`needless_collect` false positive

Lint name: needless_collect

Reduced code:

pub struct Foo {
    foo: bool,
    bar: bool,
}

pub fn test(foo: Vec<Foo>) {
    let bar: Vec<_> = foo.iter().map(|f|f.bar).collect();
    drop(foo);

    for (_, _) in bar.into_iter().enumerate() {

    }    
}

// pub fn incorrect_suggestion(foo: Vec<Foo>) {
//     
//     drop(foo);
// 
//     for (_, _) in foo.iter().map(|f|f.bar).enumerate() {
// 
//     }    
// }

I expected no warning here because foo is dropped. The incorrect suggestion also appears if foo is shadowed, potentially generating incorrect code that compiles.

Meta

• cargo clippy -V: clippy 0.1.54 (a178d032 2021-07-26)
• rustc -Vv:

rustc 1.54.0 (a178d0322 2021-07-26)
binary: rustc
commit-hash: a178d0322ce20e33eac124758e837cbd80a6f633
commit-date: 2021-07-26
host: x86_64-pc-windows-msvc
release: 1.54.0
LLVM version: 12.0.1

Start with the needless_collect lint implementation and its existing tests, then run the reduced Rust example to reproduce the false positive and incorrect suggestion. The fix is complete when an explicit drop(foo) or shadowed foo no longer produces this warning or suggestion, with a regression test covering the example.