ICE: string indexed not at a char boundary when constructing error message

[Badel2]

I'm seeing an internal compiler error on the following input, found by fuzz-rustc:

Code

// ţ must be the last character in this file, it cannot be followed by a newline
fn main() {
    0: u8(ţ

Error output

error: this file contains an unclosed delimiter
 --> src/main.rs:3:12
  |
2 | fn main() {
  |           - unclosed delimiter
3 |     0: u8(ţ
  |          - ^
  |          |
  |          unclosed delimiter

error[E0412]: cannot find type `ţ` in this scope
 --> src/main.rs:3:11
  |
3 |     0: u8(ţ
  |           ^ expecting a type here because of type ascription

thread 'rustc' panicked at 'byte index 4 is not a char boundary; it is inside 'ţ' (bytes 3..5) of `u8(ţ`', compiler/rustc_ast_lowering/src/path.rs:236:49
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Affected versions: I tried nightly 2021-11-25 and stable 1.56.1 and they both show the same ICE.

Backtrace

   Compiling playground v0.0.1 (/playground)
error: this file contains an unclosed delimiter
 --> src/main.rs:3:12
  |
2 | fn main() {
  |           - unclosed delimiter
3 |     0: u8(ţ
  |          - ^
  |          |
  |          unclosed delimiter

error[E0412]: cannot find type `ţ` in this scope
 --> src/main.rs:3:11
  |
3 |     0: u8(ţ
  |           ^ expecting a type here because of type ascription

thread 'rustc' panicked at 'byte index 4 is not a char boundary; it is inside 'ţ' (bytes 3..5) of `u8(ţ`', compiler/rustc_ast_lowering/src/path.rs:236:49
stack backtrace:
   0: rust_begin_unwind
             at /rustc/dd549dcab404ec4c7d07b5a83aca5bdd7171138f/library/std/src/panicking.rs:498:5
   1: core::panicking::panic_fmt
             at /rustc/dd549dcab404ec4c7d07b5a83aca5bdd7171138f/library/core/src/panicking.rs:107:14
   2: core::str::slice_error_fail
   3: <rustc_ast_lowering::LoweringContext>::lower_qpath
   4: <rustc_ast_lowering::LoweringContext>::lower_ty_direct
   5: <rustc_ast_lowering::LoweringContext>::lower_ty
   6: rustc_data_structures::stack::ensure_sufficient_stack::<rustc_hir::hir::Expr, <rustc_ast_lowering::LoweringContext>::lower_expr_mut::{closure#0}>
   7: <rustc_ast_lowering::LoweringContext>::lower_stmts
   8: <rustc_ast_lowering::LoweringContext>::lower_block
   9: <rustc_ast_lowering::LoweringContext>::lower_maybe_async_body
  10: <rustc_ast_lowering::LoweringContext>::with_hir_id_owner::<<rustc_ast_lowering::item::ItemLowerer as rustc_ast::visit::Visitor>::visit_item::{closure#0}>
  11: rustc_ast_lowering::lower_crate
  12: <rustc_interface::passes::boxed_resolver::BoxedResolver>::access::<rustc_interface::passes::create_global_ctxt::{closure#0}, &rustc_hir::hir::Crate>
  13: rustc_interface::passes::create_global_ctxt
  14: <rustc_interface::queries::Queries>::global_ctxt
  15: <rustc_interface::interface::Compiler>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorReported>>
  16: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#1}>
  17: rustc_interface::interface::create_compiler_and_run::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>
  18: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.58.0-nightly (dd549dcab 2021-11-25) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
end of query stack
For more information about this error, try `rustc --explain E0412`.
error: could not compile `playground` due to 2 previous errors

Thanks to the detailed error message I was able to quickly locate the issue, on this line:

rust/compiler/rustc_ast_lowering/src/path.rs

Line 236 in 6d246f0

let args = &snippet[split + 1..snippet.len() - 1];

It is assumed that the last character of snippet will be a ), so using snippet.len() - 1 as an index should be safe. However as we see in the input code, it is possible to enter that branch without a closing ), and it will trigger the panic if the last character happens to be multibyte.

[Badel2]

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