Motivation

Cargo is designed to be highly reliable by ensuring that dependencies are either pulled from a central registry (like crates.io) or explicitly defined by their location (e.g., path = "../my-crate"). While this approach provides great predictability, there are specialized use cases where developers may need more flexibility—specifically, when resolving dependencies from dynamically provided local directories without modifying the original source code.

Other build systems and environments often provide mechanisms to search for dependencies across multiple locations:

• CMake utilizes CMAKE_PREFIX_PATH for package discovery.

• pkg-config leverages PKG_CONFIG_PATH to locate libraries.

• Python uses PYTHONPATH to find modules.

pallet-patcher provides a similar workflow for the Cargo ecosystem. It is particularly useful for complex development environments where projects are composed from multiple independent local layers or for building in restricted, offline environments where dependency sources are managed externally.

By scanning a list of search paths and resolving compatible crate versions using standard SemVer rules, pallet-patcher dynamically generates the necessary Cargo configuration to patch dependencies with their discovered local counterparts. This provides a bridge between Cargo’s strict dependency management and the need for dynamic, path-based discovery in specific workflows.

Usage

usage: pallet-patcher [-h] [--output-format {args,toml}]
                      manifest_path search_path [search_path ...]

positional arguments:
  manifest_path         Path to the Cargo.toml file on disk
  search_path           List of local registry sources to search for packages

options:
  -h, --help            show this help message and exit
  --output-format {args,toml}
                        Choose the output format for Cargo configuration. 'args' (default)
                        for CLI arguments, or 'toml' for configuration file contents.

Example

Generate Cargo command-line arguments to patch dependencies using locally available crates:

pallet-patcher Cargo.toml /usr/share/cargo/registry ../another/path

Output:

--config=patch.'crates-io'.'pkg::0.1.0'.package='pkg'
--config=patch.'crates-io'.'pkg::0.1.0'.path='/path/to/local/crates/pkg'

Generate a TOML configuration for your .cargo/config.toml:

pallet-patcher --output-format toml Cargo.toml /usr/share/cargo/registry > .cargo/config.toml

Output:

[patch.'crates-io'.'pkg::0.1.0']
package = 'pkg'
path = '/usr/share/cargo/registry/pkg'