Invalid suggestion of or_fun_call causes a compiler error error[E0277]: expected a FnOnce<()> closure, found Vec<_>``

[mgacek8]

Lint name: or_fun_call

I tried this code:

use std::collections::HashMap;
fn main() {
    let mut hash_map: HashMap<_, Vec<usize>> = HashMap::new();
    let _ = hash_map.entry("test".to_owned()).or_insert(vec![]);

    println!("hash_map: {:?}", hash_map);
}

I expected to see this happen: suggestion to use or_insert_with(|| vec![]) instead of or_insert(vec![])

Instead, this happened: clippy suggested me to use or_insert_with(vec![])

warning: use of `or_insert` followed by a function call
 --> src/main.rs:4:47
  |
4 |     let _ = hash_map.entry("test".to_owned()).or_insert(vec![]);
  |                                               ^^^^^^^^^^^^^^^^^ help: try this: `or_insert_with(vec![])`
  |
  = note: `#[warn(clippy::or_fun_call)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#or_fun_call

warning: 1 warning emitted

and that suggestion causes the compiler error.

error[E0277]: expected a `FnOnce<()>` closure, found `Vec<_>`
 --> src/main.rs:4:62
  |
4 |     let _ = hash_map.entry("test".to_owned()).or_insert_with(vec![]);
  |                                                              ^^^^^^ expected an `FnOnce<()>` closure, found `Vec<_>`
  |
  = help: the trait `FnOnce<()>` is not implemented for `Vec<_>`
  = note: wrap the `Vec<_>` in a closure with no arguments: `|| { /* code */ }`
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

Playground link

I found that this only happens when the vector is empty.
When it is not, the suggestion is correct:

4 |     let _ = hash_map.entry("test".to_owned()).or_insert(vec![123]);
  |                                               ^^^^^^^^^^^^^^^^^^^^ help: try this: `or_insert_with(|| vec![123])`

Meta

• clippy 0.1.52 (5fa22fe 2021-02-14)
• rustc stable 1.50.0

[camsteffen]

Could be fixed with #3812.

[mgacek8]

Shouldn't this lint for .or_insert(vec![]) suggest using .or_insert_with(Vec::new) (instead of .or_insert_with(vec![]) that causes a compiler error)?
And then, as in #3812, there could be another link that suggest std::collections::btree_map::Entry::or_default instead of .or_insert_with(Vec::new).
Please let me know if it makes sense to fix that (.or_insert(vec![]) -> .or_insert_with(Vec::new)) . If it does I'd like to do it.

[camsteffen]

We should have both fixed actually, since vec![] could be any macro that calls any function. When #3812 is fixed, it should just suggest or_default() instead of vec![].

So @mgacek8 if you would like to just fix the this issue that would be great!

[mgacek8]

@rustbot claim