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Merge nested matches #1252

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mcarton opened this issue Oct 1, 2016 · 2 comments · Fixed by #6402
Closed

Merge nested matches #1252

mcarton opened this issue Oct 1, 2016 · 2 comments · Fixed by #6402
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A-lint Area: New lints E-medium Call for participation: Medium difficulty level problem and requires some initial experience. T-AST Type: Requires working with the AST

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@mcarton
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mcarton commented Oct 1, 2016 •
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We should have a lint that suggest to change

match foo {
    Ok(row) => match row {
        Some(pixels) => pixels,
        None => return Err(…),
    },
    Err(err) => return Err(…),
};

into

match foo {
    Ok(Some(pixels)) => pixels,
    Ok(None) => return Err(…),
    Err(...) => return Err(…),
}

Copied from https://github.com/Manishearth/rust-clippy/issues/1245#issuecomment-250666810.
@mcarton mcarton added E-medium Call for participation: Medium difficulty level problem and requires some initial experience. T-AST Type: Requires working with the AST A-lint Area: New lints labels Oct 1, 2016
@oli-obk
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oli-obk commented Oct 1, 2016 •
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5 seconds!

@mcarton
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mcarton commented Oct 1, 2016

Héhé 😄

@camsteffen camsteffen mentioned this issue Nov 29, 2020
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bors added a commit that referenced this issue Nov 30, 2020
@bors
Auto merge of #6402 - camsteffen:collapsible-match, r=llogiq
6d847c4 
Add Collapsible match lint

changelog: Add collapsible_match lint

Closes #1252
Closes #2521

This lint finds nested `match` or `if let` patterns that can be squashed together. It is designed to be very conservative to only find cases where merging the patterns would most likely reduce cognitive complexity.

Example:

```rust
match result {
    Ok(opt) => match opt {
        Some(x) => x,
        _ => return,
    }
    _ => return,
}
```
to
```rust
match result {
    Ok(Some(x)) => x,
    _ => return,
}
```

These criteria must be met for the lint to fire:

* The inner match has exactly 2 branches.
* Both the outer and inner match have a "wild" branch like `_ => ..`. There is a special case for `None => ..` to also be considered "wild-like".
* The contents of the wild branches are identical.
* The binding which "links" the matches is never used elsewhere.

Thanks to the hir, `if let`'s are easily included with this lint since they are desugared into equivalent `match`'es.

I think this would fit into the style category, but I would also understand changing it to pedantic.
@bors bors closed this as completed in e2ecc4a Dec 3, 2020
@bors bors closed this as completed in #6402 Dec 3, 2020
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A-lint Area: New lints E-medium Call for participation: Medium difficulty level problem and requires some initial experience. T-AST Type: Requires working with the AST
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Add Collapsible match lint camsteffen/rust-clippy
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@oli-obk @mcarton