alloc: Unstably add Vec::from_elem and Vec::from_elem_in

[Gelbpunkt]

These methods are of particular interest for users because they perform better due to specialization, e.g. when elem is zero. In that case, it will result in a call to calloc or equivalent, which is better than a call to malloc followed by memset.

We also forward the existing std::vec::from_elem and std::vec::from_elem_in to these new associated methods on Vec. The module-level methods could potentially be removed in a follow-up.

Zulip thread

r? @clarfonthey

[Gelbpunkt]

Is this part of an existing feature or should we create a new tracking issue for this? Also, should from_elem_in use the same feature or does it belong in allocator_ext?

View changes since the review

[clarfonthey]

I would put both of these into their own from_elem issue since I have a feeling allocator_ext will get stabilised first. But yes, should have one.

[rust-log-analyzer]

The job test-aarch64-gnu-llvm-22-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

To only update this specific test, also pass `--test-args suggestions/deref-path-method.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/suggestions/deref-path-method.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/suggestions/deref-path-method" "-Znext-solver=coherence" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2015"
stdout: none
--- stderr -------------------------------
error[E0599]: no associated function or constant named `contains` found for struct `Vec<_, _>` in the current scope
##[error]  --> /checkout/tests/ui/suggestions/deref-path-method.rs:3:10
   |
---
      and 9 others
  --> /rustc/FAKE_PREFIX/library/alloc/src/vec/mod.rs:464:4
help: the function `contains` is implemented on `[_]`
   |
LL -     Vec::contains(&vec, &0);
LL +     <[_]>::contains(&vec, &0);
   |

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0599`.

[clarfonthey]

I would just remove this since it's unstable and make the macro call the associated methods in both cases. The only reason the standalone from_elem shouldn't just be removed also is because it's currently marked as stable, and we want to see how #164053 goes.

View changes since the review

[clarfonthey]

r? ChrisDenton

Offloading a few PRs, Chris should have enough context to handle this.