warning: value assigned to x is never read not fixed by cargo fix

[jrmuizel]

I tried this code:

fn main() {
    let mut x = 0;
    x = 8;
    println!("Hello, world! {}", x);
}

which gives:

warning: value assigned to `x` is never read
 --> src/main.rs:2:13
  |
2 |     let mut x = 0;
  |             ^
  |
  = note: `#[warn(unused_assignments)]` on by default
  = help: maybe it is overwritten before being read?

It would be nice if this warning was fixable with cargo fix

[newca12]

https://doc.rust-lang.org/cargo/commands/cargo-fix.html
If you encounter any problems with cargo fix or otherwise have any questions or feature requests please don’t hesitate to file an issue at https://github.com/rust-lang/cargo

[jrmuizel]

I filed this here because I don't think the problem is in cargo fix. I think rustc needs to suggest a fix and then cargo fix will just work.

[jyn514]

Relevant code:

rust/compiler/rustc_passes/src/liveness.rs

Lines 1774 to 1793 in 3e14b68

	fn report_unsed_assign(
	&self,
	hir_id: HirId,
	spans: Vec<Span>,
	var: Variable,
	message: impl Fn(&str) -> String,
	) {
	if let Some(name) = self.should_warn(var) {
	self.ir.tcx.struct_span_lint_hir(
	lint::builtin::UNUSED_ASSIGNMENTS,
	hir_id,
	spans,
	|lint| {
	lint.build(&message(&name))
	.help("maybe it is overwritten before being read?")
	.emit();
	},
	)
	}
	}

Seems like the hard part will be coming up with a suggestion, maybe just to remove the assignment?

[lcnr]

We can't just remove the assignment here, see

fn weird<T>(v: T) -> T {
    println!("hey");
    v
}

fn main() {
    let mut x = weird(0);
    x = 8;
}

We could maybe recommend

let mut x; wierd(0); 

here, which should result in followup warnings, not sure if that's too great though

Or we only add the suggestion if the input is known to not have sideeffects, which is probably better 🤔