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Fix, document, and test parser and pretty-printer edge cases related to braced macro calls _Review note: this is a deceptively small PR because it comes with 145 lines of docs and 196 lines of tests, and only 25 lines of compiler code changed. However, I recommend reviewing it 1 commit at a time because much of the effect of the code changes is non-local i.e. affecting code that is not visible in the final state of the PR. I have paid attention that reviewing the PR one commit at a time is as easy as I can make it. All of the code you need to know about is touched in those commits, even if some of those changes disappear by the end of the stack._ This is a follow-up to https://github.com/rust-lang/rust/pull/119105. One case that is not relevant to `-Zunpretty=expanded`, but which came up as I'm porting #119105 and #118726 into `syn`'s printer and `prettyplease`'s printer where it **is** relevant, and is also relevant to rustc's `stringify!`, is statement boundaries in the vicinity of braced macro calls. Rustc's AST pretty-printer produces invalid syntax for statements that begin with a braced macro call: ```rust macro_rules! stringify_item { ($i:item) => { stringify!($i) }; } macro_rules! repro { ($e:expr) => { stringify_item!(fn main() { $e + 1; }) }; } fn main() { println!("{}", repro!(m! {})); } ``` **Before this PR:** output is not valid Rust syntax. ```console fn main() { m! {} + 1; } ``` ```console error: leading `+` is not supported --> <anon>:1:19 | 1 | fn main() { m! {} + 1; } | ^ unexpected `+` | help: try removing the `+` | 1 - fn main() { m! {} + 1; } 1 + fn main() { m! {} 1; } | ``` **After this PR:** valid syntax. ```console fn main() { (m! {}) + 1; } ``` |
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