Apply coercion rules on return expressions

[arjantop]

Asked on reddit

I have a program where I want to select from two fields (int in this example, io::Writer in practice). It is not possible to return a in a way the program compiles.

Reborrowing was suggested (&mut *self.a) but you can't do that with io::Writer references.

pub struct Foo<'a> {
    priv a: &'a mut int,
    priv b: int,
    priv take_a: bool
}

impl<'a> Foo<'a> {
    fn take(&'a mut self) -> &'a mut int {
        if self.take_a {
            self.a
        } else {
            &mut self.b
        }
    }
}

[alexcrichton]

cc @nikomatsakis

[nikomatsakis]

At present you need to write:

pub struct Foo<'a> {
    priv a: &'a mut int,
    priv b: int,
    priv take_a: bool
}

impl<'a> Foo<'a> {
    fn take(&'a mut self) -> &'a mut int {
        if self.take_a {
            &mut *self.a
        } else {
            &mut self.b
        }
    }
}

fn main() { }

Note that I returned &mut *self.a and not self.a. These two expressions are equivalent but how the type system thinks about them is not. That said, there is a bug here, because we normally automatically insert these kinds of "reborrow coercions" -- but we don't apply our coercion rules on return. I think we ought to, though. I don't seen an open bug on this, though. I think I'll repurpose this issue.

[nikomatsakis]

Nominated. Adding to meeting agenda as this would be a user-visible change, though I don't expect controversy.

[nikomatsakis]

To be clear: my suggestion is 1.0 but not backwards compat (I think this change would make the language strictly more accepting)

[flaper87]

cc @flaper87

[pnkfelix]

Assigning P-high (but not 1.0; it won't kill us if we don't have it).

[nrc]

Fixing this as part of the DST work

[nrc]

This has landed now