unnecessary_lazy_evaluations: Shouldn't warn about values with significant Drop impl

[sdroege]

Summary

The case in question that was wrongly reported is the following

warning: unnecessary closure used with `bool::then`
   --> glib/src/convert.rs:122:9
    |
122 |         (iconv as isize != -1).then(|| Self(iconv))
    |         ^^^^^^^^^^^^^^^^^^^^^^^--------------------
    |                                |
    |                                help: use `then_some(..)` instead: `then_some(Self(iconv))`
    |
    = note: `#[warn(clippy::unnecessary_lazy_evaluations)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_lazy_evaluations

Self here has a significant Drop impl, specifically it will close the isize there (it's basically like a fd and closing -1 is UB).

Lint Name

unnecessary_lazy_evaluations

Reproducer

I tried this code:

struct Foo(i32);

impl Drop for Foo {
    fn drop(&mut self) {
        println!("{}", self.0);
    }
}

fn main() {
    (0 == 1).then(|| Foo(0));
}

I saw this happen:

warning: unnecessary closure used with `bool::then`
  --> src/main.rs:10:5
   |
10 |     (0 == 1).then(|| Foo(0));
   |     ^^^^^^^^^---------------
   |              |
   |              help: use `then_some(..)` instead: `then_some(Foo(0))`
   |
   = note: `#[warn(clippy::unnecessary_lazy_evaluations)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_lazy_evaluations

I expected to see this happen: nothing because the suggestion gives different behaviour

Version

rustc 1.65.0-nightly (289279de1 2022-09-04)
binary: rustc
commit-hash: 289279de116707f28cf9c18e4bbb8c6ec84ad75b
commit-date: 2022-09-04
host: x86_64-unknown-linux-gnu
release: 1.65.0-nightly
LLVM version: 15.0.0

Additional Labels

@rustbot label +I-suggestion-causes-error

[kraktus]

@rustbot claim

[Jules-Bertholet]

This issue is not fixed, and should be reopened.

struct NoisyDrop;

impl Drop for NoisyDrop {
    fn drop(&mut self) {
        panic!();
    }
}

fn a() {
    let _ = false.then(|| NoisyDrop);
}

warns on clippy 0.1.66 (9565dfe 2022-10-28)

[kraktus]

can reproduce on master, will look at

[xFrednet]

#9750 should have fixed this issue :).

[ju1ius]

@xFrednet @kraktus I believe this issue is not fixed and should be reopened.

The following code:

extern crate ffi;
use core::ffi::c_char;

struct Buffer {
    ptr: *mut c_char,
    len: usize,
}
impl Buffer {
    unsafe fn from_owned_ptr(ptr: *mut c_char, len: usize) -> Option<Self> {
        (!ptr.is_null()).then(|| Self { ptr, len })
    }
}
impl Drop for Buffer {
    fn drop(&mut self) {
        unsafe { ffi::free(self.ptr) };
    }
}

Triggers this warning (using clippy 0.1.76):

warning: unnecessary closure used with `bool::then`
  --> buffer.rs:10:9
   |
48 |         (!ptr.is_null()).then(|| Self { ptr, len })
   |         ^^^^^^^^^^^^^^^^^--------------------------
   |                          |
   |                          help: use `then_some(..)` instead: `then_some(Self { ptr, len })`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_lazy_evaluations
   = note: `#[warn(clippy::unnecessary_lazy_evaluations)]` on by default

[y21]

Opened #12097 for that one