feat(plugin): add tsdownConfig and tsdownConfigResolved plugin hooks (#918)

* feat(plugin): add `tsdownConfig` and `tsdownConfigResolved` plugin hooks

Introduce two tsdown-specific per-plugin lifecycle hooks modeled on Vite's
`config` and `configResolved`:

- `tsdownConfig(config, env)` — called before the user config is resolved.
  May mutate in place or return a partial `UserConfig` to be deep-merged.
- `tsdownConfigResolved(resolvedConfig)` — read-only notification fired once
  per produced `ResolvedConfig` (per output format).

Both hooks are detected on user plugins via duck-typing, so existing
Rolldown plugins continue to work unchanged. A `TsdownPlugin` interface,
`TsdownConfigEnv` interface, and `flattenPlugins` helper are exported
from `tsdown/plugins`.

* refactor

* refactor(plugin): simplify `TsdownConfigEnv` to only `inlineConfig`

`watch` and `cwd` are already reachable via the first `config` argument
passed to `tsdownConfig`, so keeping them in env duplicates data and risks
going stale. Drop both — `inlineConfig` remains because it is the only
piece of context that cannot be derived from `UserConfig`.

* refactor(plugin): replace `TsdownConfigEnv` with `InlineConfig`

The env object only wrapped a single `inlineConfig` field, so pass the
`InlineConfig` directly as the second argument instead. Drops the
`TsdownConfigEnv` interface and its re-exports.

* feat(plugin): add `TsdownPluginOption` type

Mirrors Rolldown's `RolldownPluginOption` but with `TsdownPlugin` as the
atom, so that `UserConfig.plugins` entries get proper type-checking for
the new `tsdownConfig` / `tsdownConfigResolved` hooks.

* fix

* update

* refactor

Author: sxzz

__snapshots__/tsnapi/config.snapshot.d.ts

@@ -24,7 +24,7 @@ export interface UserConfig {
   checks?: ChecksOptions & {
     legacyCjs?: boolean;
   };
-  plugins?: InputOptions["plugins"];
+  plugins?: TsdownPluginOption;
   inputOptions?: InputOptions | ((_: InputOptions, _: NormalizedFormat, _: {
     cjsDts: boolean;
   }) => Awaitable<InputOptions | void | null>);

__snapshots__/tsnapi/index.snapshot.d.ts

@@ -125,6 +125,10 @@ export interface TsdownHooks {
     chunks: RolldownChunk[];
   }) => void | Promise<void>;
 }
+export interface TsdownPlugin<A = any> extends Plugin<A> {
+  tsdownConfig?: (_: UserConfig, _: InlineConfig) => Awaitable<UserConfig | void | null>;
+  tsdownConfigResolved?: (_: ResolvedConfig) => Awaitable<void>;
+}
 export interface UserConfig {
   entry?: TsdownInputOption;
   deps?: DepsConfig;
@@ -148,7 +152,7 @@ export interface UserConfig {
   checks?: ChecksOptions & {
     legacyCjs?: boolean;
   };
-  plugins?: InputOptions["plugins"];
+  plugins?: TsdownPluginOption;
   inputOptions?: InputOptions | ((_: InputOptions, _: NormalizedFormat, _: {
     cjsDts: boolean;
   }) => Awaitable<InputOptions | void | null>);
@@ -244,6 +248,9 @@ export type RolldownChunk = (OutputChunk | OutputAsset) & {
 };
 export type Sourcemap = boolean | "inline" | "hidden";
 export type TsdownInputOption = Arrayable<string | Record<string, Arrayable<string>>>;
+export type TsdownPluginOption<A = any> = Awaitable<TsdownPlugin<A> | RolldownPlugin<A> | {
+  name: string;
+} | undefined | null | void | false | TsdownPluginOption<A>[]>;
 export type UserConfigExport = Awaitable<Arrayable<UserConfig> | UserConfigFn>;
 export type UserConfigFn = (_: InlineConfig, _: {
   ci: boolean;

src/config/options.ts

@@ -10,6 +10,7 @@ import { resolveDepsConfig } from '../features/deps.ts'
 import { resolveEntry } from '../features/entry.ts'
 import { validateSea } from '../features/exe.ts'
 import { hasExportsTypes } from '../features/pkg/exports.ts'
+import { flattenPlugins } from '../features/plugin.ts'
 import { resolveTarget } from '../features/target.ts'
 import { resolveTsconfig } from '../features/tsconfig.ts'
 import {
@@ -44,6 +45,19 @@ export async function resolveUserConfig(
   userConfig: UserConfig,
   inlineConfig: InlineConfig,
 ): Promise<ResolvedConfig[]> {
+  // Dispatch `tsdownConfig` hook on user plugins before any resolution work.
+  // Plugins are snapshotted: new plugins added by a hook don't re-dispatch,
+  // preventing infinite recursion and matching Vite's `config` semantics.
+  {
+    const flat = await flattenPlugins(userConfig.plugins)
+    for (const plugin of flat) {
+      const result = await plugin.tsdownConfig?.(userConfig, inlineConfig)
+      if (result) {
+        userConfig = mergeConfig(userConfig, result)
+      }
+    }
+  }
+
   let {
     entry,
     format,
@@ -323,7 +337,7 @@ export async function resolveUserConfig(
     ? (Object.keys(format) as Format[])
     : resolveComma(toArray<Format>(format, 'esm'))
 
-  return formats.map((fmt, idx): ResolvedConfig => {
+  const resolvedConfigs = formats.map((fmt, idx): ResolvedConfig => {
     const once = idx === 0
     const overrides = objectFormat ? format[fmt] : undefined
     return {
@@ -336,6 +350,18 @@ export async function resolveUserConfig(
       ...overrides,
     }
   })
+
+  // Dispatch `tsdownConfigResolved` hook. Re-flatten from the final plugin
+  // list so plugins added during `tsdownConfig` (via fromVite or in-place
+  // mutation) still participate. Fires once per resolved format.
+  for (const resolved of resolvedConfigs) {
+    const finalPlugins = await flattenPlugins(resolved.plugins)
+    for (const plugin of finalPlugins) {
+      await plugin.tsdownConfigResolved?.(resolved)
+    }
+  }
+
+  return resolvedConfigs
 }
 
 /** filter env variables by prefixes */

src/config/types.ts

@@ -22,6 +22,7 @@ import type {
 import type { AttwOptions } from '../features/pkg/attw.ts'
 import type { ExportsOptions } from '../features/pkg/exports.ts'
 import type { PublintOptions } from '../features/pkg/publint.ts'
+import type { TsdownPlugin, TsdownPluginOption } from '../features/plugin.ts'
 import type { ReportOptions } from '../features/report.ts'
 import type { RolldownChunk, TsdownBundle } from '../utils/chunks.ts'
 import type { Logger, LogLevel } from '../utils/logger.ts'
@@ -118,6 +119,8 @@ export type {
   TreeshakingOptions,
   TsdownBundle,
   TsdownHooks,
+  TsdownPlugin,
+  TsdownPluginOption,
   UnusedOptions,
 }
 
@@ -328,7 +331,7 @@ export interface UserConfig {
     legacyCjs?: boolean
   }
 
-  plugins?: InputOptions['plugins']
+  plugins?: TsdownPluginOption
 
   /**
    * Use with caution; ensure you understand the implications.