cargo-workspace-unused-pub

This cargo subcommand tries to detect pub methods that are unused in an entire workspace, by performing workspace-wide analysis.

The dead_code Rust compiler lint ignores pub methods, given that:

• The linter operates crate-by-crate.
• Unused pub methods could be used in another workspace crate, or be part of a public API.

See also the following discussions about this problem:

• https://github.com/rust-lang/rust-clippy/issues/5828 (closed as Clippy does not perform workspace-level analyses).
• https://users.rust-lang.org/t/would-pub-workspace-be-possible/105503/2

Usage

$ cargo workspace-unused-pub --help
Detect unused pub methods in a workspace

Usage: cargo workspace-unused-pub [OPTIONS] [WORKSPACE]

Arguments:
  [WORKSPACE]  [default: current workspace]

Options:
      --scip <SCIP>
      --extensions <EXTENSIONS>  [default: rs,html]
  -h, --help                     Print help
  -V, --version                  Print version

Implementation

Detection is performed in multiple phases:

1. Open an SCIP code index, or generate one with rust-analyzer.

   Opened SCIP file with 806 documents
   

   Note that the index generation can take a significant amount of time on large workspaces.

2. Record methods and traits declarations.

   Found 6722 declarations and 286 traits
   

3. Phase 1: Find all methods that are seem never used. The following phases aim at removing false positives.

   Pass 1: 1419 candidates
   

4. Phase 2: Remove false positives stemming from main, tests and trait methods.

   Pass 2 (mains, tests, trait methods): 189 candidates
   

5. Phase 3: Perform a text search to remove false positives due to e.g. use in HTML templates:

   Pass 3 (search): 43 candidates
   Found 43 possibly unused functions
   

   The extensions that are searched are defined by the --extensions flag.

6. Display results:
   

The output of each phase (in the format above) can be viewed by setting the RUST_LOG=debug environment variable.

Alternatives

• https://github.com/est31/warnalyzer. When cargo-workspace-unused-pub was developed, warnalyzer relied on the removed nightly save-analysis feature, and therefore required an older nightly compiler (see https://github.com/est31/warnalyzer/issues/9). This issue is now addressed, and warnanalyzer has a SCIP backend.

TODO

• Automatically remove the unused functions (assuming a clean git repository is found).
• Build an approximate SCIP index with tree-sitter instead of rust-analyzer, for faster execution (e.g. in CI).
• Allow ignoring false positives with a special code comment or attribute.
• Support other objects than only methods (e.g. constants).
• Robustify the detection of test/main functions.