Stylistic warnings (as errors) in the compiler break the build

[heaths]

After pulling a new nightly version, we were surprised to see a stylistic lint caught by the compiler that, because we pass -Dwarnings, failed the build. It seems that purely stylistic lints should be done only by clippy (or even check) but not by the compiler, as extra parens is completely valid syntax and, sometimes, used by people to write more maintainable code e.g., grouping expressions while disregarding operator precedence just to future contributes are less confused.

Again, totally reasonable as a lint but seems excessive as a compiler warning.

Code

I tried this code:

let mut error = error.get_ref()? as &(dyn std::error::Error);

I expected to see this happen: nothing should happen. While unnecessary, they also don't cause any issue and are syntactically correct. I could add any number of parens and it wouldn't change the meaning.

Instead, this happened: unused_parens warning-as-error.

We just had to get rid of the parens:

- let mut error = error.get_ref()? as &(dyn std::error::Error);
+ let mut error = error.get_ref()? as &dyn std::error::Error;

Both are syntactically correct so this seems purely stylistic and not something we'd expect in the compiler.

Version it worked on

It most recently worked on a version of nightly a few weeks older. Sorry, I don't have which version it was handy but this issue is more about not making stylistic lints a warning in the compiler.

Version with regression

rustc 1.90.0-nightly (ab68b0fb2 2025-07-08)

[matthiaskrgr]

maybe same as #143653

[heaths]

Thanks. I saw that before I opened this and maybe it is a dup - or at least worth tracking as one - but this was more about putting stylistic lints in the compiler. But if you feel maybe I should just add this opinion on a comment and close this one, I'm happy to.

[compiler-errors]

This isn't really related to #143653 since it's not a false positive.

This is expected behavior under -Dwarnings. I wanted to note that -Dwarnings is a very heavy hammer and you're pretty much accepting breakage like this when you deny warnings when we tweak the scope of lints, which happens often (usually because the compiler wasn't smart enough to detect situations like the interaction btw parens and unary type operators like & in the past).

I personally don't think Rust needs to distinguish lint categories like this; if you want to add -A... for all the stylistic lints you hit regularly, you're welcome to do so.