"should not be mangling partially substituted polymorphic instance" with `impl_trait_in_assoc_type`

[Jules-Bertholet]

Code

(minimized from https://github.com/Jules-Bertholet/unsized-vec/blob/master/emplacable/src/lib.rs, can probably be minimized further)

#![feature(impl_trait_in_assoc_type)]

use core::marker::PhantomData;

struct Emplacable<T: ?Sized, F> {
    phantom: PhantomData<(*const T, F)>,
}

impl<T: ?Sized, F> Emplacable<T, F> {
    fn from_fn(_: F) -> Self {
        Emplacable {
            phantom: PhantomData,
        }
    }

    fn unsize<U: ?Sized>(self) -> Emplacable<U, impl Sized> {
        Emplacable::from_fn(|| ())
    }
}

trait IntoEmplacable<T: ?Sized> {
    type Closure;

    fn into_emplacable(self) -> Emplacable<T, Self::Closure>;
}

impl<const N: usize, F> IntoEmplacable<[()]> for Emplacable<[(); N], F> {
    type Closure = impl Sized;

    fn into_emplacable(self) -> Emplacable<[()], Self::Closure> {
        self.unsize()
    }
}

impl<F> From<Emplacable<[(); 0], F>>
    for Emplacable<[()], <Emplacable<[(); 0], F> as IntoEmplacable<[()]>>::Closure>
{
    fn from(value: Emplacable<[(); 0], F>) -> Self {
        <Emplacable<[(); 0], F> as IntoEmplacable<[()]>>::into_emplacable(value)
    }
}

fn box_new_with<T: ?Sized>(_: Emplacable<T, impl Sized>) -> Box<T> {
    loop {}
}

pub fn foo() {
    let e: Emplacable<[(); 0], ()> = Emplacable {
        phantom: PhantomData,
    };
    let _: Box<[()]> = box_new_with(e.into());
}

Meta

cargo bisect-rustc

searched toolchains nightly-2025-01-02 through nightly-2025-02-15

Regression in nightly-2025-01-10

get_commits_between returning commits, len: 7

• commit[0] 2025-01-08: Auto merge of Run borrowck tests on BIDs and emit tail-expr-drop-order lints for violations #134523 - dingxiangfei2009:issue-130836-attempt-2, r=nikomatsakis
• commit[1] 2025-01-08: Auto merge of Rollup of 6 pull requests #135260 - matthiaskrgr:rollup-8irqs72, r=matthiaskrgr
• commit[2] 2025-01-09: Auto merge of Rollup of 6 pull requests #135277 - matthiaskrgr:rollup-0k61sf8, r=matthiaskrgr
• commit[3] 2025-01-09: Auto merge of [mir-opt] GVN some more transmute cases #133324 - scottmcm:gvn-aggregate-transmute, r=cjgillot
• commit[4] 2025-01-09: Auto merge of Rollup of 5 pull requests #135279 - matthiaskrgr:rollup-ek2qere, r=matthiaskrgr
• commit[5] 2025-01-09: Auto merge of Master bootstrap update #135268 - pietroalbini:pa-bump-stage0, r=Mark-Simulacrum
• commit[6] 2025-01-09: Auto merge of Rollup of 3 pull requests #135286 - matthiaskrgr:rollup-sxuq1nh, r=matthiaskrgr

