Regression: false negative lint dead_code: dead code is not detected for struct if it implements FromStr

[safinaskar]

Code

I tried this code:

#![deny(dead_code)]

impl std::str::FromStr for Foo {
    type Err = ();
    fn from_str(_s: &str) -> Result<Self, Self::Err> {
        panic!();
    }
}

struct Foo;

fn main() {}

I expected to see this happen: compiler should report that struct Foo is never constructed

Instead, this happened: compiler didn't report this

Version it worked on

It most recently worked on: nightly-2019-08-27

Version with regression

nightly-2019-08-28

Notes

cargo-bisect-rustc reports that the following commits happened between these two versions:

get_commits_between returning commits, len: 5
commit[0] 2019-08-26: Auto merge of #63901 - estebank:unknown-receiver-type, r=zackmdavis
commit[1] 2019-08-27: Auto merge of #63926 - Centril:rollup-6kckn9n, r=Centril
commit[2] 2019-08-27: Auto merge of #63940 - Centril:rollup-47qe9gn, r=Centril
commit[3] 2019-08-27: Auto merge of #63639 - Mark-Simulacrum:rustdoc-clean-3, r=GuillaumeGomez
commit[4] 2019-08-27: Auto merge of #63922 - RalfJung:miri, r=nikomatsakis

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged +A-lints +L-dead_code +L-false-negative

[fmease]

#63317

[safinaskar]

I still believe current behavior is bug. Struct Foo is clearly dead code in my example

[workingjubilee]

This only happens in-practice with fieldless types, it seems? This is rejected:

-struct Foo;
+struct Foo(i32);

As is this:

-struct Foo;
+struct Foo {
+    a: i32,
+}

Mind, this passes, of course:

-struct Foo;
+struct Foo {
+    _a: i32,
+}

[safinaskar]

@workingjubilee , unfortunately, bug is still reproducible with a: i32. You just need to add yet another impl:

impl std::str::FromStr for Foo {
    type Err = ();
    fn from_str(_s: &str) -> Result<Self, Self::Err> {
        panic!();
    }
}

impl std::fmt::Debug for Foo {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
        write!(f, "{}", self.a)
    }
}

struct Foo {
    a: i32,
}

fn main() {}

(Reproduced on current nightly)