feat: bun dev target support

MODULE.bazel.lock

@@ -190,7 +190,7 @@
     "//bun:extensions.bzl%bun": {
       "general": {
         "bzlTransitiveDigest": "Q0uQOwFAgAU+etePCZ4TUDO+adLX7Z0EmRLaEsKgncw=",
-        "usagesDigest": "qk1PDh3WICa0VONYKXJLsmWCesNJxz3Jkb/aH/voIeI=",
+        "usagesDigest": "UC4zk8kEwWRiDG5FVQOCFysXcrZ757Jehf3sZgG893w=",
         "recordedInputs": [
           "REPO_MAPPING:,bazel_tools bazel_tools"
         ],

README.md

@@ -72,6 +72,7 @@ load(
 	"@rules_bun//bun:defs.bzl",
 	"bun_binary",
 	"bun_bundle",
+	"bun_dev",
 	"bun_test",
 	"js_library",
 	"ts_library",
@@ -101,3 +102,39 @@ Run one of your bun-backed targets, for example:
 ```bash
 bazel test //path/to:your_bun_test
 ```
+
+## Development mode (`bun_dev`)
+
+Use `bun_dev` for long-running local development with Bun watch mode.
+
+```starlark
+load("@rules_bun//bun:defs.bzl", "bun_dev")
+
+bun_dev(
+	name = "web_dev",
+	entry_point = "src/main.ts",
+)
+```
+
+Run it with:
+
+```bash
+bazel run //path/to:web_dev
+```
+
+`bun_dev` supports:
+
+- `watch_mode = "watch"` (default) for `bun --watch`
+- `watch_mode = "hot"` for `bun --hot`
+- `restart_on = [...]` to force full process restarts when specific files change
+
+### Hybrid Go + Bun + protobuf workflow
+
+For monorepos that mix Go and Bun (including FFI):
+
+1. Run Bun app with native watch/HMR via `bun_dev`.
+2. Put generated artifacts or bridge files in `restart_on` (for example generated JS/TS files from proto/go steps).
+3. Rebuild Go/proto artifacts separately (for example with `ibazel build`) so their output files change.
+4. `bun_dev` detects those `restart_on` changes and restarts Bun, while ordinary JS edits continue to use Bun watch/HMR without full Bazel restarts.
+
+This keeps the fast Bun JS loop while still supporting full restarts when non-JS dependencies change.

bun/defs.bzl

@@ -1,5 +1,7 @@
+"""Public API surface for Bun Bazel rules."""
 load("//internal:bun_binary.bzl", _bun_binary = "bun_binary")
 load("//internal:bun_bundle.bzl", _bun_bundle = "bun_bundle")
+load("//internal:bun_dev.bzl", _bun_dev = "bun_dev")
 load("//internal:bun_test.bzl", _bun_test = "bun_test")
 load("//internal:js_library.bzl", _js_library = "js_library", _ts_library = "ts_library")
 load(":toolchain.bzl", _BunToolchainInfo = "BunToolchainInfo", _bun_toolchain = "bun_toolchain")
@@ -8,6 +10,7 @@ visibility("public")
 
 bun_binary = _bun_binary
 bun_bundle = _bun_bundle
+bun_dev = _bun_dev
 bun_test = _bun_test
 js_library = _js_library
 ts_library = _ts_library

examples/basic/BUILD.bazel

@@ -1,5 +1,13 @@
+load("//bun:defs.bzl", "bun_dev")
+
 package(default_visibility = ["//visibility:public"])
 
 exports_files([
 	"README.md",
+	"main.ts",
 ])
+
+bun_dev(
+    name = "web_dev",
+    entry_point = "main.ts",
+)

examples/basic/README.md

@@ -1,3 +1,11 @@
 # basic example
 
-Placeholder for end-to-end bun rules example.
+Minimal `bun_dev` example.
+
+Run:
+
+```bash
+bazel run //examples/basic:web_dev
+```
+
+This starts Bun in watch mode for `main.ts`.

