Mordant

Lints for Rust that find the rules your code relies on but your types do not enforce.

A rule like "always build a Port through Port::new" usually lives in a comment or in somebody's head. Mordant finds rules like that and points at the type change that would make the compiler enforce them. It runs next to Clippy, not instead of it.

Run it

Mordant is built with a pinned nightly, and it lints projects on stable Rust without changing their toolchain. Install the nightly and Mordant once on each machine:

rustup toolchain install nightly-2026-09-01 --component rustc-dev --component llvm-tools-preview
cargo +nightly-2026-09-01 install --locked --git https://github.com/scarletindustries/mordant

Then run the lints from the workspace root:

cargo mordant --workspace --all-targets   # report
cargo mordant --workspace --fix           # also rewrite wildcard arms

Read a finding

Here Port::new checks that a number fits in 16 bits, but the crate also builds a Port by hand and skips the check:

mod port {
    pub struct Port {
        pub(crate) n: u16,
    }

    impl Port {
        pub fn new(n: u32) -> Result<Port, ()> {
            if n <= u16::MAX as u32 {
                Ok(Port { n: n as u16 })
            } else {
                Err(())
            }
        }
    }
}

use port::Port;

fn bypass() -> Port {
    Port { n: 0 }
}

warning: `port::Port` is built by hand here, skipping the check on `n` that `Port::new` makes
  --> src/lib.rs:24:5
   |
24 |     Port { n: 0 }
   |     ^^^^^^^^^^^^^
   |
note: the check in `Port::new`
  --> src/lib.rs:12:16
   |
12 |             if n <= u16::MAX as u32 {
   |                ^^^^^^^^^^^^^^^^^^^^
   = help: build it with `Port::new(..)`, or make `n` private so a literal like this only compiles inside `Port`'s module
   = note: `#[warn(unchecked_construction)]` on by default

The first line is the claim, the note is the evidence and the help is the fix. Here the fix makes n private, so nothing outside the module can skip Port::new:

mod port {
    pub struct Port {
        n: u16,
    }

    impl Port {
        pub fn new(n: u32) -> Result<Port, ()> {
            if n <= u16::MAX as u32 {
                Ok(Port { n: n as u16 })
            } else {
                Err(())
            }
        }
    }
}

use port::Port;

fn build(n: u32) -> Result<Port, ()> {
    Port::new(n)
}

When the code is right, silence that one finding with #[allow(unchecked_construction)] on the item.

Run it in CI

Mordant Action runs the lints on every pull request and fails the job on any finding.

name: mordant
on: [pull_request]

jobs:
  mordant:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v7
      - uses: scarletindustries/mordant-action@v1

Findings show up as annotations on the pull request. Set fail-on: never to report without failing, and ref to pin a Mordant commit.

Configuration

Settings live in the [mordant] table of a mordant.toml at the workspace root. Without the file every default lint runs, and a misspelt key stops the run with an error rather than being ignored.

[mordant]
disabled = ["unused_pub", "group:naming"]   # never run these
bool-cluster-enabled = true                 # turn on an opt-in lint

The lints

Each family is also a lint group, named in brackets, so #![allow(mordant_naming)] covers a whole family. Opt-in lints only run once mordant.toml turns them on.