Error output

Backtrace

thread 'rustc' (29529) panicked at compiler/rustc_symbol_mangling/src/legacy.rs:437:13:
should not be mangling partially substituted polymorphic instance: DefId(0:8 ~ scratch[7233]::{impl#0}) [[(); 0_usize], F/#0]
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <rustc_symbol_mangling::legacy::LegacySymbolMangler as rustc_middle::ty::print::Printer>::default_print_def_path
   3: <rustc_symbol_mangling::legacy::LegacySymbolMangler as rustc_middle::ty::print::Printer>::default_print_def_path
   4: <rustc_symbol_mangling::legacy::LegacySymbolMangler as rustc_middle::ty::print::Printer>::default_print_def_path
   5: <rustc_symbol_mangling::legacy::LegacySymbolMangler as rustc_middle::ty::print::Printer>::default_print_def_path
   6: <rustc_symbol_mangling::legacy::LegacySymbolMangler as rustc_middle::ty::print::Printer>::default_print_def_path
   7: <rustc_symbol_mangling::legacy::LegacySymbolMangler as rustc_middle::ty::print::pretty::PrettyPrinter>::pretty_print_type
   8: <rustc_symbol_mangling::legacy::LegacySymbolMangler as rustc_middle::ty::print::Printer>::default_print_def_path
   9: <rustc_symbol_mangling::legacy::LegacySymbolMangler as rustc_middle::ty::print::Printer>::default_print_def_path
  10: rustc_symbol_mangling::legacy::mangle
  11: rustc_symbol_mangling::symbol_name_provider
      [... omitted 1 frame ...]
  12: rustc_monomorphize::partitioning::assert_symbols_are_distinct::<core::slice::iter::Iter<rustc_middle::mir::mono::MonoItem>>
  13: rustc_monomorphize::partitioning::collect_and_partition_mono_items
      [... omitted 2 frames ...]
  14: rustc_codegen_ssa::back::symbol_export::exported_generic_symbols_provider_local
      [... omitted 2 frames ...]
  15: <rustc_metadata::rmeta::encoder::EncodeContext>::encode_crate_root
  16: <rustc_metadata::rmeta::encoder::encode_metadata::{closure#3} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, &std::path::Path)>>::call_once
  17: <rustc_middle::dep_graph::DepsType as rustc_query_system::dep_graph::Deps>::with_deps::<<rustc_query_system::dep_graph::graph::DepGraphData<rustc_middle::dep_graph::DepsType>>::with_task<rustc_middle::ty::context::TyCtxt, &std::path::Path, ()>::{closure#1}::{closure#0}, ()>
  18: rustc_metadata::rmeta::encoder::encode_metadata
  19: rustc_metadata::fs::encode_and_write_metadata
  20: <rustc_interface::queries::Linker>::codegen_and_build_linker
  21: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  22: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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

note: 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 make sure that you have updated to the latest nightly

note: please attach the file at `/home/julesbertholet/git/scratch/rustc-ice-2025-09-13T15_37_45-29527.txt` to your bug report

note: compiler flags: --crate-type lib -C embed-bitcode=no -C debuginfo=2 -C linker=clang -C incremental=[REDACTED] -C link-arg=-fuse-ld=/usr/bin/mold

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

query stack during panic:
#0 [symbol_name] computing the symbol for `<impl at src/lib.rs:35:1: 36:84>::from`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
#2 [exported_generic_symbols] collecting exported generic symbols for crate `0`
end of query stack

@rustbot label T-types requires-nightly F-impl_trait_in_assoc_type

[Jules-Bertholet]

#135261 looks related @compiler-errors

[cyrgani]

a bit smaller:

#![feature(impl_trait_in_assoc_type)]

use core::marker::PhantomData;

struct Emp<T, F> {
    phantom: PhantomData<(*const T, F)>,
}

impl<T, F> Emp<T, F> {
    fn from_fn(_: F) -> Emp<T, F> {
        loop {}
    }

    fn unsize(self) -> Emp<Slice, impl Sized> {
        Emp::from_fn(|| ())
    }
}

trait IntoEmplacable {
    type Closure;

    fn into_emplacable(self) -> Emp<Slice, Self::Closure>;
}

impl<F> IntoEmplacable for Emp<Arr, F> {
    type Closure = impl Sized;

    fn into_emplacable(self) -> Emp<Slice, Self::Closure> {
        self.unsize()
    }
}

impl<F> Into<Emp<Slice, <Emp<Arr, F> as IntoEmplacable>::Closure>> for Emp<Arr, F> {
    fn into(self) -> Emp<Slice, <Emp<Arr, F> as IntoEmplacable>::Closure> {
        self.into_emplacable()
    }
}

fn box_new_with(_: Emp<Slice, impl Sized>) {}

pub struct Arr;
pub struct Slice;

pub fn foo() {
    let e: Emp<Arr, ()> = Emp {
        phantom: PhantomData,
    };
    box_new_with(e.into());
}