Week 4 - The Error Model¶

4.1 Conceptual Core¶

Rust has two error mechanisms, not one:
Result<T, E>-recoverable errors, encoded in the type system.
Panics-unrecoverable, stack-unwinding (or aborting) bugs.
? is not exception handling. It is sugar for match plus From::from on the error variant. Internalize this; it is the difference between a clean error model and a re-implementation of Java exceptions.
panic = "abort" vs panic = "unwind": choose abort for binaries that own their process, unwind for libraries that may be embedded in a host that wants to catch (e.g., a Python extension).

4.2 Mechanical Detail¶

The shape of an idiomatic library error: an enum with #[non_exhaustive], thiserror::Error for derivation, and From impls for upstream errors.
The shape of an idiomatic application error: anyhow::Result at boundaries, typed errors internally. The split is deliberate: libraries owe their callers structured errors; applications owe their operators readable context.
Result::map_err and ? - chain ergonomics. The anti-pattern ofunwrap()outsidemain/tests/build.rs`.
#[track_caller]-the attribute that makes panic locations attribute to the caller rather than the panicking function. Why every helper that may panic should carry it.

4.3 Lab-"A Library With Two Faces"¶

Build parse-units: a small crate that parses strings like "3.5 GiB" into a structured Quantity. Requirements: - Public API returns Result<Quantity, ParseError> where ParseError is a thiserror enum with at least four variants. - Internally, use ? to compose. No unwrap allowed except in unit tests. - Provide a binary parse-units-cli that uses anyhow and prints rich context with .with_context(|| ...). - Ship 100% line coverage measured by cargo-llvm-cov.

4.4 Idiomatic & Clippy Drill¶

clippy::result_large_err (errors >128 bytes hurt the happy path), clippy::map_err_ignore, clippy::question_mark, clippy::unwrap_used, clippy::expect_used. Enable the last two as deny in libraries.

4.5 Production Hardening Slice¶

Add a panic = "abort" release profile and a panic = "unwind" test profile. Confirm the binary shrinks under abort. Add RUST_BACKTRACE=1 to your dev shell.
Wire up cargo audit and cargo deny check to CI. Both must pass on green main.

Month 1 Capstone Deliverable¶

A workspace foundations/ with three crates: 1. parse-units (week 4 lab) as a publishable library. 2. parse-units-cli as the application binary. 3. layout-forensics (week 2 lab) as an internal-only tool.

CI must run: cargo fmt --check, cargo clippy -D warnings, cargo test, cargo llvm-cov, cargo deny check, cargo audit. The workspace's release profile must enable lto = "fat", codegen-units = 1, panic = "abort", strip = "symbols". Document the resulting binary size in the workspace README.