p1 = alloc(layout);
let p2 = realloc(p1, layout, 32);
if p1 == p2 {
    p1.write([0; 32]);
    dealloc(p1, layout);
} else {
    dealloc(p2, layout);
}

At the very least, this isn't the case for ANSI realloc

The original pointer ptr is invalidated and any access to it is undefined behavior (even if reallocation was in-place).

and Windows HeapReAlloc is unclear at best (HEAP_REALLOC_IN_PLACE_ONLY's description may imply that the old pointer may be used if HEAP_REALLOC_IN_PLACE_ONLY is provided).

The conservative position is to just remove the parenthetical.

cc @rust-lang/wg-unsafe-code-guidelines @rust-lang/wg-allocators

[rust-highfive]

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[rust-highfive]

r? @joshtriplett

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[CAD97]

r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs

[dtolnay]

@bors r+ rollup

[bors]

📌 Commit 09dc24b has been approved by dtolnay

[bors]

⌛ Testing commit 09dc24b with merge 56d540e...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 56d540e to master...

[rust-timer]

Finished benchmarking commit (56d540e): comparison url.

Summary: This benchmark run did not return any relevant results.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression