Patterns sometimes ignores lifetimes.

[theemathas]

I tried this code:

#[derive(PartialEq)]
struct Foo<'a>(&'a ());

#[allow(dead_code)]
struct Dummy<'a>(&'a ());
impl<'a> Dummy<'a> {
    const X: Foo<'a> = Foo(&());
}

fn identity<T>(x: T) -> T { x }

pub fn weird<'a>(_: &'a ()) {
    let Dummy::<'static>::X = { Dummy::<'a>::X };
    let Dummy::<'a>::X = { Dummy::<'static>::X };
    let Dummy::<'static>::X = Dummy::<'a>::X;
    let Dummy::<'a>::X = Dummy::<'static>::X;
    let Dummy::<'static>::X = { Foo(&()) as Foo<'a> };
    let Dummy::<'a>::X = { Foo(&()) as Foo<'static> };
    let Dummy::<'static>::X = identity(Foo(&()) as Foo<'a>);
    let Dummy::<'a>::X = identity(Foo(&()) as Foo<'static>);
    let Dummy::<'static>::X = Foo(&()) as Foo<'a>;
    // For some reason, the line below doesn't compile.
    let Dummy::<'a>::X = Foo(&()) as Foo<'static>;
}

I expected all of the let statements to cause compile errors, (or half of them, due to subtyping). Instead, all of the statements compile fine, except for the last one, which produced this compile error:

error: lifetime may not live long enough
  --> src/lib.rs:23:38
   |
12 | pub fn weird<'a>(_: &'a ()) {
   |              -- lifetime `'a` defined here
...
23 |     let Dummy::<'a>::X = Foo(&()) as Foo<'static>;
   |                                      ^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`

I discovered this issue after some experimentation after @purplesyringa sent me this odd comment in the compiler:

rust/compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

Lines 506 to 511 in c0b282f

	// This *could* accept a type that isn't actually `PartialEq`, because region bounds get
	// ignored. However that should be pretty much impossible since consts that do not depend on
	// generics can only mention the `'static` lifetime, and how would one have a type that's
	// `PartialEq` for some lifetime but *not* for `'static`? If this ever becomes a problem
	// we'll need to leave some sort of trace of this requirement in the MIR so that borrowck
	// can ensure that the type really implements `PartialEq`.

Meta

Reproducible on the playground with version 1.90.0-nightly (2025-07-21 9748d87dc70a9a6725c5)

@rustbot labels +A-patterns +A-borrow-checker +A-lifetimes

[theemathas]

Another weird case:

use std::marker::PhantomData;

struct Inv<'a>(PhantomData<*mut &'a ()>);

#[derive(PartialEq)]
struct Foo<T>(PhantomData<T>);

struct Dummy<T>(T);
impl<T> Dummy<T> {
    const X: Foo<T> = Foo(PhantomData);
}

const A: Foo<for<'a> fn(Inv<'a>)> = Foo(PhantomData);
const B: Foo<fn(Inv<'static>)> = Foo(PhantomData);

pub fn weird() {
    let A = B;
    let B = A;
    let Dummy::<for<'a> fn(Inv<'a>)>::X = Dummy::<fn(Inv<'static>)>::X;
    // For some reason, the line below doesn't compile
    let Dummy::<fn(Inv<'static>)>::X = Dummy::<for<'a> fn(Inv<'a>)>::X;
}

error[E0308]: mismatched types
  --> src/lib.rs:21:9
   |
21 |     let Dummy::<fn(Inv<'static>)>::X = Dummy::<for<'a> fn(Inv<'a>)>::X;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected struct `Foo<fn(Inv<'_>)>`
              found struct `Foo<for<'a> fn(Inv<'a>)>`

For more information about this error, try `rustc --explain E0308`.

[theemathas]

Another variant:

use std::marker::PhantomData;

struct Inv<'a>(PhantomData<*mut &'a ()>);

#[derive(PartialEq)]
struct Foo<T>(PhantomData<T>);

struct Dummy<T>(T);
impl<T> Dummy<T> {
    const X: InvTy<Foo<T>> = InvTy(PhantomData);
}

#[derive(PartialEq)]
struct InvTy<T>(PhantomData<*mut T>);

const A: InvTy<Foo<for<'a> fn(Inv<'a>)>> = InvTy(PhantomData);
const B: InvTy<Foo<fn(Inv<'static>)>> = InvTy(PhantomData);

pub fn weird() {
    let A = B;
    let B = A;
    // Both lines below don't compile.
    let Dummy::<for<'a> fn(Inv<'a>)>::X = Dummy::<fn(Inv<'static>)>::X;
    let Dummy::<fn(Inv<'static>)>::X = Dummy::<for<'a> fn(Inv<'a>)>::X;
}

