ICE: Unexpected tail in unsized_info_ty with generics + traits

[DanielKeep]

Just to note: there appear to be two ICEs in here, but I don't know if they have the same underlying cause or not. The code for the second is included, just commented-out. The first error:

error: internal compiler error: src\librustc_trans\type_of.rs:157: Unexpected tail in unsized_info_ty: <Slice as Structure<Utf16>>::RefTarget for ty=SeStr<Slice, Utf16>
...
thread 'rustc' panicked at 'Box<Any>', src\librustc_errors\lib.rs:417

The second error which appears if you manually instantiate the generics:

error: internal compiler error: unexpected panic
...
thread 'rustc' panicked at 'assertion failed: tail.has_param_types() || tail.has_self_ty()', src\librustc\ty\layout.rs:1557

The code:

/*!
Compiled with `rustc run-aground.rs`.

Tested against:

- 1.15.1 (021bd294c 2017-02-08)
- 1.16.0-beta.3 (ecbf0b59e 2017-03-02)
- 1.17.0-nightly (c0b7112ba 2017-03-02)
*/
#![allow(dead_code)]

// Supporting types.  Alone, these don't appear to trigger either crash.

trait Structure<E>: Sized where E: Encoding {
    type RefTarget: ?Sized;
    type FfiPtr;
    unsafe fn borrow_from_ffi_ptr<'a>(ptr: Self::FfiPtr) -> Option<&'a Self::RefTarget>;
}

enum Slice {}

impl<E> Structure<E> for Slice where E: Encoding {
    type RefTarget = [E::Unit];
    type FfiPtr = (*const E::FfiUnit, usize);
    unsafe fn borrow_from_ffi_ptr<'a>(_ptr: Self::FfiPtr) -> Option<&'a Self::RefTarget> {
        panic!()
    }
}

trait Encoding {
    type Unit: Unit;
    type FfiUnit;
}

trait Unit {}

enum Utf16 {}

impl Encoding for Utf16 {
    type Unit = Utf16Unit;
    type FfiUnit = u16;
}

struct Utf16Unit(pub u16);

impl Unit for Utf16Unit {}

// Replace `/*` with `//*` to uncomment a block.  `Error 1` shows the first
// error.  `Error 2` shows a *different* error after I manually instantiate the
// generics.  `Works` is what happens when I also replace some of the indirect
// types with the types they should resolve to.

// Error 1:
//*
type SUtf16Str = SeStr<Slice, Utf16>;

struct SeStr<S, E> where S: Structure<E>, E: Encoding {
    _data: S::RefTarget,
}

impl<S, E> SeStr<S, E> where S: Structure<E>, E: Encoding {
    pub unsafe fn from_ptr<'a>(_ptr: S::FfiPtr) -> Option<&'a Self> {
        panic!()
    }
}
// */

// Error 2:
/*
struct SUtf16Str {
    _data: <Slice as Structure<Utf16>>::RefTarget,
}

impl SUtf16Str {
    pub unsafe fn from_ptr<'a>(ptr: <Slice as Structure<Utf16>>::FfiPtr)
    -> Option<&'a Self> {
        std::mem::transmute::<Option<&[<Utf16 as Encoding>::Unit]>, _>(
            <Slice as Structure<Utf16>>::borrow_from_ffi_ptr(ptr))
    }
}
// */

// Works:
/*
struct SUtf16Str {
    _data: [<Utf16 as Encoding>::Unit],
}

impl SUtf16Str {
    pub unsafe fn from_ptr<'a>(
        _ptr: (*const <Utf16 as Encoding>::FfiUnit, usize),
    ) -> Option<&'a Self> {
        panic!()
    }
}
// */

fn main() {
    const TEXT_U16: &'static [u16] = &[];
    let _ = unsafe { SUtf16Str::from_ptr((TEXT_U16.as_ptr(), TEXT_U16.len())).unwrap() };
}

This was mostly tested and reduced with:

rustc 1.17.0-nightly (c0b7112ba 2017-03-02)
binary: rustc
commit-hash: c0b7112ba246d96f253ba845d91f36c0b7398e42
commit-date: 2017-03-02
host: i686-pc-windows-gnu
release: 1.17.0-nightly
LLVM version: 3.9

I won't bother to include the backtraces of the crashes, since it's just "drop" over and over again... I assume backtraces don't work, not that drop is somehow now the compiler's entry point and only function.

The closest issue I could find was #16812, which is marked closed. Also, this doesn't involve an enum, and appears to depend on generics and a trait, since once you remove those, it goes away.

[arielb1]

Still affects 1.20.0

[istankovic]

Still reproducible with rustc 1.26.0-nightly (adf2135ad 2018-03-17 albeit with a different error:

error: internal compiler error: librustc_trans/context.rs:469: failed to get layout for `std::option::Option<&SeStr<Slice, Utf16>>`: the type `<Slice as Structure<Utf16>>::RefTarget` has an unknown layout

thread 'rustc' panicked at 'Box<Any>', librustc_errors/lib.rs:543:9

[Alexendoo]

Error 1 no longer ICEs as of 1.38, Error 2 as of 1.30