associated function `assumed_wf_types` repeats parent impl WF work

I tried this code:

cat > associated_fn_assumed_wf_repro.py <<'PY'
#!/usr/bin/env python3
import os
import subprocess
import time
from pathlib import Path

N = int(os.environ.get("N", "2000"))
RUSTC = os.environ.get("RUSTC", "rustc")
TIMEOUT = float(os.environ.get("TIMEOUT", "120"))


def write_many_assoc_fns(path: Path) -> None:
    params = ", ".join(f"T{i}" for i in range(N))
    phantom = ", ".join(f"*const T{i}" for i in range(N))
    methods = "\n".join(f"    pub fn m{i}(&self) {{}}" for i in range(N))
    path.write_text(
        f"""#![allow(dead_code)]
pub struct S<{params}>(core::marker::PhantomData<({phantom})>);

impl<{params}> S<{params}> {{
{methods}
}}
""",
        encoding="utf-8",
    )


src = Path(f"associated_fn_assumed_wf_{N}.rs")
out = Path(f"associated_fn_assumed_wf_{N}.rmeta")
write_many_assoc_fns(src)

start = time.perf_counter()
try:
    proc = subprocess.run(
        [
            RUSTC,
            "--edition=2024",
            "--crate-type=lib",
            "--emit=metadata",
            "-Awarnings",
            str(src),
            "-o",
            str(out),
        ],
        stdout=subprocess.DEVNULL,
        stderr=subprocess.DEVNULL,
        timeout=TIMEOUT,
        check=False,
    )
    seconds = time.perf_counter() - start
    print(f"N={N} status={proc.returncode} elapsed={seconds:.3f}s source={src.stat().st_size}")
except subprocess.TimeoutExpired:
    seconds = time.perf_counter() - start
    print(f"N={N} status=timeout>{TIMEOUT:g}s elapsed={seconds:.3f}s source={src.stat().st_size}")
PY

RUSTC=/path/to/rustc N=2000 python3 associated_fn_assumed_wf_repro.py

Here are the timings I collected with the same rustc binary:

N     source_bytes  elapsed_s     exit_status
500   28026         0.930         0
1000  56526         6.366         0
2000  118526        46.836        0
4000  242525        timeout>120   -1

Meta

rustc --version --verbose: 4008bbdf34b

<backtrace>

Start by running the provided associated_fn_assumed_wf_repro.py reproduction with the listed N values and a matching rustc binary, then investigate the compiler work associated with assumed_wf_types and parent impl well-formedness checking. Done means the reproduction still compiles successfully without the reported runtime scaling and timeout behavior.