rerun-if-changed doesn't trigger if a symlink points to a different file with the same mtime

Problem

Last week I ran into crashes working on the Python bcrypt project, which uses Rust via PyO3. Ultimately I tracked the crash down to the Rust build cache not being invalidated correctly while switching Python interpreters. See this PyO3 issue for more context.

Steps

Clone this repository and run the steps in the README.

The repository sets up symlinks in a manner analogous to the problematic symlinks that caused issues on the bcrypt CI. In the real-world case the two python symlinks were pointing at interpreters with different filesystem paths but happened to have the same mtime.

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.84.1 (66221abde 2024-11-19)
release: 1.84.1
commit-hash: 66221abdeca2002d318fde6efff516aab091df0e
commit-date: 2024-11-19
host: aarch64-apple-darwin
libgit2: 1.8.1 (sys:0.19.0 vendored)
libcurl: 8.7.1 (sys:0.4.74+curl-8.9.0 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: Mac OS 15.2.0 [64-bit]

Run the linked test-rerun-if-changed repository by following its README to reproduce the symlink behavior. Then trace Cargo's rerun-if-changed handling and filesystem metadata comparison. Done means switching a symlink to a different target triggers a rerun even when the targets have the same mtime, with regression coverage for the reproduction.