Dylint

A tool for running Rust lints from dynamic libraries

cargo install cargo-dylint dylint-link

Dylint is a Rust linting tool, similar to Clippy. But whereas Clippy runs a predetermined, static set of lints, Dylint runs lints from user-specified, dynamic libraries. Thus, Dylint allows developers to maintain their own personal lint collections.

Contents

• Quick start
  • Running Dylint
  • Writing lints
• How libraries are found
• Workspace metadata
• Conditional compilation
• Library requirements
• Utilities
• VS Code integration
• Limitations
• Resources

Quick start

Running Dylint

The next three steps install Dylint and run all of this repository's example lints on a workspace:

1. Install cargo-dylint and dylint-link:

   cargo install cargo-dylint dylint-link

2. Add the following to the workspace's Cargo.toml file:

   [workspace.metadata.dylint]
   libraries = [
       { git = "https://github.com/trailofbits/dylint", pattern = "examples/*/*" },
   ]

3. Run cargo-dylint:

   cargo dylint --all --workspace

In the above example, the libraries are found via workspace metadata (see below).

Writing lints

You can start writing your own Dylint library by running cargo dylint new new_lint_name. Doing so will produce a loadable library right out of the box. You can verify this as follows:

cargo dylint new new_lint_name
cd new_lint_name
cargo build
DYLINT_LIBRARY_PATH=$PWD/target/debug cargo dylint list --lib new_lint_name

All you have to do is implement the LateLintPass trait and accommodate the symbols asking to be filled in.

Helpful resources for writing lints appear below.

How libraries are found

Dylint tries to run all lints in all libraries named on the command line. Dylint resolves names to libraries in the following three ways:

1. Via the DYLINT_LIBRARY_PATH environment variable. If DYLINT_LIBRARY_PATH is set when Dylint is started, Dylint treats it as a colon-separated list of paths, and searches each path for files with names of the form DLL_PREFIX LIBRARY_NAME '@' TOOLCHAIN DLL_SUFFIX (see Library requirements below). For each such file found, LIBRARY_NAME resolves to that file.

2. Via workspace metadata. If Dylint is started in a workspace, Dylint checks the workspace's Cargo.toml file for workspace.metadata.dylint.libraries (see Workspace metadata below). Dylint downloads and builds each listed entry, similar to how Cargo downloads and builds a dependency. The resulting target/release directories are searched and names are resolved in the manner described in 1 above.

3. By path. If a name does not resolve to a library via 1 or 2, it is treated as a path.

It is considered an error if a name used on the command line resolves to multiple libraries.

If --lib name is used, then name is is treated only as a library name, and not as a path.

If --path name is used, then name is is treated only as a path, and not as a library name.

If --all is used, Dylint runs all lints in all libraries discovered via 1 and 2 above.

Note: Earlier versions of Dylint searched the current package's target/debug and target/release directories for libraries. This feature has been removed.

Workspace metadata

A workspace can name the libraries it should be linted with in its Cargo.toml file. Specifically, a workspace's manifest can contain a TOML list under workspace.metadata.dylint.libraries. Each list entry must have the form of a Cargo git or path dependency, with the following differences:

• There is no leading package name, i.e., no package =.
• path entries can contain glob patterns, e.g., *.
• Any entry can contain a pattern field whose value is a glob pattern. The pattern field indicates the subdirectories that contain Dylint libraries.

Dylint downloads and builds each entry, similar to how Cargo downloads and builds a dependency. The resulting target/release directories are searched for files with names of the form that Dylint recognizes (see Library requirements below).

As an example, if you include the following in your workspace's Cargo.toml file and run cargo dylint --all --workspace, Dylint will run on your workspace all of this repository's example general lints, as well as the example restriction lint try_io_result.

