chore(build): document faster rebuilds with cargo plonk (#21823)

tools/faster-rebuilds.md

@@ -0,0 +1,71 @@
+# Faster rebuilds with [`cargo-plonk`](https://crates.io/crates/cargo-plonk)
+
+This document describes how to use `cargo plonk` to speed up Deno rebuilds for
+faster dev cycles.
+
+```
+cargo install cargo-plonk
+```
+
+## How it works
+
+Plonk works by hot swapping symbols using a fresh dynamic library of the local
+crates.
+
+## Usage
+
+First, compile Deno normally.
+
+```
+cargo build -p deno [--release]
+```
+
+Run the following command to start watching for changes in `ext/webgpu` crate
+and hot swap `init_ops_and_esm` function into the previously built `deno` bin.
+
+```
+cargo plonk run \
+  --package deno_webgpu \
+  --symbol init_ops_and_esm \
+  --bin deno \
+  --watch
+```
+
+> Important:
+>
+> Currently, this will only works for symbols that have been "materialized" in
+> their crates. Cross-crate generics will not work.
+
+You can use `cargo plonk run` to re-run commands on changes.
+
+```
+cargo plonk run -v \
+  -p deno_webgpu \
+  -s init_ops_and_esm \
+  -b deno \
+  --watch \
+  -- eval "await navigator.gpu.requestAdapter()" --unstable
+```
+
+Comparing incremental compile times for `ext/webgpu` on Mac M1:
+
+| profile   | `cargo build` | `cargo plonk build` |
+| --------- | ------------- | ------------------- |
+| `debug`   | 42 s          | 0.5s                |
+| `release` | 5 mins 12 s   | 2s                  |
+
+## Debugging
+
+Use the `-v`/`--verbose` flag to turn on debug info.
+
+```
+    Finished dev [unoptimized + debuginfo] target(s) in 8.86s
+[*] Running: DYLD_INSERT_LIBRARIES="/Users/divy/gh/plonk/target/release/build/cargo-plonk-dd0f08c90ca82109/out/inject.dylib" DYLD_LIBRARY_PATH="/Users/divy/.rustup/toolchains/1.75.0-aarch64-apple-darwin/lib" NEW_SYMBOL="_ZN11deno_webgpu11deno_webgpu16init_ops_and_esm17h683ed96f45027bc1E" PLONK_BINARY="/Users/divy/gh/deno/target/debug/deno" PLONK_LIBRARY="/Users/divy/gh/deno/target/debug/libdeno_webgpu.dylib" SYMBOL="_ZN11deno_webgpu11deno_webgpu16init_ops_and_esm17h6907fcd8be7e215eE" VERBOSE="y" "/Users/divy/gh/deno/target/debug/deno" "eval" "await navigator.gpu.requestAdapter()" "--unstable"
+[*] Plonking _ZN11deno_webgpu11deno_webgpu16init_ops_and_esm17h6907fcd8be7e215eE in /Users/divy/gh/deno/target/debug/libdeno_webgpu.dylib
+[*] Old address: 0x105fcff2c
+[*] New address: 0x128511424
+===
+```
+
+Report any bugs and feature requests in the `cargo-plonk` issue tracker:
+https://github.com/littledivy/plonk/issues/new