Implement lint for obligations broken by never type fallback change

[WaffleLapkin]

This is the second (and probably last major?) lint required for the never type fallback change.

The idea is to check if the code errors with fallback = () and if it errors with fallback = ! and if it went from "ok" to "error", lint.

I'm not happy with the diagnostic, ideally we'd highlight what bound is the problem. But I'm really unsure how to do that (cc @jackh726, iirc you had some ideas?)

r? @compiler-errors

Thanks @BoxyUwU with helping with trait solver stuff when I was implementing the initial version of this lint.

Tracking:

• Tracking Issue for Rust 2024: Make ! fall back to ! #123748

[jackh726]

I'm not happy with the diagnostic, ideally we'd highlight what bound is the problem. But I'm really unsure how to do that

So, yeah, we talked about this. IIRC, what I said was basically like "we can't find diff two error sets" but as long as we only try to report ! fallback errors if () fallback errors are empty, that's probably fine...

We can chat on zulip about details here, but I would probably approach this by selecting a couple test cases where you want better diagnostics, then look at the errors that you get and what info is there, then figure out how to extract out that info. Then you can generalize across tests.

[WaffleLapkin]

Quite a few of existing tests get the warning, should I do something with it? The options I see are bless them (what I currently did), fix them and allow the warning.

[bors]

☔ The latest upstream changes (presumably #125313) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[compiler-errors]

@bors r+

[bors]

📌 Commit b73eb9a has been approved by compiler-errors

It is now in the queue for this repository.

[compiler-errors]

actually wait wait i forgot to run perf ever

@bors r-

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit b73eb9a with merge 3b63bac...

[bors]

☀️ Try build successful - checks-actions
Build commit: 3b63bac (3b63bac3a576b3585ff60fba3a402eb40d159591)

[rust-timer]

Finished benchmarking commit (3b63bac): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

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Bootstrap: 676.994s -> 679.008s (0.30%)
Artifact size: 320.03 MiB -> 320.10 MiB (0.02%)

[compiler-errors]

@bors r+

[bors]

📌 Commit b73eb9a has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit b73eb9a with merge a3baa7b...

[rust-log-analyzer]

The job rfl failed! Check out the build log: (web) (plain)

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

[RUSTC-TIMING] rustc_traits test:false 13.634
   Compiling rustc_mir_transform v0.0.0 (/checkout/compiler/rustc_mir_transform)
[RUSTC-TIMING] rustc_resolve test:false 66.318
   Compiling rustc_hir_typeck v0.0.0 (/checkout/compiler/rustc_hir_typeck)
error[E0599]: no variant or associated item named `FallbackToUnit` found for enum `DivergingFallbackBehavior` in the current scope
    |
    |
20  | pub enum DivergingFallbackBehavior {
    | ---------------------------------- variant or associated item `FallbackToUnit` not found for this enum
...
480 |         let DivergingFallbackBehavior::FallbackToUnit = behavior else { return };
    |                                        ^^^^^^^^^^^^^^ variant or associated item not found in `DivergingFallbackBehavior`
[RUSTC-TIMING] rustc_mir_build test:false 43.190
   Compiling rustc_passes v0.0.0 (/checkout/compiler/rustc_passes)
For more information about this error, try `rustc --explain E0599`.
[RUSTC-TIMING] rustc_hir_typeck test:false 5.945

[bors]

💔 Test failed - checks-actions