"variable does not need to be mutable" not detected

(sorry if this is an duplicate or the wrong repo 😇 )

fn main() {
    let mut name;
    name = format!("foo{}", "bar");
    dbg!(name);
}

(Playground)

Current behavior:
no warning

Expected behavior:

warning: variable does not need to be mutable
 --> src/main.rs:2:9
  |
2 |      let mut name;
  |          ----^
  |          |
  |          help: remove this `mut`
  |
  = note: #[warn(unused_mut)] on by default

The variable clearly doesn't need to be mutable, still, nothing is detected and no warning is emitted.

Rust 1.35.0
(Using the playground, also in 1.37.0-nightly (2019-05-31 7840a0b753a065a41999))

[czipperz]

The warning appears again if you switch to 2015 edition. Huh. Nice catch!

[czipperz]

I found that Stable 2015 edition produces the warning and everything else doesn't.

| Warning | Optimization | Channel | Edition |
|---------+--------------+---------+---------|
| Present | Debug        | Stable  |    2015 |
| Bug!    | Debug        | Beta    |    2015 |
| Bug!    | Debug        | Nightly |    2015 |
| Present | Release      | Stable  |    2015 |
| Bug!    | Release      | Beta    |    2015 |
| Bug!    | Release      | Nightly |    2015 |
| Bug!    | Debug        | Stable  |    2018 |
| Bug!    | Debug        | Beta    |    2018 |
| Bug!    | Debug        | Nightly |    2018 |
| Bug!    | Release      | Stable  |    2018 |
| Bug!    | Release      | Beta    |    2018 |
| Bug!    | Release      | Nightly |    2018 |

[jonas-schievink]

cc @matthewjasper (likely caused by NLL migrate mode, which was enabled in #59114 for the 2015 edition)

[matthewjasper]

Yes, looks like another nll bug. If someone wants to have a go at this, the issue is that we are treating DropAndReplace differently to Assign for the purposes of this lint.

[czipperz]

I can try doing it I just don't know here to begin.

[jonas-schievink]

@czipperz I don't know this code too well, but take a look at src/librustc_mir/borrow_check/used_muts.rs. The first thing I'd try is adding a case for DropAndReplace to the match in visit_statement that mirrors what's done for Assign.

[czipperz]

Makes sense thank you. I was looking at librustc_lint and couldn't find anything.