Rust interview: How do you migrate a crate that uses gen identifiers to Rust 2024?

Prompt and scope

A team is moving several crates from Rust 2021 to Rust 2024. Functions, modules, fields, and macro output use the identifier gen; Rust 2024 reserves it as a keyword, so some crates fail during parsing. Design detection, fixes, CI gates, and rollback.

Focus on edition migration. Do not assume gen blocks are stable, and keep compiler upgrades separate from the edition switch.

What the interviewer is testing

• Explain that an edition changes parsing rules while crates from different editions can still depend on one another.
• Use the keyword_idents_2024 lint and cargo fix --edition to find gen identifiers.
• Weigh renaming against raw identifiers for compatibility.
• Cover macros, public APIs, build scripts, tests, and workspace dependencies.

Clarifying questions

1. What edition, MSRV, and toolchain does each crate use?
2. Does gen appear in public APIs, serialized names, or external macro interfaces?
3. Do affected tokens come from source, a proc macro, or a build script?
4. Must the migration support Rust 2021 and 2024 simultaneously?
5. Can CI run both editions across feature combinations?

A 30-second answer

“I inventory the workspace edition, toolchain, and every source of gen, then enable keyword_idents_2024. I rename internal symbols; if a public API must retain the spelling, I use the raw identifier r#gen and verify callers in both editions. I run cargo fix --edition for mechanical changes, review macros and public interfaces, update the Cargo manifest manually, and run check, test, docs, and lint across Rust 2021, Rust 2024, and the full feature matrix. Each crate migrates in dependency order with a reversible commit and compatibility report.”

Step-by-step design

1. Separate compiler and edition changes

The compiler can build older editions; an edition selects parsing rules for a crate. Pin the toolchain and lockfile, establish a Rust 2021 baseline, and change one crate’s edition field at a time. This keeps compiler, dependency, and language changes from hiding in one diff.

2. Find every source of gen

Enable keyword_idents_2024 so identifiers valid in an older edition but conflicting in 2024 are reported. Static search must be supplemented with macro expansion, proc-macro tokens, build scripts, and tests. Fix templates before generated code rather than editing generated output.

3. Choose renaming or a raw identifier

Rename internal functions, locals, and modules to improve long-term clarity. If a public API, serialized name, or cross-crate call must retain gen, a raw identifier keeps the symbol addressable in the required edition. Treat it as a migration bridge, not an excuse to preserve confusing internal names forever.

pub fn r#gen() -> IteratorType {
    // implementation
}

fn call() {
    let _items = r#gen();
}

4. Automate fixes with review

cargo fix --edition raises the relevant lint to a warning and applies compiler suggestions such as changing gen to r#gen. It cannot decide product naming or API policy and should not overwrite uncommitted work. Run it on a clean branch, review macro, doctest, generated-code, and public-API diffs, then update the manifest edition manually.

5. Verify edition boundaries

Crates from different editions can link, so migrate workspace crates incrementally. CI should run cargo check, cargo test, cargo doc, clippy, and feature combinations for 2021 and 2024. Add compile-time examples for public APIs so callers do not regress because of a raw identifier or rename.

6. Observe, batch, and roll back

Record lint hits, edition, toolchain, crate, feature, macro source, and failed tests; never log credentials or sensitive input. Migrate leaf crates before widely depended-on crates. Keep old tags, lockfiles, and reproducible builds per batch; if downstream breaks, revert the batch rather than mixing partial fixes by hand.

Model high-quality answer

“An edition changes parsing rules, while a compiler upgrade is a separate variable, so I pin the toolchain and establish a Rust 2021 baseline. I enable keyword_idents_2024 and inventory source, macros, proc macros, build scripts, and tests that produce gen. Internal symbols are renamed; a public API that must keep the spelling uses r#gen, with serialization and cross-crate calls tested. I run cargo fix --edition, review the diff, update the manifest, and run check, test, docs, and clippy across 2021, 2024, and feature combinations. Crates switch in dependency order, with lint and failure telemetry and a reproducible rollback point.”

Common mistakes

• Upgrade only the compiler → edition parsing was never verified → pin a baseline and switch the edition separately.
• Grep only source files → macros or generated code still emit gen → lint at the token-generation source and test expansions.
• Change every name to r#gen → long-term API readability declines → rename internals and reserve raw identifiers for compatibility boundaries.
• Accept cargo fix without review → public APIs or docs can break → review every diff and run cross-edition CI.
• Switch the whole workspace at once → downstream regressions are hard to locate → migrate by dependency topology with rollback points.

Follow-up questions and responses

Can Rust 2024 use gen blocks already?

gen was reserved to leave syntax space for future gen blocks. The migration must not assume that feature is stable; verify the current toolchain documentation and feature status.

Does a raw identifier change ABI or a serialized name?

A raw identifier primarily changes how the parser reads a name. Whether ABI, reflection, serialization, or FFI changes depends on the actual public symbol and generator, so verify those boundaries. Protect external protocols with explicit wire names and compatibility tests.

Why edit Cargo.toml after cargo fix?

cargo fix --edition applies code-level lint suggestions, but the edition field still needs deliberate confirmation and commit. Review the manifest change with test results so no workspace crate is switched accidentally or omitted.