False positive: missing_const_for_fn

[snejugal]

I simplified the problem to this code:

#![deny(clippy::missing_const_for_fn)]

pub struct StringWrapper {
    string: String,
}

pub fn get_string(wrapper: StringWrapper) -> String {
    wrapper.string
}

If I run clippy, it suggests to add const to get_string signature:

snejugal ~/stuff/clippy-bug $ cargo +nightly-2019-04-26 clippy
    Checking clippy-bug v0.1.0 (/home/snejugal/stuff/clippy-bug)
error: this could be a const_fn
 --> src/lib.rs:7:1
  |
7 | / pub fn get_string(wrapper: StringWrapper) -> String {
8 | |     wrapper.string
9 | | }
  | |_^
  |
note: lint level defined here
 --> src/lib.rs:1:9
  |
1 | #![deny(clippy::missing_const_for_fn)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn

error: aborting due to previous error

error: Could not compile `clippy-bug`.

To learn more, run the command again with --verbose.

But if I add const, the code refuses to compile:

snejugal ~/stuff/clippy-bug $ cargo +nightly-2019-04-26 clippy
    Checking clippy-bug v0.1.0 (/home/snejugal/stuff/clippy-bug)
error[E0493]: destructors cannot be evaluated at compile-time
 --> src/lib.rs:7:25
  |
7 | pub const fn get_string(wrapper: StringWrapper) -> String {
  |                         ^^^^^^^ constant functions cannot evaluate destructors

error: aborting due to previous error

error: Could not compile `clippy-bug`.

To learn more, run the command again with --verbose.

The code compiles though if I use u8 instead of String.

This problem persists on both stable and nightly:

snejugal ~/stuff/clippy-bug $ cargo +nightly-2019-04-26 clippy -V
clippy 0.0.212 (8c0e038 2019-04-25)
snejugal ~/stuff/clippy-bug $ cargo clippy -V
clippy 0.0.212 (726176e 2019-04-18)

[phansch]

Thanks for the report! This is a bug in rust and should be fixed there and probably not in Clippy, so I'm going ahead and close this issue.

Until it's fixed, you can disable the lint for that specific function with:

#[allow(clippy::missing_const_for_fn)]
pub fn get_string(wrapper: StringWrapper) -> String {
    wrapper.string
}