Wrong suggestion of `const fn` if member of member implements Drop

[ArekPiekarz]

cargo clippy -V: clippy 0.0.212 (c807fbc 2019-12-29)
rustc -V: rustc 1.40.0 (73528e339 2019-12-16)

Summary

A warning "this could be a const_fn" is triggered for a function of a struct that doesn't implement Drop, but on some level of depth contains a member that does implement it. Adding const to such function results in compilation error.

Steps to reproduce

Run the following code with cargo clippy -- -W clippy::missing-const-for-fn:

#![allow(dead_code)]

struct Foo {
    field: String
}

impl Foo {
    fn take(self) -> String {
        self.field
    }
}

fn main() {
}

It results in a warning:

warning: this could be a const_fn
  --> src/main.rs:8:5
   |
8  | /     fn take(self) -> String {
9  | |         self.field
10 | |     }
   | |_____^
   |
   = note: requested on the command line with `-W clippy::missing-const-for-fn`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn

After adding const to the function Foo::take we get an error:

error[E0493]: destructors cannot be evaluated at compile-time
 --> src/main.rs:8:19
  |
8 |     const fn take(self) -> String {
  |                   ^^^^ constant functions cannot evaluate destructors

Note that Foo doesn't implement Drop, its field String also doesn't, but String's field Vec does implement it.
Also note there was already a similar fixed issue, but for structs that implement Drop: #4449

[expenses]

This is now triggering more because more things were made const in rust 1.61.0.

naga has contains the following code:

impl<W: Write> Writer<W> {
    /// Finishes writing and returns the output.
    pub fn finish(self) -> W {
        self.out
    }
...
}

This triggers the lint:

warning: this could be a `const fn`
   --> src/back/msl/writer.rs:650:5
    |
650 | /     pub fn finish(self) -> W {
651 | |         self.out
652 | |     }
    | |_____^
    |
note: the lint level is defined here
   --> src/lib.rs:199:5
    |
199 |     clippy::missing_const_for_fn
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn

but when you actually apply that you get an error:

error[E0493]: destructors cannot be evaluated at compile-time
   --> src/back/msl/writer.rs:650:25
    |
650 |     pub const fn finish(self) -> W {
    |                         ^^^^ constant functions cannot evaluate destructors
651 |         self.out
652 |     }
    |     - value is dropped here

For more information about this error, try `rustc --explain E0493`.

[samueltardieu]

This seems fixed. Can we close the issue?