ICE: in map_usage, at rust/resolve/rust-name-resolution-context.cc:258

Code

fn foo<const N: usize>(bar: impl Iterator<Item = &str>, n: usize) -> impl Iterator<Item = u8> {
    n + 42
}

Version information:

bc2a3e6

Program output:

GNU Rust (GCC) version 16.0.0 20250604 (experimental) (x86_64-pc-linux-gnu)
        compiled by GNU C version 11.4.0, GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version none
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
options passed: -frust-debug -frust-dump-ast-pretty -mtune=generic -march=x86-64 -O0 -frust-incomplete-and-experimental-compiler-do-not-use
../test.rs:1:34: error: could not resolve type path ‘Iterator’ [E0412]
    1 | fn foo<const N: usize>(bar: impl Iterator<Item = &str>, n: usize) -> impl Iterator<Item = u8> {
      |                                  ^~~~~~~~
crab1: note: popping link
crab1: note: new cursor: k: N, v: (S)[] (NS)[5] (G)[]
crab1: note: popping link
crab1: note: new cursor: k: Impl_0, v: (S)[] (NS)[88] (G)[]
crab1: note: popping link
../test.rs:1:75: error: could not resolve type path ‘Iterator’ [E0412]
    1 | fn foo<const N: usize>(bar: impl Iterator<Item = &str>, n: usize) -> impl Iterator<Item = u8> {
      |                                                                           ^~~~~~~~
crab1: internal compiler error: in map_usage, at rust/resolve/rust-name-resolution-context.cc:258
0x29abe86 internal_error(char const*, ...)
        ../../gccrs/gcc/diagnostic-global-context.cc:517
0xb48036 fancy_abort(char const*, int, char const*)
        ../../gccrs/gcc/diagnostic.cc:1803
0x913de5 Rust::Resolver2_0::NameResolutionContext::map_usage(Rust::Resolver2_0::Usage, Rust::Resolver2_0::Definition)
        ../../gccrs/gcc/rust/resolve/rust-name-resolution-context.cc:258
0xe4d308 Rust::Resolver2_0::Late::visit(Rust::AST::TypePath&)
        ../../gccrs/gcc/rust/resolve/rust-late-name-resolver-2.0.cc:572
0xe4a602 Rust::Resolver2_0::Late::visit(Rust::AST::GenericArgs&)
        ../../gccrs/gcc/rust/resolve/rust-late-name-resolver-2.0.cc:700
0xc68abf void Rust::AST::DefaultASTVisitor::visit<Rust::AST::TypePathSegment>(std::unique_ptr<Rust::AST::TypePathSegment, std::default_delete<Rust::AST::TypePathSegment> >&)
        ../../gccrs/gcc/rust/ast/rust-ast-visitor.h:416
0xc68abf Rust::AST::DefaultASTVisitor::visit(Rust::AST::TypePath&)
        ../../gccrs/gcc/rust/ast/rust-ast-visitor.cc:159
0xe4d078 Rust::Resolver2_0::Late::visit(Rust::AST::TypePath&)
        ../../gccrs/gcc/rust/resolve/rust-late-name-resolver-2.0.cc:532
0xc691ff void Rust::AST::DefaultASTVisitor::visit<Rust::AST::TypeParamBound>(std::unique_ptr<Rust::AST::TypeParamBound, std::default_delete<Rust::AST::TypeParamBound> >&)
        ../../gccrs/gcc/rust/ast/rust-ast-visitor.h:416
0xc691ff Rust::AST::DefaultASTVisitor::visit(Rust::AST::TypeBoundWhereClauseItem&)
        ../../gccrs/gcc/rust/ast/rust-ast-visitor.cc:781
0xc6c59f void Rust::AST::DefaultASTVisitor::visit<Rust::AST::WhereClauseItem>(std::unique_ptr<Rust::AST::WhereClauseItem, std::default_delete<Rust::AST::WhereClauseItem> >&)
        ../../gccrs/gcc/rust/ast/rust-ast-visitor.h:416
0xc6c59f Rust::AST::DefaultASTVisitor::visit(Rust::AST::WhereClause&)
        ../../gccrs/gcc/rust/ast/rust-ast-visitor.cc:799
0xc6c59f Rust::AST::DefaultASTVisitor::visit(Rust::AST::Function&)
        ../../gccrs/gcc/rust/ast/rust-ast-visitor.cc:886
0xe2bda1 operator()
        ../../gccrs/gcc/rust/resolve/rust-default-resolver.cc:72
0xe2bda1 scope_inner<Rust::Resolver2_0::CanonicalPathCtx::scope<Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Function&)::<lambda()> >(Rust::NodeId, std::string, Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Function&)::<lambda()>&&)::<lambda()>, Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Function&)::<lambda()> >
        ../../gccrs/gcc/rust/resolve/rust-name-resolution-context.h:470
0xe2bda1 scope<Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Function&)::<lambda()> >
        ../../gccrs/gcc/rust/resolve/rust-name-resolution-context.h:413
0xe2bda1 scope<Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Function&)::<lambda()> >
        ../../gccrs/gcc/rust/resolve/rust-name-resolution-context.h:406
0xe2bda1 operator()
        ../../gccrs/gcc/rust/resolve/rust-default-resolver.cc:75
0xe2bda1 __invoke_impl<void, Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Function&)::<lambda()>&>
        /usr/include/c++/11/bits/invoke.h:61
0xe2bda1 __invoke_r<void, Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Function&)::<lambda()>&>
        /usr/include/c++/11/bits/invoke.h:111
Please submit a full bug report, with preprocessed source (by using -freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

Start by reproducing the ICE with the Rust snippet in test.rs using the reported gccrs revision. Read rust/resolve/rust-name-resolution-context.cc:258 and the related visits in rust/resolve/rust-late-name-resolver-2.0.cc. Done means this input no longer triggers an internal compiler error and produces appropriate diagnostics.