Internal Compiler Error: 'attempt to subtract with overflow', compiler/rustc_session/src/session.rs:478:22

[dwrensha]

Build rustc with overflow checking enabled:

RUSTFLAGS="-C overflow-checks=on" ./x.py build --stage 1

Then try compiling the following program (found by fuzz-rustc):

const M;

$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc bug.rs 
error: free constant item without body
 --> bug.rs:1:1
  |
1 | const M;
  | ^^^^^^^-
  |        |
  |        help: provide a definition for the constant: `= <expr>;`

error[E0601]: `main` function not found in crate `bug`
 --> bug.rs:1:1
  |
1 | const M;
  | ^^^^^^^^ consider adding a `main` function to `bug.rs`

thread 'rustc' panicked at 'attempt to subtract with overflow', /home/dwrensha/src/rust/compiler/rustc_session/src/session.rs:475:22
note: run with `RUST_BACKTRACE=1` environment variable to display a 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 1.51.0-dev running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0601`.

[dwrensha]

The ICE is a regression, introduced in #82720.

The overflowing subtraction happens here:

rust/compiler/rustc_session/src/session.rs

Lines 470 to 479 in 0894d1e

	// FIXME(matthewjasper) Remove this method, it should never be needed.
	pub fn track_errors<F, T>(&self, f: F) -> Result<T, ErrorReported>
	where
	F: FnOnce() -> T,
	{
	let old_count = self.err_count();
	let result = f();
	let errors = self.err_count() - old_count;
	if errors == 0 { Ok(result) } else { Err(ErrorReported) }
	}

cc @matthewjasper @henryboisdequin

(src/test/ui/parser/item-free-const-no-body-semantic-fail.rs and src/test/ui/parser/item-free-static-no-body-semantic-fail.rs can also hit this when overflow-checks = true, see https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Test.20failures.20with.20.60overflow-checks.20.3D.20true.60.)

[apiraino]

Assigning priority as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

@rustbot label -I-prioritize +P-medium