## Summary
Introduces `oxc_diagnostics::Diagnostics` — a `Vec<OxcDiagnostic>` newtype that knows the difference between errors and warnings — and makes it the standard diagnostics return value across oxc.
- **New type**: `Diagnostics` with `has_errors()`, `has_warnings()`, `errors()`, `warnings()`, plus `Vec`-like ergonomics (Deref, `IntoIterator`, `Extend`, `From`/`Into<Vec>`) so most call sites are unchanged.
- **Return types**: `ParserReturn`, `SemanticBuilderReturn`, `IsolatedDeclarationsReturn`, the React Compiler `TransformResult`/`LintResult`, and `TransformerReturn` now expose `diagnostics: Diagnostics` instead of `errors: Vec<OxcDiagnostic>` (the React Compiler's separate `errors`/`warnings` are merged into one severity-tagged list).
- **Behaviour**: a React Compiler **error** now stops the transform (no output), while **warnings** flow through and the transform continues. `compile()` aborts codegen only when `diagnostics.has_errors()` — warnings alone no longer abort (previously any diagnostic did).
## Breaking change
Consumers reading `*.errors` on these return values must read `*.diagnostics`. React Compiler consumers that read `.errors`/`.warnings` separately should use `diagnostics.has_errors()` / `diagnostics.errors()` / `diagnostics.warnings()`.
Parser/semantic/isolated-declarations fatality is otherwise unchanged. Builds on the React Compiler transform feature (#23201).
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Replaces ad-hoc `SemanticBuilder::new().with_*()` chains with the `new_linter()` / `new_compiler()` presets where the preset already expands to the exact same option set, so the config lives in one place.
- linter example + linter benchmark → `new_linter()` (cfg + class table + syntax)
- semantic benchmark, cfg example, integration test util → `new_compiler()` (syntax)
Behavior-neutral — each preset matches the options the callsite set by hand. Carved out of the larger store-optional refactor so it can land independently on `main` first.
Replace `#[allow]` with `#[expect]` through the whole codebase.
This also surfaced many places where the `#[allow]` attributes were outdated/pointless and could be removed entirely.
Only places `#[allow]` remains are:
1. In generated code and macros, where lint errors may or may not be triggered depending on the generated code.
2. A few places where clippy malfunctions. These use e.g. `#[allow(unused, clippy::allow_attributes)]`.
Realized we can get the source type from the AST.
The next PR will introduce `unambiguous` to `SourceType` and directly set `Program::source_type` to either `script` or `module`.
part of #3213
We should only have one diagnostic struct instead 353 copies of them, so we don't end up choking LLVM with 50k lines of the same code due to monomorphization.
If the proposed approach is good, then I'll start writing a codemod to turn all the existing structs to plain functions.
---
Background:
Using `--timings`, we see `oxc_linter` is slow on codegen (the purple part).
The crate currently contains 353 miette errors. [cargo-llvm-lines](https://github.com/dtolnay/cargo-llvm-lines) displays
```
cargo llvm-lines -p oxc_linter --lib --release
Lines Copies Function name
----- ------ -------------
830350 33438 (TOTAL)
29252 (3.5%, 3.5%) 808 (2.4%, 2.4%) <alloc::boxed::Box<T,A> as core::ops::drop::Drop>::drop
23298 (2.8%, 6.3%) 353 (1.1%, 3.5%) miette::eyreish::error::object_downcast
19062 (2.3%, 8.6%) 706 (2.1%, 5.6%) core::error::Error::type_id
12610 (1.5%, 10.1%) 65 (0.2%, 5.8%) alloc::raw_vec::RawVec<T,A>::grow_amortized
12002 (1.4%, 11.6%) 706 (2.1%, 7.9%) miette::eyreish::ptr::Own<T>::boxed
9215 (1.1%, 12.7%) 115 (0.3%, 8.2%) core::iter::traits::iterator::Iterator::try_fold
9150 (1.1%, 13.8%) 1 (0.0%, 8.2%) oxc_linter::rules::RuleEnum::read_json
8825 (1.1%, 14.9%) 353 (1.1%, 9.3%) <miette::eyreish::error::ErrorImpl<E> as core::error::Error>::source
8822 (1.1%, 15.9%) 353 (1.1%, 10.3%) miette::eyreish::error::<impl miette::eyreish::Report>::construct
8119 (1.0%, 16.9%) 353 (1.1%, 11.4%) miette::eyreish::error::object_ref
8119 (1.0%, 17.9%) 353 (1.1%, 12.5%) miette::eyreish::error::object_ref_stderr
7413 (0.9%, 18.8%) 353 (1.1%, 13.5%) <miette::eyreish::error::ErrorImpl<E> as core::fmt::Display>::fmt
7413 (0.9%, 19.7%) 353 (1.1%, 14.6%) miette::eyreish::ptr::Own<T>::new
6669 (0.8%, 20.5%) 39 (0.1%, 14.7%) alloc::raw_vec::RawVec<T,A>::try_allocate_in
6173 (0.7%, 21.2%) 353 (1.1%, 15.7%) miette::eyreish::error::<impl miette::eyreish::Report>::from_std
6027 (0.7%, 21.9%) 70 (0.2%, 16.0%) <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter
6001 (0.7%, 22.7%) 353 (1.1%, 17.0%) miette::eyreish::error::object_drop
6001 (0.7%, 23.4%) 353 (1.1%, 18.1%) miette::eyreish::error::object_drop_front
5648 (0.7%, 24.1%) 353 (1.1%, 19.1%) <miette::eyreish::error::ErrorImpl<E> as core::fmt::Debug>::fmt
```
It's totalling more than 50k llvm lines, and is putting pressure on rustc codegen (the purple part on `oxc_linter` in the image above.
---
It's pretty obvious by looking at https://github.com/zkat/miette/blob/main/src/eyreish/error.rs, the generics can expand out to lots of code.
Boshen