False positive "field is never read" warnings

Here's a reduced test-case from Firefox source code:

struct Data {
    pub field: bool,
}

struct Foo<'a> {
    data: &'a mut Data,
}

impl<'a> Foo<'a> {
    fn parse_data(&mut self) {
        macro_rules! parse {
            () => {
                self.data.field = true;
            }
        }
        parse!()
    }
}

fn main() {
    let mut data = Data { field: false };
    {
        let mut foo = Foo { data: &mut data };
        foo.parse_data();
    }
    println!("{}", data.field);
}

Compiling that program warns with:

warning: field is never read: `data`
 --> t.rs:6:5
  |
6 |     data: &'a mut Data,
  |     ^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

Which is obviously not true.

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rustc --version --verbose:

rustc 1.51.0-nightly (04caa632d 2021-01-30)
binary: rustc
commit-hash: 04caa632dd10c2bf64b69524c7f9c4c30a436877
commit-date: 2021-01-30
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly
LLVM version: 11.0.1

Seems like this is a regression from d3c69a4.

@sanxiyn

cc @sanxiyn

Oops, sorry! Technically, Foo.data field is, in fact, never read. What went wrong is Foo.data is written and read through other ways.

I think restricting the lint to non-reference fields should work, and will submit a PR to do so quickly. Please double check whether my thinking is correct.

Ah, yeah, I somehow thought that the macro was needed to trigger the bug but it is not. That sounds good. Writes to mutable references have side effects so doing that seems ok.

Would that also cover this case here, in the idna crate? (The Mapper::errors field is a &mut reference to an Errors type, which is in fact returned to the caller of processing().)

https://github.com/servo/rust-url/pull/674/checks?check_run_id=1855869353

Yeah, I think it's the same.

Rollup merge of rust-lang#82297 - tmiasko:write-only, r=oli-obk

Ah, this was closed, but the bug leaked into beta (as mentioned in #81658) while the fix is only in nightly.

Reopening and tagging as stable-to-beta regression

As noted on #81473 (comment) , we're going to resolve the beta issue by reverting PR #81473 on beta (which I have assigned myself to take care of).

Auto merge of rust-lang#83171 - pnkfelix:revert-81473-warn-write-only…

This was a regression affecting beta 1.51, closing this.

Rollup merge of rust-lang#145214 - notJoon:fix/enable-self-assignment…

Unrolled build for #145214

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False positive "field is never read" warnings #81626

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False positive "field is never read" warnings #81626

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emilio commented on Feb 1, 2021

emilio commented on Feb 1, 2021

sanxiyn commented on Feb 2, 2021

emilio commented on Feb 2, 2021

djc commented on Feb 8, 2021

emilio commented on Feb 8, 2021

pnkfelix commented on Mar 10, 2021

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pnkfelix commented on Mar 15, 2021

emilyalbini commented on Apr 30, 2021

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