mirror of https://github.com/rust-lang/rust.git
89e2160c4c
Fix parenthesization of subexprs containing statement boundary This PR fixes a multitude of false negatives and false positives in the AST pretty printer's parenthesis insertion related to statement boundaries — statements which terminate unexpectedly early if there aren't parentheses. Without this fix, the AST pretty printer (including both `stringify!` and `rustc -Zunpretty=expanded`) is prone to producing output which is not syntactically valid Rust. Invalid output is problematic because it means Rustfmt is unable to parse the output of `cargo expand`, for example, causing friction by forcing someone trying to debug a macro into reading poorly formatted code. I believe the set of bugs fixed in this PR account for the most prevalent reason that `cargo expand` produces invalid output in real-world usage. Fixes #98790. ## False negatives The following is a correct program — `cargo check` succeeds. ```rust macro_rules! m { ($e:expr) => { match () { _ => $e } }; } fn main() { m!({ 1 } - 1); } ``` But `rustc -Zunpretty=expanded main.rs` produces output that is invalid Rust syntax, because parenthesization is needed and not being done by the pretty printer. ```rust fn main() { match () { _ => { 1 } - 1, }; } ``` Piping this expanded code to rustfmt, it fails to parse. ```console error: unexpected `,` in pattern --> <stdin>:1:38 | 1 | fn main() { match () { _ => { 1 } - 1, }; } | ^ | help: try adding parentheses to match on a tuple... | 1 | fn main() { match () { _ => { 1 } (- 1,) }; } | + + help: ...or a vertical bar to match on multiple alternatives | 1 | fn main() { match () { _ => { 1 } - 1 | }; } | ~~~~~ ``` Fixed output after this PR: ```rust fn main() { match () { _ => ({ 1 }) - 1, }; } ``` ## False positives Less problematic, but worth fixing (just like #118726). ```rust fn main() { let _ = match () { _ => 1 } - 1; } ``` Output of `rustc -Zunpretty=expanded lib.rs` before this PR. There is no reason parentheses would need to be inserted there. ```rust fn main() { let _ = (match () { _ => 1, }) - 1; } ``` After this PR: ```rust fn main() { let _ = match () { _ => 1, } - 1; } ``` ## Alternatives considered In this PR I opted to parenthesize only the leading subexpression causing the statement boundary, rather than the entire statement. Example: ```rust macro_rules! m { ($e:expr) => { $e }; } fn main() { m!(loop { break [1]; }[0] - 1); } ``` This PR produces the following pretty-printed contents for fn main: ```rust (loop { break [1]; })[0] - 1; ``` A different equally correct output would be: ```rust (loop { break [1]; }[0] - 1); ``` I chose the one I did because it is the *only* approach used by handwritten code in the standard library and compiler. There are 4 places where parenthesization is being used to prevent a statement boundary, and in all 4, the developer has chosen to parenthesize the smallest subexpression rather than the whole statement: |
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