depub: minimise visibility

Overview

When working on medium or large sized Rust code bases, it can be hard to know whether the visibility of functions, structs, and so on are still at the minimum required. For example, sometimes functions that once needed to be pub now only need to be pub(crate), pub(super), or simply private.

depub minimises the visibility of such items in files passed to it, using a user-specified command (e.g. cargo check) as an oracle to tell if its reduction of an item's visibility is valid or not. Note that depub is entirely guided by the oracle command: if the code it compiles happens not to use part of an intentionally public interface, then depub is likely to suggest reducing its visibility even though that's not what you want. The broader the coverage of your oracle, the less this is an issue.

In essence, depub does a string search for pub, replaces it with pub crate and sees if a test command still succeeds. If it does, it keeps that visibility, otherwise it replaces with the original and tries the next item. Note that depub is inherently destructive: it overwrites files as it operates, so do not run it on source code that you do not want altered!

The list of visibilities that depub considers is, in order: pub, pub(crate), pub(super), and private (i.e. no pub keyword at all). depub searches for pub/pub(crate)/pub(super) instances, reduces their visibility by one level, and tries the oracle command. If it succeeds, it tries the next lower level until private visibility has been reached.

Since reducing the visibility of one item can enable other items' visibility to be reduced, depub keeps running "rounds" until a fixed point has been reached. The maximum number of rounds is equal to the number of visible items in the code base, though in practise 2 or 3 rounds are likely to be all that is needed.

Usage

depub's usage is as follows:

depub -c <command> file_1 [... file_n]

where <command> is a string to be passed to /bin/sh -c for execution to determine whether the altered source code is still valid.

To reduce the visibility of a normal Rust project, cd to your Rust code base and execute:

$ find . -name "*.rs" | \
    xargs /path/to/depub -c "cargo check && cargo check --test"

depub informs you of its progress. After it is finished, diff your code base, and accept those of its recommendations you think appropriate. Note that depub currently uses string search and replace, so it will merrily change the string pub in a comment into pub(crate) -- you should not expect to accept its recommendations without at least a cursory check.

Using with libraries

Running depub on a library will tend to reduce all its intentionally pub functions to private visibility. You can weed these out manually after depub has run, but this can be tedious, and may also have reduced the visibility of a cascade of other items.

To avoid this, use one or more users of the library in the oracle command as part of your oracle. Temporarily alter their Cargo.toml to point to the local version of your libary and use a command such as:

$ find . -name "*.rs" | \
    xargs /path/to/depub -c " \
      cargo check && cargo check --test && \
      cd /path/to/lib && cargo check && cargo check --test"