False dead_code warning on struct pattern match

[nvzqz]

The following code will emit a dead_code warning (playground):

use std::mem;

fn main() {
    struct Value {
        a: i32,
        b: i32,
    };

    let Value { a, b } = unsafe { mem::zeroed() };
    println!("{} {}", a, b);
}

It states that Value is never constructed when in fact it has been within the let Value { a, b } pattern match.

   Compiling playground v0.0.1 (/playground)
warning: struct is never constructed: `Value`
 --> src/main.rs:4:5
  |
4 |     struct Value {
  |     ^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.90s
     Running `target/debug/playground`

[zackmdavis]

@nvzqz Thanks for the report! We recently changed the wording to say "never constructed" rather than "never used" in the hopes of clarifying that the liveness analysis doesn't consider destructuring patterns as "live," but the existence of this Issue would seem to suggest that this still isn't clear enough. 🙁

Part of me was tempted to call this #wontfix (as it wasn't obvious to me why it's useful to have a struct pattern without the struct being constructed anywhere else—what would realistically be on the other side of the match/assignment?), but your linked commit being a real-world counterexample makes me more sympathetic to changing the analysis to consider patterns "live." (As you point out, unsafe bytes can be on the other side of the match, and you might want to have a struct to name them.)

[nvzqz]

I noticed the warning for Value not being constructed but in my reasoning, it actually is when returned from mem::zeroed() and then immediately deconstructed in the pattern. I know that's a very implicit way of viewing it. I still think that code like this that's correct shouldn't emit warnings despite the warning being generated in unsafe code.

Thanks for taking my real-world case in that commit into account!

[danielhenrymantilla]

Using a struct only for it to be destructured right away does indeed have its use cases, see also: https://github.com/danielhenrymantilla/fstrings-rs/blob/2de1a3db8d6bf6f9e33ea6eeef8468d75b329c83/src/proc_macro/mod.rs#L91-L94:

let Args {
    format_literal,
    mut extra_args,
} = parse_macro_input!(input); // Args : Parse

• My Args struct implements Parse logic, but then I just want to extract its fields right away.

• Thus my Args struct is definitely not dead_code.

[Spoonbender]

Triage: no change