examples/basic/main.ts

@@ -0,0 +1 @@
+console.log("rules_bun bun_dev example");

internal/BUILD.bazel

@@ -3,6 +3,7 @@ package(default_visibility = ["//visibility:public"])
 exports_files([
 	"bun_binary.bzl",
 	"bun_bundle.bzl",
+	"bun_dev.bzl",
 	"bun_install.bzl",
 	"bun_test.bzl",
 	"js_library.bzl",

internal/bun_dev.bzl

@@ -0,0 +1,142 @@
+"""Rule for running JS/TS scripts with Bun in watch mode for development."""
+
+
+def _bun_dev_impl(ctx):
+    toolchain = ctx.toolchains["//bun:toolchain_type"]
+    bun_bin = toolchain.bun.bun_bin
+    entry_point = ctx.file.entry_point
+
+    restart_watch_paths = "\n".join([path.short_path for path in ctx.files.restart_on])
+
+    launcher = ctx.actions.declare_file(ctx.label.name)
+    ctx.actions.write(
+        output = launcher,
+        is_executable = True,
+        content = """#!/usr/bin/env bash
+set -euo pipefail
+
+runfiles_dir="${{RUNFILES_DIR:-$0.runfiles}}"
+bun_bin="${{runfiles_dir}}/_main/{bun_short_path}"
+entry_point="${{runfiles_dir}}/_main/{entry_short_path}"
+cd "${{runfiles_dir}}/_main"
+
+watch_mode="{watch_mode}"
+if [[ "${{watch_mode}}" == "hot" ]]; then
+    dev_flag="--hot"
+else
+    dev_flag="--watch"
+fi
+
+run_dev() {{
+    exec "${{bun_bin}}" "${{dev_flag}}" run "${{entry_point}}" "$@"
+}}
+
+if [[ {restart_count} -eq 0 ]]; then
+    run_dev "$@"
+fi
+
+readarray -t restart_paths <<'EOF_RESTART_PATHS'
+{restart_watch_paths}
+EOF_RESTART_PATHS
+
+file_mtime() {{
+    local p="$1"
+    if stat -f '%m' "${{p}}" >/dev/null 2>&1; then
+        stat -f '%m' "${{p}}"
+        return 0
+    fi
+    stat -c '%Y' "${{p}}"
+}}
+
+declare -A mtimes
+for rel in "${{restart_paths[@]}}"; do
+    if [[ -e "${{rel}}" ]]; then
+        mtimes["${{rel}}"]="$(file_mtime "${{rel}}")"
+    else
+        mtimes["${{rel}}"]="missing"
+    fi
+done
+
+child_pid=""
+restart_child() {{
+    if [[ -n "${{child_pid}}" ]] && kill -0 "${{child_pid}}" 2>/dev/null; then
+        kill "${{child_pid}}"
+        wait "${{child_pid}}" || true
+    fi
+    "${{bun_bin}}" "${{dev_flag}}" run "${{entry_point}}" "$@" &
+    child_pid=$!
+}}
+
+cleanup() {{
+    if [[ -n "${{child_pid}}" ]] && kill -0 "${{child_pid}}" 2>/dev/null; then
+        kill "${{child_pid}}"
+        wait "${{child_pid}}" || true
+    fi
+}}
+
+trap cleanup EXIT INT TERM
+
+restart_child "$@"
+
+while true; do
+    sleep 1
+    changed=0
+    for rel in "${{restart_paths[@]}}"; do
+        if [[ -e "${{rel}}" ]]; then
+            current="$(file_mtime "${{rel}}")"
+        else
+            current="missing"
+        fi
+        if [[ "${{current}}" != "${{mtimes[${{rel}}]}}" ]]; then
+            mtimes["${{rel}}"]="${{current}}"
+            changed=1
+        fi
+    done
+    if [[ "${{changed}}" -eq 1 ]]; then
+        restart_child "$@"
+    fi
+done
+""".format(
+            bun_short_path = bun_bin.short_path,
+            entry_short_path = entry_point.short_path,
+            watch_mode = ctx.attr.watch_mode,
+            restart_count = len(ctx.files.restart_on),
+            restart_watch_paths = restart_watch_paths,
+        ),
+    )
+
+    transitive_files = []
+    if ctx.attr.node_modules:
+        transitive_files.append(ctx.attr.node_modules[DefaultInfo].files)
+
+    runfiles = ctx.runfiles(
+        files = [bun_bin, entry_point] + ctx.files.data + ctx.files.restart_on,
+        transitive_files = depset(transitive = transitive_files),
+    )
+
+    return [
+        DefaultInfo(
+            executable = launcher,
+            runfiles = runfiles,
+        ),
+    ]
+
+
+bun_dev = rule(
+    implementation = _bun_dev_impl,
+    attrs = {
+        "entry_point": attr.label(
+            mandatory = True,
+            allow_single_file = [".js", ".ts", ".jsx", ".tsx", ".mjs", ".cjs"],
+        ),
+        "watch_mode": attr.string(
+            default = "watch",
+            values = ["watch", "hot"],
+        ),
+        "restart_on": attr.label_list(allow_files = True),
+        "node_modules": attr.label(),
+        "data": attr.label_list(allow_files = True),
+    },
+    executable = True,
+    toolchains = ["//bun:toolchain_type"],
+)