ICE when formatting trait declaration with non-NFC name

[Jules-Bertholet]

trait Foó: Bar {}

The ó above is two codepoints, ASCII o followed by U+0301 COMBINING ACUTE ACCENT. It NFC-normalizes to ó, U+00F3 LATIN SMALL LETTER O WITH ACUTE.

Trying to format the above results in:

thread 'main' panicked at src/tools/rustfmt/src/source_map.rs:31:13:
bad span: `Foó`: `trait Foó: Bar {}`
stack backtrace:
   0:     0x7f3c6b9a06f6 - std::backtrace_rs::backtrace::libunwind::trace::hbee8a7973eeb6c93
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/../../backtrace/src/backtrace/libunwind.rs:104:5
   1:     0x7f3c6b9a06f6 - std::backtrace_rs::backtrace::trace_unsynchronized::hc8ac75eea3aa6899
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f3c6b9a06f6 - std::sys_common::backtrace::_print_fmt::hc7f3e3b5298b1083
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x7f3c6b9a06f6 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hbb235daedd7c6190
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f3c6b9f2f40 - core::fmt::rt::Argument::fmt::h76c38a80d925a410
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/fmt/rt.rs:142:9
   5:     0x7f3c6b9f2f40 - core::fmt::write::h3ed6aeaa977c8e45
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/fmt/mod.rs:1120:17
   6:     0x7f3c6b99453f - std::io::Write::write_fmt::h78b18af5775fedb5
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/io/mod.rs:1810:15
   7:     0x7f3c6b9a04d4 - std::sys_common::backtrace::_print::h5d645a07e0fcfdbb
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f3c6b9a04d4 - std::sys_common::backtrace::print::h85035a511aafe7a8
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f3c6b9a3267 - std::panicking::default_hook::{{closure}}::hcce8cea212785a25
  10:     0x7f3c6b9a2fc9 - std::panicking::default_hook::hf5fcb0f213fe709a
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:292:9
  11:     0x7f3c6e62961c - std[79729d9c385e1623]::panicking::update_hook::<alloc[6df67106ebca92c0]::boxed::Box<rustc_driver_impl[66ed8fdbde15dc6c]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f3c6b9a39b6 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::hbc5ccf4eb663e1e5
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/alloc/src/boxed.rs:2029:9
  13:     0x7f3c6b9a39b6 - std::panicking::rust_panic_with_hook::h095fccf1dc9379ee
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:783:13
  14:     0x7f3c6b9a3702 - std::panicking::begin_panic_handler::{{closure}}::h032ba12139b353db
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:657:13
  15:     0x7f3c6b9a0bf6 - std::sys_common::backtrace::__rust_end_short_backtrace::h9259bc2ff8fd0f76
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/sys_common/backtrace.rs:171:18
  16:     0x7f3c6b9a3460 - rust_begin_unwind
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:645:5
  17:     0x7f3c6b9ef645 - core::panicking::panic_fmt::h784f20a50eaab275
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/panicking.rs:72:14
  18:     0x55e4891a72ac - <rustfmt_nightly[9b2d04b1e1d23002]::visitor::SnippetProvider as rustfmt_nightly[9b2d04b1e1d23002]::source_map::SpanUtils>::span_after
  19:     0x55e4891bee27 - <rustfmt_nightly[9b2d04b1e1d23002]::visitor::FmtVisitor>::visit_item
  20:     0x55e48919f146 - <rustfmt_nightly[9b2d04b1e1d23002]::visitor::FmtVisitor>::visit_items_with_reordering
  21:     0x55e4891c6e7b - <rustfmt_nightly[9b2d04b1e1d23002]::visitor::FmtVisitor>::format_separate_mod
  22:     0x55e489038a85 - <rustfmt_nightly[9b2d04b1e1d23002]::Session<std[79729d9c385e1623]::io::stdio::Stdout>>::format_input_inner::{closure#0}
  23:     0x55e48904d3c0 - rustfmt[c449a577f49ce32d]::format_and_emit_report::<std[79729d9c385e1623]::io::stdio::Stdout>
  24:     0x55e48904bc5e - rustfmt[c449a577f49ce32d]::execute
  25:     0x55e4890474c6 - rustfmt[c449a577f49ce32d]::main
  26:     0x55e489032533 - std[79729d9c385e1623]::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>
  27:     0x55e489034749 - std[79729d9c385e1623]::rt::lang_start::<()>::{closure#0}
  28:     0x7f3c6b984cb1 - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::h37600b1e5eea4ecd
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/ops/function.rs:284:13
  29:     0x7f3c6b984cb1 - std::panicking::try::do_call::hb4bda49fa13a0c2b
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:552:40
  30:     0x7f3c6b984cb1 - std::panicking::try::h8bbf75149211aaaa
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:516:19
  31:     0x7f3c6b984cb1 - std::panic::catch_unwind::h8c78ec68ebea34cb
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panic.rs:142:14
  32:     0x7f3c6b984cb1 - std::rt::lang_start_internal::{{closure}}::hffdf44a19fd9e220
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/rt.rs:148:48
  33:     0x7f3c6b984cb1 - std::panicking::try::do_call::hcb3194972c74716d
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:552:40
  34:     0x7f3c6b984cb1 - std::panicking::try::hcdc6892c5f0dba4c
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:516:19
  35:     0x7f3c6b984cb1 - std::panic::catch_unwind::h4910beb4573f4776
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panic.rs:142:14
  36:     0x7f3c6b984cb1 - std::rt::lang_start_internal::h6939038e2873596b
                               at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/rt.rs:148:20
  37:     0x55e48904e5d5 - main
  38:     0x7f3c6b590083 - __libc_start_main
  39:     0x55e4890212c9 - <unknown>
  40:                0x0 - <unknown>

