Turn the ExactSizeIterator trait in core::iter const-compatible.

[alejandro-vaz]

This PR is the implementation PR for #161094.

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[alejandro-vaz]

@rustbot review

[alejandro-vaz]

@rustbot review

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

LL | const impl ExactSizeIterator for MyType {}
   |            ^^^^^^^^^^^^^^^^^ trait is not stable as const yet
   |
   = note: see issue #161094 <https://github.com/rust-lang/rust/issues/161094> for more information
   = help: add `#![feature(const_exact_size_iterator)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0658`.

---
To only update this specific test, also pass `--test-args feature-gates/feature-gate-const_exact_size_iterator.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/feature-gates/feature-gate-const_exact_size_iterator.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/feature-gates/feature-gate-const_exact_size_iterator" "-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"
stdout: none
--- stderr -------------------------------
error[E0658]: use of unstable const library feature `const_exact_size_iterator`
##[error]  --> /checkout/tests/ui/feature-gates/feature-gate-const_exact_size_iterator.rs:19:12
   |
LL | const impl ExactSizeIterator for MyType {}
   |            ^^^^^^^^^^^^^^^^^ trait is not stable as const yet
   |
   = note: see issue #161094 <https://github.com/rust-lang/rust/issues/161094> for more information
   = help: add `#![feature(const_exact_size_iterator)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0658`.
------------------------------------------