Inconsistent behavior when importing type implementation from local crate vs. cross-crate

[edwardw]

Consider the following:

mod a {
    pub struct Foo;
    impl Foo { pub fn bar() {} }
}

fn main() {
    use std::io::File::open;
    use a::Foo::bar;
}

Only the import from non-std module is reported as "error: Cannot import from a trait or type implementation". It is inconsistent and both should be recognized as error.

Some of the rustc internal code actually presume that is the case. Failing to do that leads to bug such as #15528.

This is probably a general cross-crate vs local crate problem, not specific to std.

[edwardw]

Great point, the title has been changed as such.