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unused_assignments warning: false negative for dead stores in struct fields #49256

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matthiaskrgr opened this issue Mar 22, 2018 · 1 comment
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A-diagnostics Area: Messages for errors, warnings, and lints A-lint Area: Lints (warnings about flaws in source code) such as unused_mut. C-enhancement Category: An issue proposing an enhancement or a PR with one. L-unused_assignments Lint: unused_assignments T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@matthiaskrgr
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matthiaskrgr commented Mar 22, 2018

code like this causes a proper warning

fn main() {
	let mut a;
	let b = 4;
	let c = 5;

    	a = b;
    	a = c;

	println!("{}", a);
}

=>

warning: value assigned to `a` is never read
 --> src/main.rs:6:5
  |
6 |     a = b;
  |     ^
  |
  = note: #[warn(unused_assignments)] on by default

however if a is a struct field, I get no warning at all.

struct Foo {
    x: i32,
}

fn main() {
	let mut strct = Foo {
	    x: 0, // maybe warn here, too?
	};
	let b = 4;
	let c = 5;

  	strct.x = b; // please warn!
   	strct.x = c;

	println!("{}", strct.x);
}

playground: https://play.rust-lang.org/?gist=c904ff202754b2691e2968977620fd7a&version=nightly

I ran into bugs in my code that could have been prevented by warning about this :(

@matthiaskrgr matthiaskrgr changed the title unused_assignments: false negative for dead stores in struct fields unused_assignments warning: false negative for dead stores in struct fields Mar 22, 2018
@pietroalbini pietroalbini added C-enhancement Category: An issue proposing an enhancement or a PR with one. A-diagnostics Area: Messages for errors, warnings, and lints labels Mar 27, 2018
@Boscop
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Boscop commented Apr 13, 2018

I also ran into a bug where it was a situation like this, also no warning that a is never read:

struct Foo {
    a: i32
}

impl Foo {
    fn bar(&mut self) {
        self.a = 42;
    }
}

fn main() {
    let mut foo = Foo { a: 1 };
    foo.bar();
}

https://play.rust-lang.org/?gist=dbe81aa3431e467557fc2bf930827c52&version=stable

@jonas-schievink jonas-schievink added A-lint Area: Lints (warnings about flaws in source code) such as unused_mut. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 20, 2020
JohnTitor added a commit to JohnTitor/rust that referenced this issue Jan 30, 2021
@fmease fmease added the L-unused_assignments Lint: unused_assignments label Aug 30, 2024
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Labels
A-diagnostics Area: Messages for errors, warnings, and lints A-lint Area: Lints (warnings about flaws in source code) such as unused_mut. C-enhancement Category: An issue proposing an enhancement or a PR with one. L-unused_assignments Lint: unused_assignments T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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