GlobalAlloc and Allocator traits missing `base + size <= usize::MAX` from the allocation conditions

[susitsm]

Location (URL)

GlobalAlloc and Allocator trait documentation

Summary

The core::ptr module's allocation paragraph states that base + size <= usize::MAX must hold for an allocation, which is later reinforced by the statement in pointer::add: vec.as_ptr().add(vec.len()) is always safe. This is not documented as a requirement for allocations in the GlobalAlloc and Allocator traits.

An allocator following the current trait documentation could cause UB by allocating 1 byte at usize::MAX.

[lms0806]

@rustbot claim

[lms0806]

Let's add this to the Safety section of the Allocator documentation.

[eggyal]

@rustbot label +A-allocators +C-bug +E-easy +T-libs-api +T-opsem -needs-triage

[RalfJung]

I fixed the issue title; it said < instead of <=.