Provided that the iterator has finite length and does not trigger user-provided code, this is safe.
What follows is a full list of the DoubleEndedIterators in liballoc/libcore and whether this optimization is safe, and if not, why not.
src/liballoc/boxed.rs
Box: Pass through to avoid defeating optimization of the underlying DoubleIterator implementation. This has no correctness impact.
src/liballoc/collections/binary_heap.rs
Iter: Pass through to avoid defeating optimizations on slice::Iter
IntoIter: Not safe, changes Drop order
Drain: Not safe, changes Drop order
src/liballoc/collections/btree/map.rs
Iter: Safe to call next_back, invokes no user defined code.
IterMut: ditto
IntoIter: Not safe, changes Drop order
Keys: Safe to call next_back, invokes no user defined code.
Values: ditto
ValuesMut: ditto
Range: ditto
RangeMut: ditto
src/liballoc/collections/btree/set.rs
Iter: Safe to call next_back, invokes no user defined code.
IntoIter: Not safe, changes Drop order
Range: Safe to call next_back, invokes no user defined code.
src/liballoc/collections/linked_list.rs
Iter: Safe to call next_back, invokes no user defined code.
IterMut: ditto
IntoIter: Not safe, changes Drop order
src/liballoc/collections/vec_deque.rs
Iter: Safe to call next_back, invokes no user defined code.
IterMut: ditto
IntoIter: Not safe, changes Drop order
Drain: ditto
src/liballoc/string.rs
Drain: Safe because return type is a primitive (char)
src/liballoc/vec.rs
IntoIter: Not safe, changes Drop order
Drain: ditto
Splice: ditto
src/libcore/ascii.rs
EscapeDefault: Safe because return type is a primitive (u8)
src/libcore/iter/adapters/chain.rs
Chain: Not safe, invokes user defined code (Iterator impl)
src/libcore/iter/adapters/flatten.rs
FlatMap: Not safe, invokes user defined code (Iterator impl)
Flatten: ditto
FlattenCompat: ditto
src/libcore/iter/adapters/mod.rs
Rev: Not safe, invokes user defined code (Iterator impl)
Copied: ditto
Cloned: Not safe, invokes user defined code (Iterator impl and T::clone)
Map: Not safe, invokes user defined code (Iterator impl + closure)
Filter: ditto
FilterMap: ditto
Enumerate: Not safe, invokes user defined code (Iterator impl)
Skip: ditto
Fuse: ditto
Inspect: ditto
src/libcore/iter/adapters/zip.rs
Zip: Not safe, invokes user defined code (Iterator impl)
src/libcore/iter/range.rs
ops::Range: Not safe, changes Drop order, but ALREADY HAS SPECIALIZATION
ops::RangeInclusive: ditto
src/libcore/iter/sources.rs
Repeat: Not safe, calling last should iloop.
Empty: No point, iterator is at most one item long.
Once: ditto
OnceWith: ditto
src/libcore/option.rs
Item: No point, iterator is at most one item long.
Iter: ditto
IterMut: ditto
IntoIter: ditto
src/libcore/result.rs
Iter: No point, iterator is at most one item long
IterMut: ditto
IntoIter: ditto
src/libcore/slice/mod.rs
Split: Not safe, invokes user defined closure
SplitMut: ditto
RSplit: ditto
RSplitMut: ditto
Windows: Safe, already has specialization
Chunks: ditto
ChunksMut: ditto
ChunksExact: ditto
ChunksExactMut: ditto
RChunks: ditto
RChunksMut: ditto
RChunksExact: ditto
RChunksExactMut: ditto
src/libcore/str/mod.rs
Chars: Safe, already has specialization
CharIndices: ditto
Bytes: ditto
Lines: Safe to call next_back, invokes no user defined code.
LinesAny: Deprecated
Everything that is generic over P: Pattern: Not safe because Pattern invokes user defined code.
SplitWhitespace: Safe to call next_back, invokes no user defined code.
SplitAsciiWhitespace: ditto
Stabilize support for Profile-guided Optimization
This PR makes profile-guided optimization available via the `-C profile-generate` / `-C profile-use` pair of commandline flags and adds end-user documentation for the feature to the [rustc book](https://doc.rust-lang.org/rustc/). The PR thus ticks the last two remaining checkboxes of the [stabilization tracking issue](https://github.com/rust-lang/rust/issues/59913).
From the tracking issue:
> Profile-guided optimization (PGO) is a common optimization technique for ahead-of-time compilers. It works by collecting data about a program's typical execution (e.g. probability of branches taken, typical runtime values of variables, etc) and then uses this information during program optimization for things like inlining decisions, machine code layout, or indirect call promotion.
If you are curious about how this can be used, there is a rendered version of the documentation this PR adds available [here](
https://github.com/michaelwoerister/rust/blob/stabilize-pgo/src/doc/rustc/src/profile-guided-optimization.md).
r? @alexcrichton
cc @rust-lang/compiler
