Consumer Experience Review

Consumer-experience review

You are auditing a library as the people who will cargo add / npm install / pip install it — not as its author. Internal tests pass through the crate's own privacy and import context and therefore cannot see the gaps a real consumer hits (missing root re-exports, trait-import friction, method-name collisions, feature-flag dead-ends, error types you can't name). The only reliable instrument is a separate project that depends on the library externally and uses it the way a reasonable-but-not-omniscient user would.

Core principle

Write the code a competent user would write on their first try — reaching for conventional names and paths — and let it fail. The natural call that doesn't compile is the finding. Do not pre-correct your consumer code with insider knowledge; that hides exactly what you're looking for. Evidence is a captured compiler error, not an assertion.

Procedure

1. Map the public surface (don't trust memory — read it)

Enumerate what a consumer actually sees:

• Crate-root re-exports vs what's only reachable via deeper paths (crate::stores::Foo). Asymmetry here is the #1 source of findings.
• Traits and every method signature — look for two traits that define the same method name (collides when both are in scope; common with sync/async pairs), and for &mut self where a consumer would expect &self.
• Constructors / builders — is Type::new() returning Type or a builder? Is the fallible path build() or try_build()? Is it consistent across sibling types?
• Error types — can the error returned by a public function be named via the same path the type came from? (cached::Foo but only cached::stores::FooBuildError is a classic gap.)
• Feature flags — what's gated, and does a natural feature combination leave a type unreferenceable or a macro unusable?
• Macros — re-exported at the crate root (use lib::thing;) or only under a submodule? Ecosystem convention is root.

2. Scaffold a throwaway external consumer

• Put it in $TMPDIR (or another scratch path) — never inside the repo, never in git. Do not pollute the project's examples/, tests/, or working tree.
• Depend on the library by path with a realistic feature set:
  • Rust: lib = { path = "/abs/path", features = [...] }
  • npm: npm link / file: dependency. Python: pip install -e. Same idea — external module resolution, real package boundary.
• Exercise the breadth: each macro, direct store/type use, sync and async paths, builders, error handling, and at least one less-common feature combination.

3. Write naive-but-reasonable consumer code

Deliberately reach for conventions first:

• Import the macro from the crate root (use lib::macro;) before trying submodule paths.
• Bring the obvious traits into scope together (or glob use lib::*;) — exactly what a real user does.
• Name the error type returned by a builder/constructor in a let _: Option<lib::TheError> = None; or a function signature.
• Guess builder method names by convention (max_size, capacity, with_*) — a wrong guess that the compiler can't help disambiguate is a discoverability finding.

4. Compile, run, capture exact errors

cargo build/run (or the ecosystem equivalent). For every failure, record the exact diagnostic code and message (error[E0034]: multiple applicable items, E0432: unresolved import, E0599, etc.). That verbatim error is the evidence in your report.

5. Isolate each gap in a minimal probe

For each suspected issue, create a tiny separate binary/module that triggers only that issue with the smallest natural snippet. This (a) proves it's real and not a side effect, and (b) gives you a clean before/after to re-run once a fix is proposed/applied.

6. Severity — weight the release window

Rank findings, and explicitly factor in the version context:

• Pre-1.0 / major bump: a hard compile error on a common operation, or any public-surface inconsistency, is high — this is the only non-breaking window to fix the method/type surface. Say so.
• Post-1.0 minor: a breaking fix is itself a cost; lean toward additive fixes (new re-exports, #[doc(alias)], deprecations) and documentation.
• Always separate: 🔴 blocks/forces ugly workarounds on common paths · 🟠 awkward but workable · 🟡 docs/discoverability.

7. Report — evidence first, fixes proposed not applied

For each finding: the natural code that failed → the exact compiler error → root cause (cite file:line) → concrete proposed fix (and whether it's breaking or additive). Recommend a subset to act on given the release window. Do not implement fixes unless the user asks — this skill produces a review. If asked to fix, re-run the isolated probes (step 5) afterward to prove the natural code now works, and run the project's full check (make check/tests/clippy/doctests, golden/snapshot drift).

8. Clean up

Remove or abandon the scratch consumer crate. Never leave it in the repo or stage it.

Gap classes to actively check (the checklist that catches the real ones)

• Root re-export gaps / asymmetry — type at root but its builder/error only via crate::sub::; one sibling exports *Builder at root, another doesn't.
• Trait method-name collisions — two in-scope traits with the same method name (sync foo vs async foo) → E0034, forces UFCS. Fix: prefix/rename one (e.g. async_-prefix the async variant), settle it pre-1.0.
• Unnameable error types — Type::build() returns Result<_, E> but E isn't reachable from the path Type is.
• Constructor/builder inconsistency — new()→value vs new()→builder; build vs try_build; one shared error enum vs per-type ones. Often documentation-only at 1.0, but flag it.
• Macro not at crate root — every mainstream crate puts its attribute/derive macro at the root; submodule-only is friction and a migration cost.
• Feature-flag dead-ends — a plausible feature set where a needed type/trait/macro is unreferenceable.
• &mut self for reads — forces a lock/wrapper for shared use; note it even if intentional.
• Discoverability — natural names that don't exist and have no #[doc(alias)]; missing trait-import hints in errors.

Cautions

• Testing from inside the crate (its own tests/, examples/, or a workspace member) is not a consumer test — it inherits the crate's import/privacy context and will miss the gaps. External path dependency is mandatory.
• Sandboxed builds may hit registry/network errors; this skill only needs the path-dependency and std/tokio-class deps. If a build fails for sandbox/network reasons (not a real API gap), retry with the sandbox disabled before concluding.
• A finding is the user's natural code failing, not "the API is unusable" — the API may work fine once you know the trick. The cost being measured is "knowing the trick."
• Generalize beyond Rust where relevant: npm (root export maps, exports field, dual ESM/CJS), Python (__init__.py re-exports, optional-extra imports) — same methodology, same gap classes.