unused variable: warn about std::collections that are pushed to but not used afterwards

[matthiaskrgr]

fn main() {
        let mut var: Vec<i64> = Vec::new();
        var.push(1);
}

I would expect a warning here that var is unused.
I get that it is "used" in the sense that we push to it, however the resulting vector is still not done anything with.

Some kind of dead-store warning about the collection being unused/never queried after receiving an item might be helpful.

[matthiaskrgr]

Just for the record, if I rewrite the code as follows

fn main() {
    let var: Vec<i64> = vec![1];
}

I get a warning

warning: unused variable: `var`
 --> src/main.rs:2:9
  |
2 |     let var: Vec<i64> = vec![1];
  |         ^^^ help: consider using `_var` instead
  |
  = note: #[warn(unused_variables)] on by default

[durka]

I don't see any way to do this in a generalizable way.

let mut var = Missile::new();
var.launch();

Yeah, var is not consumed, but...

[matthiaskrgr]

Hmm, I mostly had the std::collections in mind, which we might be able to savely reason about.

[abonander]

What about an attribute for methods like #[does_not_use] that tells the unused-variables lint that those method calls shouldn't mark a value used. That could cause false-positives like here:

fn add_something(vals: &mut Vec<i32>) {
    vals.push(1); // WARN unused variable: `vals`
}
  
fn main() {
    let mut vals = Vec::new();
    add_something(vals);
    println!("{:?}", vals);
}

But we could have an exception for mutable references (though I'm not sure if that's easy to deduce during the linting pass), or just make it really conservative (only lint against local variable bindings)

[oli-obk]

This feels very much like something that clippy can solve nicely for now by blacklisting certain types and methods