never patterns: It is correct to lower `!` to `_`. #120517

Nadrieril · 2024-01-31T00:49:25Z

This is just a comment update but a non-trivial one: it is correct to lower ! patterns as _. The reasoning is that ! matches all the possible values of the type, since the type is empty. Moreover, we do want to warn that the Err is redundant in:

match x {
  !,
  Err(!),
}

which is consistent with ! behaving like a wildcard.

I did try to introduce Constructor::Never and it ended up needing to behave exactly like Constructor::Wildcard.

r? @compiler-errors

compiler-errors · 2024-01-31T00:55:23Z

Can you remind me where are we enforcing that we're only matching ! against an uninhabited type?

Nadrieril · 2024-01-31T00:58:09Z

In check_match, before running exhaustiveness:

rust/compiler/rustc_mir_build/src/thir/pattern/check_match.rs

Lines 291 to 294 in cb4d9a1

    
               err = err.and(check_never_pattern(cx, pat)); 
        
           }); 
        
           err?; 
        
           Ok(cx.pattern_arena.alloc(cx.lower_pat(pat)))

compiler-errors · 2024-01-31T14:10:41Z

@bors r+

bors · 2024-01-31T14:10:44Z

📌 Commit ee2cddd has been approved by compiler-errors

It is now in the queue for this repository.

Nadrieril · 2024-01-31T15:10:38Z

Ty!

@bors rollup

… r=compiler-errors never patterns: It is correct to lower `!` to `_`. This is just a comment update but a non-trivial one: it is correct to lower `!` patterns as `_`. The reasoning is that `!` matches all the possible values of the type, since the type is empty. Moreover, we do want to warn that the `Err` is redundant in: ```rust match x { !, Err(!), } ``` which is consistent with `!` behaving like a wildcard. I did try to introduce `Constructor::Never` and it ended up needing to behave exactly like `Constructor::Wildcard`. r? `@compiler-errors`

… r=compiler-errors never patterns: It is correct to lower `!` to `_`. This is just a comment update but a non-trivial one: it is correct to lower `!` patterns as `_`. The reasoning is that `!` matches all the possible values of the type, since the type is empty. Moreover, we do want to warn that the `Err` is redundant in: ```rust match x { !, Err(!), } ``` which is consistent with `!` behaving like a wildcard. I did try to introduce `Constructor::Never` and it ended up needing to behave exactly like `Constructor::Wildcard`. r? ``@compiler-errors``

…iaskrgr Rollup of 9 pull requests Successful merges: - rust-lang#120484 (Avoid ICE when is_val_statically_known is not of a supported type) - rust-lang#120516 (pattern_analysis: cleanup manual impls) - rust-lang#120517 (never patterns: It is correct to lower `!` to `_`.) - rust-lang#120523 (Improve `io::Read::read_buf_exact` error case) - rust-lang#120528 (Store SHOULD_CAPTURE as AtomicU8) - rust-lang#120529 (Update data layouts in custom target tests for LLVM 18) - rust-lang#120530 (Be less confident when `dyn` suggestion is not checked for object safety) - rust-lang#120531 (Remove a bunch of `has_errors` checks that have no meaningful or the wrong effect) - rust-lang#120533 (Correct paths for hexagon-unknown-none-elf platform doc) r? `@ghost` `@rustbot` modify labels: rollup

…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#120484 (Avoid ICE when is_val_statically_known is not of a supported type) - rust-lang#120516 (pattern_analysis: cleanup manual impls) - rust-lang#120517 (never patterns: It is correct to lower `!` to `_`.) - rust-lang#120523 (Improve `io::Read::read_buf_exact` error case) - rust-lang#120528 (Store SHOULD_CAPTURE as AtomicU8) - rust-lang#120529 (Update data layouts in custom target tests for LLVM 18) - rust-lang#120531 (Remove a bunch of `has_errors` checks that have no meaningful or the wrong effect) - rust-lang#120533 (Correct paths for hexagon-unknown-none-elf platform doc) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#120517 - Nadrieril:lower-never-as-wildcard, r=compiler-errors never patterns: It is correct to lower `!` to `_`. This is just a comment update but a non-trivial one: it is correct to lower `!` patterns as `_`. The reasoning is that `!` matches all the possible values of the type, since the type is empty. Moreover, we do want to warn that the `Err` is redundant in: ```rust match x { !, Err(!), } ``` which is consistent with `!` behaving like a wildcard. I did try to introduce `Constructor::Never` and it ended up needing to behave exactly like `Constructor::Wildcard`. r? ```@compiler-errors```

It is correct to lower ! to _.

ee2cddd

Nadrieril added the F-never_patterns `#![feature(never_patterns)]` label Jan 31, 2024

rustbot assigned compiler-errors Jan 31, 2024

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 31, 2024

Nadrieril mentioned this pull request Jan 31, 2024

Tracking Issue for Never Patterns (never_patterns) #118155

Open

5 tasks

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 31, 2024

matthiaskrgr mentioned this pull request Jan 31, 2024

Rollup of 9 pull requests #120534

Closed

matthiaskrgr mentioned this pull request Jan 31, 2024

Rollup of 9 pull requests #120536

Closed

matthiaskrgr mentioned this pull request Feb 3, 2024

Rollup of 8 pull requests #120624

Merged

bors merged commit ceeaa8a into rust-lang:master Feb 4, 2024

rustbot added this to the 1.78.0 milestone Feb 4, 2024

Nadrieril deleted the lower-never-as-wildcard branch February 4, 2024 23:10

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never patterns: It is correct to lower `!` to `_`. #120517

never patterns: It is correct to lower `!` to `_`. #120517

Nadrieril commented Jan 31, 2024

compiler-errors commented Jan 31, 2024

Nadrieril commented Jan 31, 2024

compiler-errors commented Jan 31, 2024

bors commented Jan 31, 2024

Nadrieril commented Jan 31, 2024

never patterns: It is correct to lower ! to _. #120517

never patterns: It is correct to lower ! to _. #120517

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Nadrieril commented Jan 31, 2024

compiler-errors commented Jan 31, 2024

Nadrieril commented Jan 31, 2024

compiler-errors commented Jan 31, 2024

bors commented Jan 31, 2024

Nadrieril commented Jan 31, 2024

never patterns: It is correct to lower `!` to `_`. #120517

never patterns: It is correct to lower `!` to `_`. #120517