Rust Abstraction

rust-abstraction

Decide whether a long Rust file has seams worth extracting, and if there are, how to extract each.

Check the cheap fixes first

Before looking for abstractions, see if a mechanical fix handles it.

• Tests inline? Measure prod vs. test lines. If tests are >20% of the file and live in #[cfg(test)] mod ... blocks, lift them to foo/tests.rs per the project convention. That alone often resolves "this file is too long."
• Hand-written Clone/Debug that could derive? Replace and stop.
• Dead code from a half-done refactor? Delete and stop.

Seam smells

Look for these if there are no cheap fixes. One strong signal is enough; three weak ones is not.

• Inline struct/enum in a function body. The author already factored the concept but didn't lift the boundary. Promote it.
• Repeated loop or match shape with varying bodies. N near-identical for/match walks differing only in body or predicate → consolidate.
• Read-only narrow access to a wide type. A method that touches one or two fields of a large struct and never the rest. That slice is the boundary.
• Load-bearing invariant living as a docstring. A rule that must be preserved (cache-safety, ordering, "don't recurse here") is one rename away from being lost — promote it to a type name.

When picking a shape

Your goal is to make the code easier to read and safer to edit. Try to encapsulate data and invariants.

• No empty wrappers. If Foo::new(thing).do_x() just renames Thing::do_x(thing), reject it. The test: can you state in one sentence what the new type guarantees? If not, it's a wrapper.

Workflow

1. Read the file end to end (not skim).
2. Run it by the cheap-fixes list, then the seam smells list. Give honest judgment - weak signals are not seams.
3. If you find one or more seams, report the name of the proposed abstraction and (briefly) how it improves the code.
4. f you don't find a seam: report "no seams found" and move on.

Source: jackhall/koan — distributed by TomeVault.