Clarify the 'use a constant in a pattern' error message #108548

jamen · 2023-02-28T00:46:03Z

use std::borrow::Cow;

const ERROR_CODE: Cow<'_, str> = Cow::Borrowed("23505");

fn main() {
    let x = Cow::from("23505");
    
    match x {
        ERROR_CODE => {}
    }
}

error: to use a constant of type `Cow` in a pattern, `Cow` must be annotated with `#[derive(PartialEq, Eq)]`
 --> src/main.rs:9:9
  |
9 |         ERROR_CODE => {}
  |         ^^^^^^^^^^

error: could not compile `playground` due to previous error

It seems helpful to link to StructuralEq in this message. I was a little confused, because Cow<'_, str> implements PartialEq and Eq, but they're not derived, which I learned is necessary for structural equality and using constants in patterns (thanks to the Rust community Discord server)

For tests, should I update every occurrence of this message? I see tests where this is still a warning and I'm not sure if I should update those.

rustbot · 2023-02-28T00:46:11Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @petrochenkov (or someone else) soon.

Please see the contribution instructions for more information.

compiler-errors · 2023-02-28T04:09:54Z

I don't think that this error message should mention StructuralEq, but regardless, it should be done as a separate "note:" section on the diagnostic, and not in the primary message of the error.

jamen · 2023-02-28T05:01:53Z

Ok I attempted that

petrochenkov · 2023-02-28T10:03:38Z

CI is failing.
x.py test --bless tests/ui should update the affected test outputs.
@rustbot author

jamen · 2023-02-28T21:13:48Z

fixed an accidental edit. tests unaffected

petrochenkov · 2023-03-01T08:00:12Z

@rustbot ready
r? @compiler-errors

jamen · 2023-03-02T20:07:29Z

I don't think that this error message should mention StructuralEq

Why not? To me the message explains the what but not the why. Seems like a helpful piece of information.

compiler-errors · 2023-03-07T05:47:05Z

My only concern is I'm not sure if it's conventional to document never-stable, compiler implementation details in user-facing (non-nightly-feature-related) error messages...

Maybe someone else from @rust-lang/wg-diagnostics can help make a call here.

This also could really use a test demonstrating the note. otherwise r=me on the implementation though.

oli-obk · 2023-03-07T07:47:54Z

I think we could mention StructualEq as "extended reading material", but in order to fix your issue, the more important thing to me is to have a note saying "the traits must be derived, manual impls are not sufficient" first.

bors · 2023-03-12T07:31:49Z

☔ The latest upstream changes (presumably #108682) made this pull request unmergeable. Please resolve the merge conflicts.

compiler-errors · 2023-03-14T03:54:13Z

This has a conflict

Would appreciate an additional note (maybe above the one you added) that mentions that manual impls of PartialEq and Eq are not sufficient

compiler-errors · 2023-03-25T07:05:27Z

compiler/rustc_mir_build/messages.ftl

@@ -315,7 +315,7 @@ mir_build_moved = value is moved into `{$name}` here
 mir_build_union_pattern = cannot use unions in constant patterns

 mir_build_type_not_structural =
-     to use a constant of type `{$non_sm_ty}` in a pattern, `{$non_sm_ty}` must be annotated with `#[derive(PartialEq, Eq)]`
+     to use a constant of type `{$non_sm_ty}` in a pattern, `{$non_sm_ty}` must be annotated with `#[derive(PartialEq, Eq)]`. the traits must be derived, manual `impl`s are not sufficient


please make this into a note, not the primary message. same below.

also, why did you take away the link to Structural{Partial}Eq? That was fine, as long as it was paired with an additional note that explains what those traits means in practice.

estebank · 2023-03-25T18:05:04Z

Can we also make sure that there's a test exercising this message?

jamen · 2023-03-26T23:58:00Z

I see tests in tests/ui/consts/const_in_pattern but they're unaffected by adding notes (they are affected when changing the main message). How do I go about that?

compiler-errors · 2023-03-26T23:59:02Z

You need to give the second note another name and declare it in the diagnostic struct -- the first is being overwritten. Fluent file slug names are not deduplicated.

rustbot · 2023-03-27T15:08:48Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

jamen · 2023-03-27T20:39:31Z

src/tools/clippy/tests/ui/crashes/ice-6254.stderr

@@ -6,6 +6,8 @@ LL |         FOO_REF_REF => {},
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #62411 <https://github.com/rust-lang/rust/issues/62411>
+   = note: the traits must be derived, manual `impl`s are not sufficient
+   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralEq.html for details
   = note: `-D indirect-structural-match` implied by `-D warnings`


unsure why this appears in the middle of these other notes

I don't think it really matters.

compiler-errors · 2023-03-27T23:36:31Z

@bors r+ rollup

bors · 2023-03-27T23:36:34Z

📌 Commit 73c34cb has been approved by compiler-errors

It is now in the queue for this repository.

…iaskrgr Rollup of 9 pull requests Successful merges: - rust-lang#108548 (Clarify the 'use a constant in a pattern' error message) - rust-lang#109565 (Improve documentation for E0223) - rust-lang#109661 (Fix LVI test post LLVM 16 update) - rust-lang#109667 (Always set `RUSTC_BOOTSTRAP` with `x doc`) - rust-lang#109669 (Update books) - rust-lang#109678 (Don't shadow the `dep_node` var in `incremental_verify_ich_failed`) - rust-lang#109682 (Add `#[inline]` to CStr trait implementations) - rust-lang#109685 (Make doc comment a little bit more accurate) - rust-lang#109687 (Document the heuristics IsTerminal uses on Windows) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

Clarify the 'use a constant in a pattern' error message ```rs use std::borrow::Cow; const ERROR_CODE: Cow<'_, str> = Cow::Borrowed("23505"); fn main() { let x = Cow::from("23505"); match x { ERROR_CODE => {} } } ``` ``` error: to use a constant of type `Cow` in a pattern, `Cow` must be annotated with `#[derive(PartialEq, Eq)]` --> src/main.rs:9:9 | 9 | ERROR_CODE => {} | ^^^^^^^^^^ error: could not compile `playground` due to previous error ``` It seems helpful to link to StructuralEq in this message. I was a little confused, because `Cow<'_, str>` implements PartialEq and Eq, but they're not derived, which I learned is necessary for structural equality and using constants in patterns (thanks to the Rust community Discord server) For tests, should I update every occurrence of this message? I see tests where this is still a warning and I'm not sure if I should update those.

rustbot assigned petrochenkov Feb 28, 2023

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 Feb 28, 2023

jamen marked this pull request as draft February 28, 2023 00:48

petrochenkov marked this pull request as ready for review February 28, 2023 10:02

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 28, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 1, 2023

rustbot assigned compiler-errors and unassigned petrochenkov Mar 1, 2023

compiler-errors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 14, 2023

compiler-errors reviewed Mar 25, 2023

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Add notes to non-structural const in pattern error message

73c34cb

jamen commented Mar 27, 2023

View reviewed changes

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 27, 2023

matthiaskrgr mentioned this pull request Mar 28, 2023

Rollup of 9 pull requests #109690

Merged

bors merged commit a7c07cf into rust-lang:master Mar 28, 2023

rustbot added this to the 1.70.0 milestone Mar 28, 2023

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Clarify the 'use a constant in a pattern' error message #108548

Clarify the 'use a constant in a pattern' error message #108548

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Clarify the 'use a constant in a pattern' error message #108548

Clarify the 'use a constant in a pattern' error message #108548

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