Fix ICE #48984

[sinkuu]

• fbf6423 The tail type was not normalized.
• d0839d5 The method had a wrong assumption that something whose parent is a trait is an associated item. Fixes ICE: rustc panics on trait bound with array associated type (stable) #48984.

[rust-highfive]

r? @pnkfelix

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[eddyb]

r? @nikomatsakis For the record, I don't like HasParamEnv but I'm not sure if I can suggest anything better. I think this is not enough in case struct_tail returns a projection that normalizes to a struct.

[nikomatsakis]

@bors r+

[bors]

📌 Commit d0839d5 has been approved by nikomatsakis

[eddyb]

@bors r-

I believe #48010 isn't fixed, and that this is a pervasive problem with uses of struct_tail.
The followed modified testcase likely still exhibits an ICE with this PR:

#![crate_type = "lib"]
struct Foo;
pub struct Path<T: Bar> {
    inner: T::Slice,
}
pub trait Bar: Sized {
    type Slice: ?Sized;
    fn open(_: &Path<Self>);
}
impl Bar for Foo {
    type Slice = ([u8],);
    fn open(_: &Path<Self>) {
        unimplemented!()
    }
}

[sinkuu]

Thanks, I should have noticed that. I removed fbf6423 from this PR (I don't have time to fix for now).

[eddyb]

@bors r+

[bors]

📌 Commit 9fc2595 has been approved by eddyb

[bors]

⌛ Testing commit 9fc2595 with merge f76f6fb...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing f76f6fb to master...