Add const_param_ty_unchecked gate

[zedddie]

Add const_param_ty_unchecked internal feature gate to skip ConstParamTy_ trait enforcement on type. Provides an escape hatch for writing tests and examples that use const generics without needing to ensure all fields implement ConstParamTy_.

r? BoxyUwU

[rustbot]

BoxyUwU is currently at their maximum review capacity.
They may take a while to respond.

[rustbot]

changes to the core type system

cc @lcnr

[BoxyUwU]

thx yeah this is what i was looking for :3

View changes since this review

[BoxyUwU]

I wouldn't expect this to actually do anything 🤔 do you have an example that needs this code? otherwise we should drop this

[zedddie]

Without it Vec<[u8]> works :< (const_param_ty_unchecked-unsupported.rs pass); I am not completely sure here but without scoping for ConstParamTys it seem to skip all obligations of inner type, not only ConstParamTy related. So ill formed type also pass (even with register_wf_obligations in wfcheck).

[BoxyUwU]

can you double check this, and if it is definitely a problem can you try look into why wf::obligations isn't returning anything when proving the wf goal in wfcheck 🤔 I don't understand what's going on here and won't have time to actively look into it myself. Feel free to reach out on zulip while looking into this

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[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)

Executing "/scripts/stage_2_test_set1.sh"
+ /scripts/stage_2_test_set1.sh
PR_CI_JOB set; skipping tidy
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
+ ../x.py --stage 2 test --skip tidy --skip compiler --skip src
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
downloading https://static.rust-lang.org/dist/2026-03-05/rustfmt-nightly-aarch64-unknown-linux-gnu.tar.xz
---
To only update this specific test, also pass `--test-args cfg/suggest-alternative-name-on-target.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/cfg/suggest-alternative-name-on-target.rs" "-Zthreads=1" "-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/cfg/suggest-alternative-name-on-target" "-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: unexpected `cfg` condition value: `arm`
##[error]  --> /checkout/tests/ui/cfg/suggest-alternative-name-on-target.rs:5:7
   |
---
LL | #![deny(unexpected_cfgs)]
   |         ^^^^^^^^^^^^^^^
help: `arm` is an expected value for `target_arch`
   |
LL - #[cfg(target_abi = "arm")]
LL + #[cfg(target_arch = "arm")]
   |

error: unexpected `cfg` condition value: `gnu`
##[error]  --> /checkout/tests/ui/cfg/suggest-alternative-name-on-target.rs:12:7
   |
LL | #[cfg(target_arch = "gnu")]
   |       ^^^^^^^^^^^^^^^^^^^
   |
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
help: `gnu` is an expected value for `target_env`
   |
LL - #[cfg(target_arch = "gnu")]
LL + #[cfg(target_env = "gnu")]
   |

error: unexpected `cfg` condition value: `openbsd`
##[error]  --> /checkout/tests/ui/cfg/suggest-alternative-name-on-target.rs:19:7
   |

[zedddie]

its fine locally... :_<