mirror of
https://github.com/rust-lang/rust.git
synced 2025-11-17 11:36:26 +00:00
b12bb2530b
Implement asymmetrical precedence for closures and jumps I have been through a series of asymmetrical precedence designs in Syn, and finally have one that I like and is worth backporting into rustc. It is based on just 2 bits of state: `next_operator_can_begin_expr` and `next_operator_can_continue_expr`. Asymmetrical precedence is the thing that enables `(return 1) + 1` to require parentheses while `1 + return 1` does not, despite `+` always having stronger precedence than `return` [according to the Rust Reference](https://doc.rust-lang.org/1.83.0/reference/expressions.html#expression-precedence). This is facilitated by `next_operator_can_continue_expr`. Relatedly, it is the thing that enables `(return) - 1` to require parentheses while `return + 1` does not, despite `+` and `-` having exactly the same precedence. This is facilitated by `next_operator_can_begin_expr`. **Example:** ```rust macro_rules! repro { ($e:expr) => { $e - $e; $e + $e; }; } fn main() { repro!{return} repro!{return 1} } ``` `-Zunpretty=expanded` **Before:** ```console fn main() { (return) - (return); (return) + (return); (return 1) - (return 1); (return 1) + (return 1); } ``` **After:** ```console fn main() { (return) - return; return + return; (return 1) - return 1; (return 1) + return 1; } ```
The rustc_ast crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), along with some definitions for tokens and token streams, data structures/traits for mutating ASTs, and shared definitions for other AST-related parts of the compiler (like the lexer and macro-expansion).
For more information about how these things work in rustc, see the rustc dev guide: