Incorrect let_underscore_drop warning on type without Drop implementation

[Elrendio]

I tried this code:

#![deny(let_underscore_drop)]

fn main() {
    let _ = MyStruct {
        field: vec![1, 2, 3],
    };
}

struct MyStruct {
    field: Vec<usize>,
}

I got this warning:

error: non-binding let on a type that implements `Drop`
 --> src/main.rs:4:5
  |
4 | /     let _ = MyStruct {
5 | |         field: vec![1, 2, 3],
6 | |     };
  | |______^
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![deny(let_underscore_drop)]
  |         ^^^^^^^^^^^^^^^^^^^
help: consider binding to an unused variable to avoid immediately dropping the value
  |
4 |     let _unused = MyStruct {
  |         ~~~~~~~
help: consider immediately dropping the value
  |
4 ~     drop(MyStruct {
5 |         field: vec![1, 2, 3],
6 ~     });
  |

However, there is no Drop impl on the type, so the error message is misleading.
It is true however that there is a destructor, as defined here: https://doc.rust-lang.org/stable/std/ops/trait.Drop.html

That being said, it's insiginificant for all underlying types (only #[may_dangle] destructor of Vec and Copy type in the Vec), so ideally the warning still wouldn't even show.

Meta

rustc --version --verbose:

rustc 1.80.1 (3f5fd8dd4 2024-08-06)
binary: rustc
commit-hash: 3f5fd8dd41153bc5fdca9427e9e05be2c767ba23
commit-date: 2024-08-06
host: x86_64-unknown-linux-gnu
release: 1.80.1
LLVM version: 18.1.7

Backtrace

   Compiling MinimalReproNonBindingLetDrop v0.1.0 (/home/elrendio/Stockly/MinimalReproNonBindingLetDrop)
warning: field `field` is never read
  --> src/main.rs:10:5
   |
9  | struct MyStruct {
   |        -------- field in this struct
10 |     field: Vec<usize>,
   |     ^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

error: non-binding let on a type that implements `Drop`
 --> src/main.rs:4:5
  |
4 | /     let _ = MyStruct {
5 | |         field: vec![1, 2, 3],
6 | |     };
  | |______^
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![deny(let_underscore_drop)]
  |         ^^^^^^^^^^^^^^^^^^^
help: consider binding to an unused variable to avoid immediately dropping the value
  |
4 |     let _unused = MyStruct {
  |         ~~~~~~~
help: consider immediately dropping the value
  |
4 ~     drop(MyStruct {
5 |         field: vec![1, 2, 3],
6 ~     });
  |

warning: `MinimalReproNonBindingLetDrop` (bin "MinimalReproNonBindingLetDrop") generated 1 warning
error: could not compile `MinimalReproNonBindingLetDrop` (bin "MinimalReproNonBindingLetDrop") due to 1 previous error; 1 warning emitted

[compiler-errors]

Well, even if the T parameter of Vec<T> is #[may_dangle], it still has a side-effect (deallocates memory). I don't think we need this special-casing. I do think the diagnostic is a bit misleading and should just say "has a destructor" or "has side-effects when dropped".

Is there a more practical code example for why you'd want to suppress this error? The example you gave is a bit artificial, so it's a hard to motivate why we'd want to suppress this code since it really could be rewritten to be clearer.

[makai410]

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