[Prototype] Feature-gate heavy derives (MallocSizeOf, ToShmem, SpecifiedValueInfo, ToTyped) to improve compile times

[nicoburns]

Summary

Prototype / experiment — makes the heavy trait derives in the stylo crate optional via cargo features, to reduce compile times for embedders that don't need them. Defaults are unchanged: all features stay on by default (and are forced on by gecko), so existing consumers compile the exact same code.

New features on the stylo crate:

Feature	Gates	Needed for
malloc_size_of	derive(MallocSizeOf) expansions (~860 sites) + memory-reporting fns	memory reporting
to_shmem	derive(ToShmem) expansions + selectors/to_shmem + ToShmem impls	Gecko shared-memory UA sheets
specified_value_info	derive(SpecifiedValueInfo) + the per-property SUPPORTED_TYPES/completion-keyword tables (supports_type, collect_property_completion_keywords)	devtools value autocomplete
typed_om	derive(ToTyped), the typed_om module, and all reification code	Typed OM

default = ["servo", "malloc_size_of", "to_shmem", "specified_value_info", "typed_om"]. To disable the gates: default-features = false, features = ["servo"].

Mechanism

Rather than sprinkling #[cfg_attr(feature = ..., derive(...))] over ~940 derive sites (huge diff, and awkward to keep in sync with upstream), this exploits the existing central mod derives re-export hub in style/lib.rs: when a feature is off, the derive name is re-pointed at a no-op derive macro (added to style_derive) that accepts the same helper attributes (#[shmem(...)], #[value_info(...)], #[typed(...)], #[ignore_malloc_size_of]) but expands to nothing:

#[cfg(feature = "malloc_size_of")]
pub(crate) use malloc_size_of_derive::MallocSizeOf;
#[cfg(not(feature = "malloc_size_of"))]
pub(crate) use style_derive::NoopMallocSizeOf as MallocSizeOf;

This skips both the derive expansion and the type-checking of the expanded impls, which is where the compile time goes. The rest of the diff is #[cfg(feature = ...)] on the handwritten trait impls, imports, and call sites that require the derived impls to exist (typed_om reification code, PropertyDeclarationBlock: ToShmem, the property value-info tables in properties.mako.rs, memory-reporting entry points, etc.). The traits themselves remain defined unconditionally, so generic bounds still compile.

Notes:

• selectors/to_shmem moved from the servo feature into the new to_shmem feature.
• CalcNodeLeaf's ToTyped supertrait is replaced by a CalcLeafToTyped alias that is literally ToTyped when typed_om is on, and a blanket-implemented empty trait when off.
• With gates off, some imports/helpers become unused; a crate-level cfg_attr(..., allow(unused_imports, dead_code)) (active only when any gate is off) keeps that config warning-free without churning hundreds of import lines. Default builds keep full warnings.
• All four derives were tractable to gate, including MallocSizeOf — the no-op-derive approach avoided the per-site yak-shave.

Compile-time measurements

2-core Linux VM, rustc 1.95.0, debug profile. "Gated off" = -p stylo --no-default-features --features servo.

Measurement	Default (all on)	Gated off	Δ
Cold cargo build (workspace)	98.2 s	91.0 s	−7%
stylo crate unit (cargo --timings)	55.6 s	48.7 s	−12%
Cold cargo check (workspace)	42.7 s	39.2 s	−8%
touch style/lib.rs && cargo check	9.0 s	7.7 s	−15%

Default-feature times are identical to pre-change baseline (98 s / 42 s / 9 s), confirming default behavior is unchanged. The win is real but more modest than the derive counts might suggest — the remaining frontend time is dominated by the always-on derives (ToCss, ToComputedValue, ToResolvedValue, Parse, Animate, ...) and general type-checking of the ~276k-line crate.

Validation

• cargo check (default features): clean
• cargo check -p stylo --no-default-features --features servo: clean (no warnings)
• cargo fmt --check: clean

The gecko configuration compiles the same code as before (it force-enables all four features) but wasn't buildable here (needs bindgen/mozbuild). Intermediate feature combinations (e.g. typed_om off but specified_value_info on) haven't been exhaustively tested.

Link to Devin session: https://app.devin.ai/sessions/e8fbaf225d8a40cea95584e606d17c72
Requested by: @nicoburns

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring