no_effect_underscore_binding fires when ignoring a generic argument

[lopopolo]

Summary

When accepting a T: IntoIterator argument, no_effect_underscore_binding requires .into_iter() be called on the argument when assigning to an _ prefixed variable.

Lint Name

no_effect_underscore_binding

Reproducer

I tried this code:

#![warn(clippy::all)]
#![warn(clippy::pedantic)]

/// # Errors
/// not implemented
pub fn iter_good<T>(args: T) -> Result<String, &'static str>
where
    T: IntoIterator<Item = String>,
{
    let _ignored_while_unimplemented = args.into_iter();
    Err("good")
}

/// # Errors
/// not implemented
pub fn iter_bad<T>(args: T) -> Result<String, &'static str>
where
    T: IntoIterator<Item = String>,
{
    let _ignored_while_unimplemented = args;
    Err("bad")
}

I saw this happen:

    Checking playground v0.0.1 (/playground)
warning: binding to `_` prefixed variable with no side-effect
  --> src/lib.rs:20:5
   |
20 |     let _ignored_while_unimplemented = args;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> src/lib.rs:2:9
   |
2  | #![warn(clippy::pedantic)]
   |         ^^^^^^^^^^^^^^^^
   = note: `#[warn(clippy::no_effect_underscore_binding)]` implied by `#[warn(clippy::pedantic)]`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#no_effect_underscore_binding

warning: `playground` (lib) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.57s

I expected to see this happen: no warnings

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3ffd5b85bf0c41df91d23f7eb9e1eea9

Version

playground rust 1.58.0

Clippy 0.1.60 (2022-01-15 ec4bcaa)

Additional Labels

No response

[oxalica]

The lint also falsely triggers when the binding actually has effects on drop ordering, like to move a generic guard into an async block.

#![warn(clippy::pedantic)]

fn work(_: impl std::future::Future<Output = ()>) { todo!() }

pub fn foo<T>(guard: T) {
    let fut = async move {
        let _guard = guard;
        // work...
    };
    work(fut);
}