perf(handler): centralize HTML attribute writing in ResponseWriter

[Mohamed Mansour (mohamedmansour)]

What

Adds hidden write_attribute / write_boolean_attribute methods to the ResponseWriter trait, backed by centralized append helpers and four macro_rules! generators in a new crates/webui-handler/src/response_writer.rs.

Buffered hosts adopt the macros and drop their hand-written writer implementations. Six duplicated ResponseWriter impls are removed (webui-ffi, webui-python, webui-press, webui-cli, webui-wasm, webui, plus webui-node's BufferedWriter and the demo server's shell writer).

Writers with distinct semantics — StreamingSink, BufferSink, StreamingWriter, and the Node CallbackWriter — implement the methods directly so threshold flushing and stream forwarding are preserved.

Why

Emitting one HTML attribute previously cost five write() calls through a dyn ResponseWriter vtable (" ", name, "=\"", value, "\""). For a buffered host, all five collapse into a direct push_str on the backing String/Vec<u8>.

This is the structural prerequisite for the later handler render-pipeline layers, which are attribute-bound.

Evidence

Correctness — byte-identical output (deterministic)

A temporary harness rendered a mixed protocol (component attrs, attribute templates, boolean attrs, nested for loop with a per-item attribute) at 10 / 100 / 2,000 items, on the merge base and on this branch, using a generic writer that implements only write():

items	revision	write() calls	bytes	FNV-1a
10	origin/main	112	433	489c33c436b03492
10	this branch	112	433	489c33c436b03492
100	origin/main	922	3043	e48fab1207c9a292
100	this branch	922	3043	e48fab1207c9a292
2000	origin/main	18022	63943	ec91d5c009de1b8a
2000	this branch	18022	63943	ec91d5c009de1b8a

Identical hash, byte count, and call count — the write()-based defaults are behaviorally indistinguishable from the previous inline code, so external ResponseWriter implementors are unaffected.

Dispatch reduction — the actual change (deterministic)

Same render, generic writer vs. a macro-generated writer, output asserted equal:

items	generic write() calls	macro write() calls	eliminated	reduction
10	112	52	60	−53.6%
100	922	412	510	−55.3%
2000	18022	8012	10010	−55.5%

Exactly 5 virtual dispatches removed per attribute (2,002 attributes × 5 = 10,010 at the 2,000-item scale).

Wall-clock — no claim made

Commands:

cargo bench -p microsoft-webui-handler --bench handler_bench
cargo bench -p microsoft-webui        --bench contact_book_bench

Ran 3 interleaved base/branch cycles with prebuilt binaries at high priority (24-core dev machine, ~22% background load). The measurements are not usable: run-to-run spread across identical repeated runs of the same binary reached 31%, exceeding every observed between-variant delta (max 13%). contact_book_protocol_parse — pure protobuf decode, untouched by this PR — "regressed" 43% on one pass, confirming the noise floor.

No performance improvement is claimed from timing data. The deterministic dispatch counts above are the evidence for this PR. Wall-clock benchmarking should be repeated on a quiet machine in the layer where it is load-bearing.

Allocation behavior is unchanged: no new allocations are introduced on any path, and the macro writers append into the caller's existing buffer.

Tests

• New generated_string_writer_methods_preserve_exact_attribute_output asserts exact output (" data-id=\"42\" disabled") through the macro path.
• cargo test -p microsoft-webui-handler — 428 + 29 passing.
• cargo xtask check: license-headers, fmt, clippy, proto, deny, test, build, build (wasm), build (examples), bench (validate) all pass.
• The docs step fails with PROJ-C013: Adapter module graph is incomplete or inconsistent from the esbuild projection adapter. This reproduces identically on a pristine origin/main checkout and is unrelated to this PR, which changes no JavaScript.

Notes

• ResponseWriter gains only #[doc(hidden)] methods with defaults — no breaking change for external implementors.
• DESIGN.md documents the macro contract and when a host should implement the methods directly.
• No dependency, Cargo.lock, or pnpm-lock.yaml changes.

[Copilot]

Pull request overview

This PR centralizes HTML attribute emission in webui-handler by adding hidden ResponseWriter methods for quoted and boolean attributes, plus shared append helpers and macro generators to let buffered hosts inline optimized attribute writing and reduce virtual dispatch overhead in the render pipeline.

Changes:

• Added ResponseWriter::write_attribute / write_boolean_attribute (hidden, defaulted) and switched handler attribute emission to call the new method.
• Introduced centralized attribute append helpers + macro_rules! generators, and migrated multiple buffered host writers to macro-generated implementations.
• Updated streaming/callback writers (streaming, WASM, Node) to implement the hidden attribute methods directly to preserve flushing/forwarding semantics; added/updated benches and tests and updated DESIGN.md.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
examples/demo/server/src/shell.rs	Replaces a local ResponseWriter impl with a macro-defined string writer.
DESIGN.md	Documents the new attribute-writing contract and when to override hidden methods.
crates/webui/src/streaming.rs	Implements optimized attribute methods for StreamingWriter while preserving flush behavior.
crates/webui/src/server.rs	Replaces an internal MemWriter with a macro-defined string writer.
crates/webui/benches/contact_book_bench.rs	Fixes bench command docs and adds macro-generated attribute methods to bench writer.
crates/webui-wasm/src/handler.rs	Uses macro-defined string writer and adds optimized attribute methods to CallbackWriter.
crates/webui-python/src/lib.rs	Replaces bytes writer impl with macro-defined bytes writer and updates construction callsite.
crates/webui-press/src/build.rs	Replaces local string writer impl with a macro-defined string writer.
crates/webui-node/src/lib.rs	Replaces buffered writer impl with macro-defined writer and adds optimized attribute methods to callback writer.
crates/webui-handler/src/streaming/owned.rs	Adds optimized attribute methods to BufferSink.
crates/webui-handler/src/streaming/mod.rs	Adds optimized attribute methods to StreamingSink, including buffered component-opening handling.
crates/webui-handler/src/response_writer.rs	New module providing append helpers and macros for defining/augmenting buffered writers.
crates/webui-handler/src/lib.rs	Wires in the new module/exports, adds hidden trait methods, switches write_attr to call them, and adds a regression test.
crates/webui-handler/benches/streaming_hydration_bench.rs	Adds macro-generated attribute methods to bench writer.
crates/webui-handler/benches/handler_bench.rs	Adds macro-generated attribute methods to bench writer.
crates/webui-handler/benches/bootstrap_state_bench.rs	Adds macro-generated attribute methods to bench writer.
crates/webui-ffi/src/lib.rs	Replaces local string writer impl with macro-defined writer and updates construction callsite.
crates/webui-cli/src/commands/serve.rs	Replaces local in-memory writer impl with macro-defined writer and adjusts ownership of output buffer.

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