rust/compiler/rustc_ast
Jonas Schievink f66af28641
Rollup merge of #79016 - fanzier:underscore-expressions, r=petrochenkov
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: #71126). This PR is the third and final part of #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````
2020-11-15 13:39:48 +01:00
..
src Rollup merge of #79016 - fanzier:underscore-expressions, r=petrochenkov 2020-11-15 13:39:48 +01:00
Cargo.toml mv compiler to compiler/ 2020-08-30 18:45:07 +03:00
README.md mv compiler to compiler/ 2020-08-30 18:45:07 +03:00

The rustc_ast crate contains those things concerned purely with syntax that is, the AST ("abstract syntax tree"), parser, pretty-printer, lexer, macro expander, and utilities for traversing ASTs.

For more information about how these things work in rustc, see the rustc dev guide: