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Rollup merge of rust-lang#79016 - fanzier:underscore-expressions, r=p…
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…etrochenkov

Make `_` an expression, to discard values in destructuring assignments

This is the third and final step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: rust-lang#71126). This PR is the third and final part of rust-lang#71156, which was split up to allow for easier review.

With this PR, an underscore `_` is parsed as an expression but is allowed *only* on the left-hand side of a destructuring assignment. There it simply discards a value, similarly to the wildcard `_` in patterns. For instance,
```rust
(a, _) = (1, 2)
```
will simply assign 1 to `a` and discard the 2. Note that for consistency,
```
_ = foo
```
is also allowed and equivalent to just `foo`.

Thanks to ````@varkor```` who helped with the implementation, particularly around pre-expansion gating.

r? ````@petrochenkov````
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jonas-schievink authored Nov 15, 2020
2 parents 0c7a48c + 864e554 commit 27a1572
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1 change: 1 addition & 0 deletions clippy_lints/src/utils/sugg.rs
Original file line number Diff line number Diff line change
Expand Up @@ -170,6 +170,7 @@ impl<'a> Sugg<'a> {
| ast::ExprKind::MacCall(..)
| ast::ExprKind::MethodCall(..)
| ast::ExprKind::Paren(..)
| ast::ExprKind::Underscore
| ast::ExprKind::Path(..)
| ast::ExprKind::Repeat(..)
| ast::ExprKind::Ret(..)
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