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parser diagnostics: Suggest adding a body for a match arm followed by a comma - #160787
jakubadamw wants to merge 4 commits into
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The parser was modified, potentially altering the grammar of (stable) Rust cc @fmease |
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rustbot has assigned @dingxiangfei2009. Use Why was this reviewer chosen?The reviewer was selected based on:
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| // Neither parenthesising the patterns nor joining them with `|` would give the arm a | ||
| // body, so the comma is far more likely to stand in for a missing `=> <body>`. | ||
| self.restore_snapshot(snapshot); | ||
| return Ok(()); |
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the return here make it miss adding the parentheses suggestion and also introduced some other noises.
for code:
fn main() {
let pair = (0, 1);
match pair {
0, 1 = {}
}
}nightly output:
error: unexpected `,` in pattern
--> src/main.rs:4:10
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4 | 0, 1 = {}
| ^
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help: try adding parentheses to match on a tuple...
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4 | (0, 1) = {}
| + +
help: ...or a vertical bar to match on alternatives
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4 - 0, 1 = {}
4 + 0 | 1 = {}
|while with this PR will output:
error: expected one of `...`, `..=`, `..`, `=>`, `if`, or `|`, found `=`
--> /tmp/pr160787-almost-arrow.rs:4:14
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4 | 0, 1 = {}
| ^ expected one of `...`, `..=`, `..`, `=>`, `if`, or `|`
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help: use a fat arrow to start a match arm
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4 | 0, 1 => {}
| +
error: `match` arm with no body
--> /tmp/pr160787-almost-arrow.rs:4:9
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4 | 0, 1 = {}
| ^
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help: add a body after the pattern
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4 | 0 => { todo!() }, 1 = {}
| ++++++++++++++
error[E0308]: mismatched types
--> /tmp/pr160787-almost-arrow.rs:4:9
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3 | match pair {
| ---- this expression has type `({integer}, {integer})`
4 | 0, 1 = {}
| ^ expected `({integer}, {integer})`, found integer
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= note: expected tuple `({integer}, {integer})`
found type `{integer}`
error[E0308]: mismatched types
--> /tmp/pr160787-almost-arrow.rs:4:12
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3 | match pair {
| ---- this expression has type `({integer}, {integer})`
4 | 0, 1 = {}
| ^ expected `({integer}, {integer})`, found integer
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= note: expected tuple `({integer}, {integer})`
found type `{integer}`
error: aborting due to 4 previous errorsseems a regression for this case.
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@chenyukang, ah, interesting, good catch… Addressed it in another commit. Thanks!
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| help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown | ||
| | | ||
| LL ~ None => {}, | ||
| LL + Some(_) => todo!() |
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Maybe Some(_) => { todo!() },?
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Well, I tried making this change and it affected results for 95 tests. So it seems rather out of scope, honestly, as the current behaviour is to produce a comma-less suggestion?
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@ermakov0, apologies, I realise I never thanked you for your review! This is ready for re-review if you had a moment. |
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Only suggest a tuple or an or-pattern when the comma-separated patterns are followed by a `=>` or a guard. Otherwise the arm is just missing a body, which `parse_arm` already reports.
A `match` arm pattern can also be followed by a token that is a typo for `=>`, such as `=` or `->`, which `parse_arm` recovrs from. Treat those like a `=>` so that the comma is not taken for a missing body.
Avoid the extra blank lines. Make the `cfg`-gated `match` in the never-pattern test exhaustive too, so that the test keeps reporting just the parse errors it is about.
Suggest `_ => { todo!() }` rather than `_ => todo!()`, matching the body the
parser suggests for a match arm that has none. A block body needs no trailing
comma, so the arm that fills in an empty match body no longer gets one either.
Claude-Session: https://claude.ai/code/session_01FmLL3kw6eDHEs6soUo7sxz
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Some changes occurred in match checking cc @Nadrieril |
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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. |
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parser diagnostics: only suggest a tuple or an or-pattern when the comma-separated patterns are followed by a
=>or a guard. Otherwise the arm is just missing a body, which will then be reported byparse_arm.No new tests, because existing tests cover this, I’ve extended with one more specific example and updated their expected results.
Closes #159686.