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Incorrect 'never used' warning for struct when only use is in pattern match #52325
Description
I ran across this implementing a one-time-use struct for a diesel query.
I incorrectly get a warning:
warning: struct is never used: `Foo`
when compiling
trait SomeTrait {}
struct Foo {
s: String
}
impl SomeTrait for Foo {}
fn main() {
let Foo { s } = get_thing();
println!("{}", s);
}
fn get_thing<T: SomeTrait>() -> T {
unimplemented!()
}
I got this is on 1.27.0 stable, and verified still exists on 1.29.0-nightly (2018-07-10). I didn't find another issue that looked like the same thing, so hopefully this isn't a duplicate :).
Reacted by David James
Activity
stearnsc commentedon Jul 12, 2018
Looks like the warning goes away if the trait impl constructs the struct, which I imagine happens almost always. In my case, the struct was deriving a trait instead of impl'ing something manually. My actual use was something more like:
csmoe commentedon Jul 12, 2018
let Foo { s } = get_thing();You're assigningswith pattern matching here, theFooisn't' used indeed.zackmdavis commentedon Jul 13, 2018
@stearnsc Thanks for the report! You are right that the issue is that the lint is firing because the struct isn't constructed even if it's used as a match pattern. The message wording was changed for unused enum variants not too long ago for exactly this reason; perhaps we should do the same for structs as well. (Unless someone objects to the consonance of "struct is never constructed"??) Pull request: #52332.
dead-code lint: say "constructed", "called" for structs, functions
Rollup merge of rust-lang#52332 - zackmdavis:dead_code_lint_should_sa…
stearnsc commentedon Jul 13, 2018
Thank's for the explanation! Is the consensus view that the warning is "correct" then, in that there ought to be a warning for this case? This use of structs doesn't seem particularly hacky or weird to me, so it feels weird that my code should require a fair number of
#[allow(dead_code)]s sprinkled throughout in order to compile without warnings.Or, maybe rephrasing a bit, should the "bug" be considered that the linter doesn't check derived code to see if a struct is constructed?
I haven't looked directly at the derived output, but I'm assuming somewhere in there my type is being explicitly constructed.
dead-code lint: say "constructed" for structs
zackmdavis commentedon Jul 22, 2018
@stearnsc I'm not aware of this having been debated enough for there to be a consensus view. The dead-code analysis lives in src/librustc/middle/dead.rs if you're curious how it works.
Rollup merge of rust-lang#52332 - zackmdavis:dead_code_lint_should_sa…
Rollup merge of rust-lang#52332 - zackmdavis:dead_code_lint_should_sa…
Auto merge of #52332 - zackmdavis:dead_code_lint_should_say_2_electri…