Mordant

Lints that find code where the type system is not enforcing the invariants the code depends on.

Documentation

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Mordant is a lint pack for Rust. It finds rules your code depends on but its types do not enforce, such as two flags that must never both be true, and suggests the type that would. A lint that cannot prove its claim stays silent.

Install

Mordant lints stable Rust projects. It is built with the nightly pinned in rust-toolchain, and your own toolchain does not change.

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

Add --rev <commit> to pin the lints to one version.

Run

cargo mordant --workspace --all-targets
cargo mordant --workspace --fix

--fix applies the fixes Mordant is sure of, and every other flag goes to cargo check. cargo mordant --list prints every lint.

Configure

Settings go in a [mordant] table in mordant.toml at the workspace root:

[mordant]
disabled = ["unit_mismatch", "group:duplication"]
parallel-params-enabled = true

[[mordant.forbidden-reach]]
from = "sched::pick"
never = ["std::vec::Vec::push"]

disabled takes a lint name or a whole family. A few lints are off until their *-enabled key is set, and the lint reference names each one.

Baseline

A baseline records how many findings each lint has in each file, so a run says nothing about a file until it has more than that:

[mordant]
baseline = "mordant-baseline.toml"

MORDANT_BASELINE_WRITE=1 cargo mordant --workspace

The baseline holds a count, not which findings. So a file that goes over shows every finding of that lint, with the count the baseline allows: the new one is among them.

Regenerate and commit it after fixing a finding. When any crate goes over the baseline, cargo mordant prints one closing error and exits with status 101, so a CI job fails on the run itself. It also lists those crates in target/mordant/over-baseline.txt, rewritten on every run that gets that far, including one where cargo compiles nothing again, and deleted when no crate is over:

cargo mordant --workspace --keep-going

Lints

There are 49, in families. Each family is a lint group, so #![allow(mordant_naming)] silences every naming lint. The lint reference has an example and a fix for each.

Family	Group	Lints
State	mordant_state	options_as_enum parallel_bools bool_cluster runtime_typestate always_unwrapped_option derived_field field_valid_only_when bool_beside_option parallel_vecs parallel_params stringly_state tuple_wants_struct some_still_unchecked
Checks	mordant_checks	unchecked_construction defaulted_failure unchecked_input_len guard_blind_to_action stale_across_reentry error_collapsed_to_bool narrowed_two_ways cast_bypasses_from sentinel_integer bound_checked_only_in_debug
Errors	mordant_errors	stringly_error stringified_error discarded_error unread_error_variant
Enums	mordant_enums	wildcard_over_own_enum param_wider_than_callers return_wider_than_body
Duplication	mordant_duplication	same_match_twice reimplemented_helper generic_body_not_generic
Naming	mordant_naming	bare_bool_args arg_named_like_other_param interchangeable_aliases index_of_other_kind unit_mismatch
Keys and locks	mordant_keys_locks	key_not_identity insert_then_unwrap lock_order lock_taken_twice
Comments	mordant_comments	stale_safety_comment stale_panic_message
Unsafe	mordant_unsafe	safe_fn_returns_shared_mut unsafe_send_covers_more_than_pointers freed_while_borrowed
Unused	mordant_unused	unused_pub
Custom	mordant_custom	forbidden_reach

Develop

CI runs these four, and the last one is Mordant linting itself:

cargo fmt --check
cargo test
cargo clippy --all-targets -- -D warnings
target/debug/cargo-mordant mordant --all-targets

Each ui/*.rs fixture must print exactly the .stderr file beside it, so a change to what a lint reports means updating that file.

Name

Stroud dyed wool scarlet, and a mordant is the compound that binds the dye to the fiber so it holds.

License

MIT or Apache-2.0, at your option.