Diagnostic deduplication breaks with incr comp

I tried this code:

tests/ui/allocator/not-an-allocator.rs

#[global_allocator]
static A: usize = 0;
//~^ ERROR E0277
//~| ERROR E0277
//~| ERROR E0277
//~| ERROR E0277

fn main() {}

I expected to see this happen: Both without and with incr comp a single diagnostic.

Instead, this happened: Without incr comp a single diagnostic. With incr comp 4 duplicated diagnostics. With -Zdeduplicate-diagnostics=no in both cases 4 diagnostics are shown, but with incr comp it misses the message that the diagnostics were deduplicated.

Adding some debug prints shows that each diagnostic hashes to a different value with incr comp enabled, yet the debug print of the diagnostics are identical as is the DiagInner::keys() debug print.

Meta

rustc --version --verbose:

rustc 1.100.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.100.0-dev
LLVM version: 23.1.1

c999cef531ea9059e189e82fe0e82c5daf249bc9

Start with tests/ui/allocator/not-an-allocator.rs and run the reproduction with and without incremental compilation, including -Zdeduplicate-diagnostics=no. Compare diagnostic counts, hashes, and the deduplication message; the fix is complete when incremental compilation matches the expected single-diagnostic behavior and the explicit no-deduplication mode remains consistent.