[workspace.metadata.dylint]
libraries = [
    { git = "https://github.com/trailofbits/dylint", pattern = "examples/general/*" },
    { git = "https://github.com/trailofbits/dylint", pattern = "examples/restriction/try_io_result" },
]

Conditional compilation

For each library that Dylint uses to check a crate, Dylint passes the following to the Rust compiler:

--cfg=dylint_lib="LIBRARY_NAME"

You can use this feature to allow a lint when Dylint is used, but also avoid an "unknown lint" warning when Dylint is not used. Specifically, you can do the following:

#[cfg_attr(dylint_lib = "LIBRARY_NAME", allow(LINT_NAME))]

Note that LIBRARY_NAME and LINT_NAME may be the same. For an example involving non_thread_safe_call_in_test, see dylint/src/lib.rs in this repository.

Also note that the just described approach does not work for pre-expansion lints. The only known workaround for pre-expansion lints is allow the compiler's built-in unknown_lints lint. Specifically, you can do the following:

#[allow(unknown_lints)]
#[allow(PRE_EXPANSION_LINT_NAME)]

For an example involving env_cargo_path, see internal/src/examples.rs in this repository.

Library requirements

A Dylint library must satisfy four requirements. Note: Before trying to satisfy these explicitly, see Utilities below.

1. Have a filename of the form:

   DLL_PREFIX LIBRARY_NAME '@' TOOLCHAIN DLL_SUFFIX
   

   The following is a concrete example on Linux:

   libquestion_mark_in_expression@nightly-2021-04-08-x86_64-unknown-linux-gnu.so
   

   The filename components are as follows:

   • DLL_PREFIX and DLL_SUFFIX are OS-specific strings. For example, on Linux, they are lib and .so, respectively.
   • LIBRARY_NAME is a name chosen by the library's author.
   • TOOLCHAIN is the Rust toolchain for which the library is compiled, e.g., nightly-2021-04-08-x86_64-unknown-linux-gnu.

2. Export a dylint_version function:

   extern "C" fn dylint_version() -> *mut std::os::raw::c_char

   This function should return 0.1.0. This may change in future versions of Dylint.

3. Export a register_lints function:

   fn register_lints(sess: &rustc_session::Session, lint_store: &mut rustc_lint::LintStore)

   This is a function called by the Rust compiler. It is documented here.

4. Link against the rustc_driver dynamic library. This ensures the library uses Dylint's copies of the Rust compiler crates. This requirement can be satisfied by including the following declaration in your library's lib.rs file:

   extern crate rustc_driver;

Dylint provides utilities to help meet the above requirements. If your library uses the dylint-link tool and the dylint_library! macro, then all you should have to do is implement the register_lints function.

Utilities

The following utilities can be helpful for writing Dylint libraries:

• dylint-link is a wrapper around Rust's default linker (cc) that creates a copy of your library with a filename that Dylint recognizes.
• dylint_library! is a macro that automatically defines the dylint_version function and adds the extern crate rustc_driver declaration.
• ui_test is a function that can be used to test Dylint libraries. It provides convenient access to the compiletest_rs package.
• clippy_utils is a collection of utilities to make writing lints easier. It is generously made public by the Rust Clippy Developers. Note that, like rustc, clippy_utils provides no stability guarantees for its APIs.

VS Code integration

Dylint results can be viewed in VS Code using rust-analyzer. To do so, add the following to your VS Code settings.json file:

    "rust-analyzer.checkOnSave.overrideCommand": [
        "cargo",
        "dylint",
        "--all",
        "--workspace",
        "--",
        "--all-targets",
        "--message-format=json"
    ]

If you want to use rust-analyzer inside a lint library, you need to add the following to your VS Code settings.json file:

    "rust-analyzer.rustcSource": "discover",

And add the following to the library's Cargo.toml file:

[package.metadata.rust-analyzer]
rustc_private = true

Limitations

To run a library's lints on a package, Dylint tries to build the package with the same toolchain used to build the library. So if a package requires a specific toolchain to build, Dylint may not be able to apply certain libraries to that package.

One way this problem can manifest itself is if you try to run one library's lints on the source code of another library. That is, if two libraries use different toolchains, they may not be applicable to each other.

Resources

Helpful resources for writing lints include the following:

• Adding a new lint (targeted at Clippy but still useful)
• Author lint
• Common tools for writing lints
• Guide to Rustc Development
• Crate rustc_hir
• Crate rustc_middle
• Struct rustc_lint::LateContext
  • Method typeck_results
  • Field tcx
    • Method hir