Proposal

This would stabilize two related options:

• --json=unused-externs
• --json=unused-externs-silent

--json=unused-externs was introduced about 3 years ago in rust-lang/rust#73945.

When using json diagnostics, this flag causes rustc to emit extra diagnostics about unused crates. Specifically, it lists all the crates specified by an --extern cratename option which never had any symbols referenced when compiling the current crate.

This is distinct from a normal diagnostic message because there's no actual problem with the Rust code per-se - it simply means that the build system provided extra dependencies which were not used. In particular, there's no source file or line number a diagnostic can reference, because that's the whole point - we're reporting an absence.

Typically a user will want to remove those dependencies because they're either the result of cut-and-paste, no longer needed after code changes, and so on. The intended use of this message is that the build system itself will consume it, and turn it into a form which makes sense to the user with respect to the build system. For example, Cargo could consume it, make sure that none the crates in a given Cargo package require the unused crate(s), and then reference specific lines in Cargo.toml which should be removed or altered. Alternatively one could auto-fix such dependencies.

unused-externs and unused-externs-silent are identical except for how they interact with lint levels. Firstly neither does anything unless the unused-crate-dependencies lint is enabled. If it's set to deny or forbid level, then unused-externs will cause the build to fail as expected. unused-externs-silent suppresses this, and leaves it to the build system to present a build failure. This is because Cargo needs to determine whether a given dependency is unused over multiple crates, so its only an error if it is unused in all crates.

This feature has been used extensively with a non-Cargo build system (buck2) with great success - it is used as part of tooling to automatically remove unused dependencies across a very large codebase. The only change required in functionality since the original introduction was making unused-externs honor lint levels and introducting unused-externs-silent in order to resolve issue rust-lang/rust#96068.

Mentors or Reviewers

@est31 @ehuss

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

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.