False Positive on or_fun_call lint with enum constructor.

[CryZe]

or_fn_call shouldn't warn in case the function is just an enum constructor wrapping over local variables.

Here's the code:

pub enum Error<'a> {
    Path(&'a str),
}

pub fn question_mark(path: &str) -> Result<(), Error> {
    None.ok_or(Error::Path(path))?;
    
    Ok(())
}

pub fn try(path: &str) -> Result<(), Error> {
    try!(None.ok_or(Error::Path(path)));
    
    Ok(())
}

Warning:

Compiling playground v0.0.1 (file:///playground)
warning: use of `ok_or` followed by a function call, #[warn(or_fun_call)] on by default
 --> src/main.rs:6:5
  |
6 |     None.ok_or(Error::Path(path))?;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
help: try this
  |     None.ok_or_else(|| Error::Path(path))?;
  = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#or_fun_call

Here's the link to the code in the custom playground: Playground

[Manishearth]

cause it didn't do that before with the try! macro, so something is inconsistent here.

A lot of the clippy lints get turned off within macros.

[Manishearth]

This isn't a false positive fwiw, since clippy has no idea how expensive that function call is.

[CryZe]

Can it not check that it's just an enum constructor? I feel like this should definitely be improved, as this is something that will come up a lot more often, as you often have Options that you want to turn into proper Results before using the ? operator on them.

[Manishearth]

Fair point. I'm editing the bug report.

[llogiq]

So how could we determine something is a struct/enum constructor? fn name == return type name?

[oli-obk]

here's how we do it in miri: https://github.com/solson/miri/blob/ac2bf50f9d853a56ff1a236d456619b2d3b65b64/src/terminator/mod.rs#L236

[ishitatsuyuki]

This also applies to Some, which should instead pointed to unwrap_or():

None.or(Some(x))

EDIT: maybe not, this yields a different type.

[Manishearth]

This should have been fixed by #919, but wasn't, because that only deals with const promotable things.

I'll make a PR.