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

note: we would appreciate a bug report: https:/rust-lang/rustfmt/issues/new?labels=bug

note: rustc 1.76.0 (07dca489a 2024-02-04) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack

[ytmimi]

@Jules-Bertholet you mentioned back in #6058 that the compiler is now normalizing idents? Has that been a thing for a while now? Is there a way to get the unnormalized ident? The only way I can think to do so is to use the ident's span.

Let me know if I'm understanding this one correctly. I believe what you mentioned above is that Foó above is actually Foo\u{301}, which is getting interpreted as Foó. At least from looking into this it seems that calling item.ident.as_str() returns Foó. From what I can tell this is causing issues because rustfmt is now trying to find Foó in trait Foo\u{301}: Bar {}, and panics! when it can't do so.

This is where the panic happens:

rustfmt/src/items.rs

Lines 1169 to 1171 in 8486837

	let ident_hi = context
	.snippet_provider
	.span_after(item.span, item.ident.as_str());

[Jules-Bertholet]

Let me know if I'm understanding this one correctly

@ytmimi You are.

Has that been a thing for a while now?

Identifier normalization has been a thing since Rust started supporting non-ASCII idents, changing normalization behavior would have been a breaking change (it would change whether two identifiers are considered to refer to the same thing). I can't speak to the history of rustc's exact implementation though

[ytmimi]

Cool! This should be a straightforward fix. As mentioned, this is where the panic originates:

rustfmt/src/items.rs

Lines 1169 to 1171 in 8486837

	let ident_hi = context
	.snippet_provider
	.span_after(item.span, item.ident.as_str());

And I think the fix here is to ensure we're using the right needle when searching for the end of the ident:

+        // Idents are NFC normalized by the compiler. To make sure we provide the right needle to
+        // `span_after` we grab the ident from the source file. See #6069
+        let source_ident = context.snippet(item.ident.span);
         let ident_hi = context
             .snippet_provider
-            .span_after(item.span, item.ident.as_str());
+            .span_after(item.span, source_ident);

Then we should add a test case with the snippet posted above to a file like tests/target/issue_6069.rs, and to further test that the panic is resolved we can add tests/target/issue_6069.rs to this list:

rustfmt/tests/rustfmt/main.rs

Lines 176 to 186 in 8486837

	#[test]
	#[allow(non_snake_case)]
	fn dont_emit_ICE() {
	let files = ["tests/target/issue_5728.rs", "tests/target/issue_5729.rs"];
	for file in files {
	let args = [file];
	let (_stdout, stderr) = rustfmt(&args);
	assert!(!stderr.contains("thread 'main' panicked"));
	}
	}