Fix parenthesization of chained comparisons by pretty-printer
Example:
```rust
macro_rules! repro {
() => {
1 < 2
};
}
fn main() {
let _ = repro!() == false;
}
```
Previously `-Zunpretty=expanded` would pretty-print this syntactically invalid output: `fn main() { let _ = 1 < 2 == false; }`
```console
error: comparison operators cannot be chained
--> <anon>:8:23
|
8 | fn main() { let _ = 1 < 2 == false; }
| ^ ^^
|
help: parenthesize the comparison
|
8 | fn main() { let _ = (1 < 2) == false; }
| + +
```
With the fix, it will print `fn main() { let _ = (1 < 2) == false; }`.
Making `-Zunpretty=expanded` consistently produce syntactically valid Rust output is important because that is what makes it possible for `cargo expand` to format and perform filtering on the expanded code.
## Review notes
According to `rg '\.fixity\(\)' compiler/` the `fixity` function is called only 3 places:
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| .github | ||
| compiler | ||
| library | ||
| LICENSES | ||
| src | ||
| tests | ||
| .clang-format | ||
| .editorconfig | ||
| .git-blame-ignore-revs | ||
| .gitattributes | ||
| .gitignore | ||
| .gitmodules | ||
| .ignore | ||
| .mailmap | ||
| Cargo.lock | ||
| Cargo.toml | ||
| CODE_OF_CONDUCT.md | ||
| config.example.toml | ||
| configure | ||
| CONTRIBUTING.md | ||
| COPYRIGHT | ||
| INSTALL.md | ||
| LICENSE-APACHE | ||
| license-metadata.json | ||
| LICENSE-MIT | ||
| README.md | ||
| RELEASES.md | ||
| REUSE.toml | ||
| rust-bors.toml | ||
| rustfmt.toml | ||
| triagebot.toml | ||
| x | ||
| x.ps1 | ||
| x.py | ||
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