ICE: begin <= end (12 <= 11) when slicing 'Self::Assoc<'_>'

[cushionbadak]

Note

This issue is almost identical to closed issue #121807 , but ICE-triggering input is slightly differnent from previous one.

• ICE-loc link

Code

trait MemoryUnit {
    extern "C" fn read_word(&mut self) -> u8;
    extern "C" fn read_dword(Self::Assoc<'_>) -> u16;
}

struct ROM {}

impl MemoryUnit for ROM {
    extern "C" fn read_dword(&'_ self) -> u16 {
        let a16 = self.read_word() as u16;
        let b16 = self.read_word() as u16;

        (b16 << 8) | a16
    }
}

pub fn main() {}

original test-case's read_dword(&'s self) is mutated to read_dword(&'_ self).

Meta

rustc --version --verbose:

rustc 1.79.0-nightly (88c2f4f5f 2024-04-02)
binary: rustc
commit-hash: 88c2f4f5f50ace5ddc7655ea311435104d3659bd
commit-date: 2024-04-02
host: x86_64-apple-darwin
release: 1.79.0-nightly
LLVM version: 18.1.2

Error output

warning: anonymous parameters are deprecated and will be removed in the next edition
 --> reduced_87933A3C2BEEFEEAFED61025DEE0BD738BBBDEBB1F116B588A791E327AE11916.rs:3:30
  |
3 |     extern "C" fn read_dword(Self::Assoc<'_>) -> u16;
  |                              ^^^^^^^^^^^^^^^ help: try naming the parameter or explicitly ignoring it: `_: Self::Assoc<'_>`
  |
  = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
  = note: for more information, see issue #41686 <https://github.com/rust-lang/rust/issues/41686>
  = note: `#[warn(anonymous_parameters)]` on by default

error[E0220]: associated type `Assoc` not found for `Self`
 --> reduced_87933A3C2BEEFEEAFED61025DEE0BD738BBBDEBB1F116B588A791E327AE11916.rs:3:36
  |
3 |     extern "C" fn read_dword(Self::Assoc<'_>) -> u16;
  |                                    ^^^^^ associated type `Assoc` not found

error[E0185]: method `read_dword` has a `&self` declaration in the impl, but not in the trait
 --> reduced_87933A3C2BEEFEEAFED61025DEE0BD738BBBDEBB1F116B588A791E327AE11916.rs:9:5
  |
3 |     extern "C" fn read_dword(Self::Assoc<'_>) -> u16;
  |     ------------------------------------------------- trait method declared without `&self`
...
9 |     extern "C" fn read_dword(&'_ self) -> u16 {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&self` used in impl

error[E0046]: not all trait items implemented, missing: `read_word`
 --> reduced_87933A3C2BEEFEEAFED61025DEE0BD738BBBDEBB1F116B588A791E327AE11916.rs:8:1
  |
2 |     extern "C" fn read_word(&mut self) -> u8;
  |     ----------------------------------------- `read_word` from trait
...
8 | impl MemoryUnit for ROM {
  | ^^^^^^^^^^^^^^^^^^^^^^^ missing `read_word` in implementation

Backtrace

thread 'rustc' panicked at compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs:1366:48:
begin <= end (12 <= 11) when slicing `Self::Assoc<'_>`
stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::str::slice_error_fail_rt
   3: core::str::slice_error_fail
   4: rustc_borrowck::diagnostics::mutability_errors::suggest_ampmut_self
   5: <rustc_borrowck::MirBorrowckCtxt>::suggest_make_local_mut
   6: <rustc_borrowck::MirBorrowckCtxt>::report_mutability_error
   7: <rustc_borrowck::MirBorrowckCtxt>::access_place
   8: <rustc_borrowck::MirBorrowckCtxt>::consume_rvalue
   9: rustc_borrowck::do_mir_borrowck
  10: rustc_borrowck::mir_borrowck
      [... omitted 1 frame ...]
  11: <rustc_middle::hir::map::Map>::par_body_owners::<rustc_interface::passes::analysis::{closure#1}::{closure#0}>::{closure#0}
  12: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  13: <rustc_interface::queries::QueryResult<&rustc_middle::ty::context::GlobalCtxt>>::enter::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}::{closure#1}::{closure#3}>
  14: rustc_interface::interface::run_compiler::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#0}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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

note: it seems that this compiler `1.79.0-nightly (88c2f4f5f 2024-04-02)` is outdated, a newer nightly should have been released in the mean time
  |
  = note: please consider running `rustup update nightly` to update the nightly channel and check if this problem still persists
  = note: if the problem still persists, we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please attach the file at `/Volumes/T7/workspace/placeholder_rustexec/0403/rustc-ice-2024-04-03T14_18_05-88650.txt` to your bug report

query stack during panic:
#0 [mir_borrowck] borrow-checking `<impl at reduced_87933A3C2BEEFEEAFED61025DEE0BD738BBBDEBB1F116B588A791E327AE11916.rs:8:1: 8:24>::read_dword`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 3 previous errors; 1 warning emitted

Some errors have detailed explanations: E0046, E0185, E0220.
For more information about an error, try `rustc --explain E0046`.

[jieyouxu]

Similar variants:

trait MemoryUnit {
    extern "C" fn read_word(&mut self) -> u8;
    extern "C" fn read_dword(x: &Self<'_>) -> u16;
}

struct ROM {}

impl MemoryUnit for ROM {
    extern "C" fn read_dword(&'_ self) -> u16 {
        let a16 = self.read_word() as u16;
        let b16 = self.read_word() as u16;

        (b16 << 8) | a16
    }
}

the suggestion that's triggering the ICE is not ready for HIR typeck to not fail before MIR borrowck is reached.

rust/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

Lines 1075 to 1078 in 385fa9d

	LocalInfo::User(mir::BindingForm::ImplicitSelf(_)) => {
	let suggestion = suggest_ampmut_self(self.infcx.tcx, decl_span);
	Some((true, decl_span, suggestion))
	}

Since now it doesn't fail in HIR typeck, the diagnostic can no longer use suggest_ampmut_self (it can no longer assume the trait's item's first local is some variant of simple implicit self)

[cushionbadak]

searched nightlies: from nightly-2024-01-01 to nightly-2024-04-04
regressed nightly: nightly-2024-02-09
searched commit range: 8ace7ea...98aa362
regressed commit: af88f7d

bisected with cargo-bisect-rustc v0.6.8

Host triple: x86_64-apple-darwin
Reproduce with:

cargo bisect-rustc --start=2024-01-01 --end=2024-04-04 --preserve --regress=ice