Isolate tracing from wrap domain logic (#309)

Cargo.toml

@@ -9,6 +9,11 @@ description = """
 `mdtablefix` unb0rks and reflows Markdown tables so that each column has a uniform width. When \
 the `--wrap` option is used, it also wraps paragraphs and list items to 80 columns."""
 
+[features]
+# Exposes internal inline-wrapping shims and fixtures used only by
+# `benches/wrap_observer.rs`. Never enable in production; it is not part of the
+# public API and only affects `#[doc(hidden)]` benchmark support code.
+bench-internals = []
 [package.metadata.binstall]
 
 [package.metadata.binstall.overrides.'cfg(all(target_os = "linux", any(target_arch = "x86_64", target_arch = "aarch64"), target_env = "gnu"))']
@@ -44,6 +49,11 @@ predicates = "3"
 trybuild = "1"
 tracing-test = "0.2"
 test-macros = { path = "test-macros" }
+criterion = { version = "0.5", default-features = false, features = ["cargo_bench_support"] }
 
+[[bench]]
+name = "wrap_observer"
+harness = false
+required-features = ["bench-internals"]
 [lints.clippy]
 pedantic = "warn"

Makefile

@@ -1,4 +1,4 @@
-.PHONY: help all clean test build release lint typecheck fmt check-fmt check-ripgrep check-static-regexes markdownlint nixie
+.PHONY: help all clean test bench build release lint typecheck fmt check-fmt check-ripgrep check-static-regexes markdownlint nixie
 
 APP ?= mdtablefix
 CARGO ?= $(or $(shell command -v cargo 2>/dev/null),$(HOME)/.cargo/bin/cargo)
@@ -19,6 +19,9 @@ clean: ## Remove build artifacts
 test: ## Run tests with warnings treated as errors
 	RUSTFLAGS="-D warnings" $(CARGO) test --all-targets --all-features $(BUILD_JOBS)
 
+bench: ## Run the observer-boundary wrapping benchmarks
+	$(CARGO) bench --features bench-internals --bench wrap_observer
+
 target/%/$(APP): ## Build binary in debug or release mode
 	$(CARGO) build $(BUILD_JOBS) $(if $(findstring release,$(@)),--release) --bin $(APP)
 

benches/wrap_observer.rs

@@ -0,0 +1,57 @@
+//! Benchmarks for the inline-wrapping observer boundary.
+//!
+//! These benchmarks protect the performance invariant introduced with the
+//! observer/tracing adapter (see `docs/adrs/0006-observer-boundary-for-tracing.md`):
+//! with tracing disabled, `TracingObserver` must add no derived-payload work —
+//! Unicode length counts or snippet truncation — beyond a single
+//! `tracing::enabled!` branch per event. Comparing the `observer_none` and
+//! `tracing_observer_disabled` inline cases makes any regression in that
+//! invariant visible as a growing gap between the two.
+//!
+//! Run with:
+//!
+//! ```sh
+//! cargo bench --features bench-internals --bench wrap_observer
+//! ```
+
+use std::hint::black_box;
+
+use criterion::{Criterion, criterion_group, criterion_main};
+use mdtablefix::{
+    wrap::bench_internals::{
+        BENCH_WIDTH,
+        large_inline_paragraph,
+        realistic_markdown_document,
+        wrap_with_tracing_observer,
+        wrap_without_observer,
+    },
+    wrap_text,
+};
+
+/// Benchmarks the public `wrap_text` path over a large realistic document.
+///
+/// `wrap_text` always drives the wrapping pipeline through `TracingObserver`,
+/// so this measures the library boundary with no subscriber installed.
+fn bench_public_document(c: &mut Criterion) {
+    let document = realistic_markdown_document();
+    c.bench_function("wrap_text_realistic_document", |b| {
+        b.iter(|| wrap_text(black_box(&document), black_box(BENCH_WIDTH)));
+    });
+}
+
+/// Benchmarks the inline hot path with the observer disabled (`None`) and with
+/// `TracingObserver` while no subscriber is installed.
+fn bench_inline_observer_paths(c: &mut Criterion) {
+    let paragraph = large_inline_paragraph();
+    let mut group = c.benchmark_group("inline_wrapping");
+    group.bench_function("observer_none", |b| {
+        b.iter(|| wrap_without_observer(black_box(&paragraph), black_box(BENCH_WIDTH)));
+    });
+    group.bench_function("tracing_observer_disabled", |b| {
+        b.iter(|| wrap_with_tracing_observer(black_box(&paragraph), black_box(BENCH_WIDTH)));
+    });
+    group.finish();
+}
+
+criterion_group!(benches, bench_public_document, bench_inline_observer_paths);
+criterion_main!(benches);