Cleanup from_cycle_error.rs

[zetanumbers]

A cleanup based and blocked on #153694.

This PR changes panic messages in fn_sig's value_from_cycle_error to more fitting ones. Also this PR makes variances_of's value_from_cycle_error to utilize new query key argument instead of extracting it from CycleError.

Closes #127971

r? @nnethercote

[rustbot]

⚠️ Warning ⚠️

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  Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.
  • 5986684
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[nnethercote]

Seems plausible, but please add a short description that explains the current situation and how/why this PR improves things. Currently the only descriptive word is "cleanup" which tells the reader very little.

cc @Zoxc

[zetanumbers]

Seems plausible, but please add a short description that explains the current situation and how/why this PR improves things. Currently the only descriptive word is "cleanup" which tells the reader very little.

Updated the top comment. Is it good?

[nnethercote]

It's an improvement, but it still doesn't explain what was going wrong in #127971 and how that has been fixed. The new fn_sig calls default, is that important?

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

##[endgroup]
Executing "/scripts/stage_2_test_set1.sh"
+ /scripts/stage_2_test_set1.sh
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
+ ../x.py --stage 2 test --skip tidy --skip compiler --skip src
PR_CI_JOB set; skipping tidy
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.05s
##[endgroup]
---

4 LL |     fn g() -> B;
5    |               ^
6    |
- help: `B` is dyn-incompatible, use `impl B` to return an opaque type, as long as you return a single underlying type
8    |
9 LL |     fn g() -> impl B;
10    |               ++++


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args parallel-rustc/fn-sig-cycle-ice-153391.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/parallel-rustc/fn-sig-cycle-ice-153391.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/parallel-rustc/fn-sig-cycle-ice-153391" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2024" "-Z" "threads=16"
stdout: none
--- stderr -------------------------------
error[E0782]: expected a type, found a trait
##[error]  --> /checkout/tests/ui/parallel-rustc/fn-sig-cycle-ice-153391.rs:13:23
   |
LL |     fn bar(&self, x: &A);
   |                       ^
   |
   = note: `A` is dyn-incompatible, otherwise a trait object could be used
help: use a new generic type parameter, constrained by `A`
   |
LL -     fn bar(&self, x: &A);
LL +     fn bar<T: A>(&self, x: &T);
   |
help: you can also use an opaque type, but users won't be able to specify the type parameter when calling the `fn`, having to rely exclusively on type inference
   |
LL |     fn bar(&self, x: &impl A);
   |                       ++++
---
LL |     fn g() -> impl B;
   |               ++++
help: alternatively, you can return an owned trait object
   |
LL |     fn g() -> Box<dyn B>;
   |               +++++++  +

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0782`.
------------------------------------------

[zetanumbers]

It's an improvement, but it still doesn't explain what was going wrong in #127971 and how that has been fixed.

This PR basically eliminates panic that have caused #127971 and avoids fn_sig's issue like #153391 to happen to variances_of.

The new fn_sig calls default, is that important?

Previously there was:

tcx.dcx().abort_if_errors();
unreachable!()

Now compare it to default's definition:

let Some(guar) = tcx.sess.dcx().has_errors() else {
    bug!(
        "`from_cycle_error_default` on query `{query_name}` called without errors: {:#?}",
        cycle_error.cycle,
    );
};
guar.raise_fatal()

These are essentially the same. I've only changed unreachable! to more descriptive bug!.

[rust-bors]

☔ The latest upstream changes (presumably #153741) made this pull request unmergeable. Please resolve the merge conflicts.

[nnethercote]

A better PR title would be something like "Fix ICE in variances_of." That makes it clear that it's not just a cleanup, but also fixing a bug.

A better commit message would be something like this:

variances_of currently used search_for_cycle_permutation, which can fail and abort when <... describe circumstances... >. This commit changes variances_of to receive a def_id which means it can compute a value without using search_for_cycle_permutation, avoiding the possible abort.

Fixes #127971

This explains the current situation and the change in a way that is meaningful to a reader who isn't already familiar with #127971.

[nnethercote]

Needs rebasing over #153694, and the title/description/commit messages updated as described above.