[theemathas]

This code causes an ICE:

#[allow(dead_code)]
struct Inv<'a>(*mut &'a ());

type F1 = for<'a> fn(Inv<'a>);
type F2 = fn(Inv<'static>);

trait Trait {
    type Assoc: PartialEq;
}
impl Trait for F1 {
    type Assoc = i32;
}
impl Trait for F2 {
    type Assoc = i64;
}

#[derive(PartialEq)]
struct InvTy<T: Trait>(<T as Trait>::Assoc);

const A: InvTy<F1> = InvTy(1i32);
const B: InvTy<F2> = InvTy(1i64);

pub fn main() {
    let A = B else { panic!(); };
    let B = A else { panic!(); };
}

Error output

warning: conflicting implementations of trait `Trait` for type `for<'a> fn(Inv<'a>)`
  --> src/main.rs:13:1
   |
10 | impl Trait for F1 {
   | ----------------- first implementation here
...
13 | impl Trait for F2 {
   | ^^^^^^^^^^^^^^^^^ conflicting implementation for `for<'a> fn(Inv<'a>)`
   |
   = warning: the behavior may change in a future release
   = note: for more information, see issue #56105 <https://github.com/rust-lang/rust/issues/56105>
   = note: this behavior recently changed as a result of a bug fix; see rust-lang/rust#56105 for details
   = note: `#[warn(coherence_leak_check)]` on by default

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: broken MIR in DefId(0:18 ~ playground[c760]::main) (_4): bad field access (i64: i32): NoSolution
  --> src/main.rs:24:9
   |
24 |     let A = B else { panic!(); };
   |         ^
   |
note: delayed at compiler/rustc_borrowck/src/type_check/mod.rs:1832:21
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
         4: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_operand
         5: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_body
         6: rustc_borrowck::type_check::type_check
         7: rustc_borrowck::do_mir_borrowck
         8: rustc_borrowck::mir_borrowck
         9: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
        10: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        11: rustc_query_impl::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
        12: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
        13: rustc_interface::passes::analysis
        14: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
        15: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        16: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
        17: 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}
        18: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        19: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
        20: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
        21: std::sys::pal::unix::thread::Thread::new::thread_start
        22: <unknown>
        23: clone
  --> src/main.rs:24:9
   |
24 |     let A = B else { panic!(); };
   |         ^

error: internal compiler error: broken MIR in DefId(0:18 ~ playground[c760]::main) (_8): bad field access (i32: i64): NoSolution
  --> src/main.rs:25:9
   |
25 |     let B = A else { panic!(); };
   |         ^
   |
note: delayed at compiler/rustc_borrowck/src/type_check/mod.rs:1832:21
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
         4: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_operand
         5: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_body
         6: rustc_borrowck::type_check::type_check
         7: rustc_borrowck::do_mir_borrowck
         8: rustc_borrowck::mir_borrowck
         9: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
        10: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        11: rustc_query_impl::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
        12: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
        13: rustc_interface::passes::analysis
        14: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
        15: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        16: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
        17: 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}
        18: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        19: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
        20: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
        21: std::sys::pal::unix::thread::Thread::new::thread_start
        22: <unknown>
        23: clone
  --> src/main.rs:25:9
   |
25 |     let B = A else { panic!(); };
   |         ^

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 `/playground/rustc-ice-2025-07-23T03_34_37-26.txt` to your bug report

note: compiler flags: --crate-type bin -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

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

query stack during panic:
end of query stack
warning: `playground` (bin "playground") generated 1 warning
error: could not compile `playground` (bin "playground"); 1 warning emitted

Note that this hits the FCW in #56105

Given that the compiler seems to be thinking that two different types are the same type, I think that this is likely to be able to cause unsoundness somehow, even though I haven't figured out how to actually cause UB.

@rustbot labels +I-ICE +I-unsound

[purplesyringa]

I don't think you can actually trigger unsoundness here. I've experimented with this for a bit, and whenever you find a snippet that works around the broken MIR error, you get "one type is more general than the other". I think this is because the MIR validator compares the two types for invariance, so unrelated types immediately fail MIR and related but non-equal types fail the generality check later.