diagnostics: dead_code false negative on never constructed enum variant

[matthiaskrgr]

use std::fmt;

trait CargoCacheError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result;
}

impl fmt::Display for dyn CargoCacheError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.fmt(f)
    }
}

#[derive(Debug, Eq, PartialEq)]
pub(crate) enum CacheError<'a> {
    SomeCache(&'a str), // WARNING HERE
    NoCache, // BUT NOT HERE
}

impl fmt::Display for CacheError<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match &self {
            Self::NoCache => write!(f, ""),
            Self::SomeCache(string) => write!(f, "oh no, {}", string),
        }
    }
}

pub fn main() {}

playground

I expected to see this happen:
There should be a warning that CacheError::SomeCache as well as CacheError::NoCache are never constructed.

Instead, this happened:
Instead, there only was a warning about dead code for CacheError::NoCache

warning: variant is never constructed: `SomeCache`
  --> src/main.rs:27:5
   |
27 |     SomeCache(&'a str), // WARNING HERE
   |     ^^^^^^^^^^^^^^^^^^

rustc --version --verbose:

rustc 1.48.0-nightly (6af1bdda5 2020-09-15)
binary: rustc
commit-hash: 6af1bdda54abc9e919fc1137411dfc4311e05649
commit-date: 2020-09-15
host: x86_64-unknown-linux-gnu
release: 1.48.0-nightly
LLVM version: 11.0