Add repro-check tool for build reproducibility

[windsunil]

Proposal

I propose adding a new in-tree tool, repro-check, to the rust-lang/rust repository. It allows developers to easily verify if Rust compiler builds are reproducible.

This is an MCP because it adds a new tool to src/tools/ and integrates with the bootstrap system (x.py).

The implementation is available in PR rust-lang/rust#149888.

Motivation

Currently, checking if compiler builds are reproducible is a manual, error-prone process. We lack an in-tree tool to automatically compare builds and pinpoint differences. This tool aims to solve that by automating the comparison.

Design and Usage

The source is located at src/tools/repro-check.

The tool is registered in x.py, allowing it to run within the existing workflow. When executed, it:

1. Builds the compiler twice (Stage 2 by default) in separate directories.
2. Compares artifacts byte-for-byte (SHA-256).
3. Generates an HTML report highlighting identical vs. differing files.

Impact and Maintenance

• Status: Experimental, opt-in tool for local development.
• CI Impact: None. It does not run on PR CI.
• Future direction: Eventually, we want to run this automatically to detect regressions (e.g., in Nightly or Release builds). We will gather data and evaluate infrastructure costs before proposing any automation.
• Maintenance: Discussed with @Kobzol and the Infra team. We plan to merge this as an experiment.

Non-goals (for now)

• No automatic execution in CI.
• Does not block PRs or releases.
• Does not analyze the root cause of differences, only reports them.

References

• Implementation PR: #149888
• Tracking Issues: #134589, #139793
• Zulip: Checking reproducibility of rustc

[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that.
Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections can formally be registered here by adding a comment.

@rustbot concern reason-for-concern
<description of the concern>

Concerns can be lifted with:

@rustbot resolve reason-for-concern

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler