ICE with const generics over a &'static str

[moxian]

#![feature(const_generics)]

pub fn function_with_str<const STRING: &'static str>() {
}

pub fn use_it() {
    function_with_str::<"Hello, world!">()
}

Results in

   Compiling playground v0.0.1 (/playground)
warning: the feature `const_generics` is incomplete and may cause the compiler to crash
 --> src/lib.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^

error: internal compiler error: broken MIR in DefId(0:14 ~ playground[d52b]::use_it[0]) (CanonicalUserTypeAnnotation { user_ty: Canonical { max_universe: U0, variables: [], value: TypeOf(DefId(0:12 ~ playground[d52b]::function_with_str[0]), UserSubsts { substs: [Const { ty: &'static str, val: Unevaluated(DefId(0:15 ~ playground[d52b]::use_it[0]::{{constant}}[0]), []) }], user_self_ty: None }) }, span: src/lib.rs:7:5: 7:41, inferred_ty: fn() {function_with_str::<Const { ty: &str, val: Slice(Ptr(Pointer { alloc_id: AllocId(0), offset: Size { raw: 0 }, tag: () }), 13) }>} }): bad user type AscribeUserType(fn() {function_with_str::<Const { ty: &str, val: Slice(Ptr(Pointer { alloc_id: AllocId(0), offset: Size { raw: 0 }, tag: () }), 13) }>}, DefId(0:12 ~ playground[d52b]::function_with_str[0]) UserSubsts { substs: [Const { ty: &'static str, val: Unevaluated(DefId(0:15 ~ playground[d52b]::use_it[0]::{{constant}}[0]), []) }], user_self_ty: None }): NoSolution

error: internal compiler error: broken MIR in DefId(0:14 ~ playground[d52b]::use_it[0]) (const function_with_str): constant const function_with_str should have type fn() {function_with_str::<Const { ty: &str, val: Slice(Ptr(Pointer { alloc_id: AllocId(0), offset: Size { raw: 0 }, tag: () }), 13) }>} but has fn() {function_with_str::<Const { ty: &str, val: Slice(Ptr(Pointer { alloc_id: AllocId(0), offset: Size { raw: 0 }, tag: () }), 13) }>} (NoSolution)
 --> src/lib.rs:7:5
  |
7 |     function_with_str::<"Hello, world!">()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: broken MIR in DefId(0:14 ~ playground[d52b]::use_it[0]) (const function_with_str): bad constant user type CanonicalUserTypeAnnotation { user_ty: Canonical { max_universe: U0, variables: [], value: TypeOf(DefId(0:12 ~ playground[d52b]::function_with_str[0]), UserSubsts { substs: [Const { ty: &'static str, val: Unevaluated(DefId(0:15 ~ playground[d52b]::use_it[0]::{{constant}}[0]), []) }], user_self_ty: None }) }, span: src/lib.rs:7:5: 7:41, inferred_ty: fn() {function_with_str::<Const { ty: &str, val: Slice(Ptr(Pointer { alloc_id: AllocId(0), offset: Size { raw: 0 }, tag: () }), 13) }>} } vs fn() {function_with_str::<Const { ty: &str, val: Slice(Ptr(Pointer { alloc_id: AllocId(0), offset: Size { raw: 0 }, tag: () }), 13) }>}: NoSolution
 --> src/lib.rs:7:5
  |
7 |     function_with_str::<"Hello, world!">()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:356:17
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   6: std::panicking::begin_panic
   7: <rustc_errors::Handler as core::ops::drop::Drop>::drop
   8: core::ptr::real_drop_in_place
   9: core::ptr::real_drop_in_place
  10: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  11: core::ptr::real_drop_in_place
  12: rustc_interface::interface::run_compiler_in_existing_thread_pool
  13: std::thread::local::LocalKey<T>::with
  14: scoped_tls::ScopedKey<T>::set
  15: syntax::with_globals
query stack during panic:
end of query stack

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.36.0-nightly (a9ec99f42 2019-05-13) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

Playground link

cc @Serentty

[hellow554]

@rustbot modify labels: I-ICE C-Bug T-compiler A-const-generics

[yodaldevoid]

I'm looking into this currently.

[yodaldevoid]

Partially as a note to myself and partially as an update to anyone interested

The root cause of this bug seems to be the fact that we currently don't explicitly handle slice const values in super_relate_consts, and therefore we always say they cannot match.

rust/src/librustc/ty/relate.rs

Line 630 in 538e17a

_ => Err(TypeError::ConstMismatch(expected_found(relation, &a, &b))),

This error eventually bubbles up to to produce all three errors.