Add --stream flag to canonicalize subcommand (#19796)

## Summary

- Adds `--stream` flag to `tailwindcss canonicalize` that reads
candidate groups from stdin line by line and writes canonicalized
results to stdout
- Keeps the design system loaded across requests, making it suitable as
a long-running sidecar process
- Empty lines pass through, keeping request/response pairs aligned

## Motivation

Non-JS tools (formatters, editor plugins, etc.) currently have no
lightweight way to canonicalize Tailwind classes. The existing batch
mode works for one-off use, but tools that need to canonicalize
repeatedly pay the cost of loading the design system each time.

With `--stream`, a tool can start `tailwindcss canonicalize --stream`
once and send candidate groups over stdin as needed:

```sh
$ echo -e "py-3 p-1 px-3\nmt-2 mr-2 mb-2 ml-2" | tailwindcss canonicalize --stream
p-3
m-2
```

Related discussion:
https://github.com/tailwindlabs/tailwindcss/discussions/19736

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

packages/@tailwindcss-cli/src/commands/canonicalize/canonicalize.test.ts

@@ -1,7 +1,8 @@
 import path from 'node:path'
+import { PassThrough, Readable } from 'node:stream'
 import { fileURLToPath } from 'node:url'
 import { describe, expect, test } from 'vitest'
-import { runCommandLine } from '.'
+import { runCommandLine, streamStdin } from '.'
 import { normalizeWindowsSeparators } from '../../utils/test-helpers'
 
 let css = normalizeWindowsSeparators(
@@ -114,4 +115,61 @@ describe('runCommandLine', { timeout: 30_000 }, () => {
     expect(result.stderr).toBe('No candidate groups provided')
     expect(result.stdout).toContain('Usage:')
   })
+
+  test('streams canonicalized output line by line', async () => {
+    let input = Readable.from('py-3 p-1 px-3\nmt-2 mr-2 mb-2 ml-2\n')
+    let { stream: output, collect: collectOutput } = createOutput()
+
+    await streamStdin({ css, cwd: path.dirname(css), format: 'text', input, output })
+
+    expect(collectOutput()).toBe('p-3\nm-2\n')
+  })
+
+  test('streams empty lines as empty lines', async () => {
+    let input = Readable.from('py-3 p-1 px-3\n\nmt-2 mr-2 mb-2 ml-2\n')
+    let { stream: output, collect: collectOutput } = createOutput()
+
+    await streamStdin({ css, cwd: path.dirname(css), format: 'text', input, output })
+
+    expect(collectOutput()).toBe('p-3\n\nm-2\n')
+  })
+
+  test('streams json output when requested', async () => {
+    let input = Readable.from('py-3 p-1 px-3\nmt-2 mr-2 mb-2 ml-2\n')
+    let { stream: output, collect: collectOutput } = createOutput()
+
+    await streamStdin({ css, cwd: path.dirname(css), format: 'json', input, output })
+
+    expect(JSON.parse(collectOutput())).toEqual([
+      {
+        input: 'py-3 p-1 px-3',
+        output: 'p-3',
+        changed: true,
+      },
+      {
+        input: 'mt-2 mr-2 mb-2 ml-2',
+        output: 'm-2',
+        changed: true,
+      },
+    ])
+  })
+
+  test('streams jsonl output when requested', async () => {
+    let input = Readable.from('py-3 p-1 px-3\nmt-2 mr-2 mb-2 ml-2\n')
+    let { stream: output, collect: collectOutput } = createOutput()
+
+    await streamStdin({ css, cwd: path.dirname(css), format: 'jsonl', input, output })
+
+    expect(collectOutput()).toBe(
+      '{"input":"py-3 p-1 px-3","output":"p-3","changed":true}\n' +
+        '{"input":"mt-2 mr-2 mb-2 ml-2","output":"m-2","changed":true}\n',
+    )
+  })
 })
+
+function createOutput() {
+  let stream = new PassThrough()
+  let chunks: Buffer[] = []
+  stream.on('data', (chunk) => chunks.push(chunk))
+  return { stream, collect: () => Buffer.concat(chunks).toString() }
+}

packages/@tailwindcss-cli/src/commands/canonicalize/index.ts

@@ -1,6 +1,8 @@
 import { __unstable__loadDesignSystem } from '@tailwindcss/node'
 import fs from 'node:fs/promises'
 import path from 'node:path'
+import { createInterface } from 'node:readline'
+import type { Readable, Writable } from 'node:stream'
 import { compare } from '../../../../tailwindcss/src/utils/compare'
 import { segment } from '../../../../tailwindcss/src/utils/segment'
 import { args, type Arg } from '../../utils/args'
@@ -37,6 +39,10 @@ function usageWithCss() {
   return 'tailwindcss canonicalize --css input.css [classes...]'
 }
 
+function usageWithStream() {
+  return 'tailwindcss canonicalize --stream [--css input.css]'
+}
+
 export function options() {
   return {
     '--css': {
@@ -50,6 +56,11 @@ export function options() {
       default: 'text',
       values: ['text', 'json', 'jsonl'],
     },
+    '--stream': {
+      type: 'boolean',
+      description: 'Read candidate groups from stdin line by line and write results to stdout',
+      default: false,
+    },
   } satisfies Arg
 }
 
@@ -78,15 +89,27 @@ export async function runCommandLine({
       argv,
     )
 
+    let format = parseFormat(flags['--format'] ?? 'text')
+
     if ((stdoutIsTTY && argv.length === 0) || flags['--help']) {
       return {
         exitCode: 0,
-        stdout: helpMessage(),
+        stdout: helpMessage() ?? '',
         stderr: '',
       }
     }
 
-    let format = parseFormat(flags['--format'])
+    if (flags['--stream']) {
+      await streamStdin({
+        css: flags['--css'],
+        cwd,
+        format,
+        input: process.stdin,
+        output: process.stdout,
+      })
+      return { exitCode: 0, stdout: '', stderr: '' }
+    }
+
     let inputs = flags._.length > 0 ? flags._ : await readCandidateGroups({ stdin, stdinIsTTY })
 
     if (inputs.length === 0) {
@@ -113,6 +136,60 @@ export async function runCommandLine({
   }
 }
 
+export async function streamStdin({
+  css: cssFile,
+  cwd,
+  format,
+  input,
+  output,
+}: {
+  css: string | null
+  cwd: string
+  format: OutputFormat
+  input: Readable
+  output: Writable
+}): Promise<void> {
+  let designSystem = await loadDesignSystem(cssFile, cwd)
+  let rl = createInterface({ input })
+  let first = true
+
+  if (format === 'json') {
+    output.write('[')
+  }
+
+  for await (let line of rl) {
+    let result = createCandidateGroupResult(designSystem, line)
+
+    switch (format) {
+      case 'text': {
+        output.write(result.output + '\n')
+        break
+      }
+
+      case 'jsonl': {
+        output.write(JSON.stringify(result) + '\n')
+        break
+      }
+
+      case 'json': {
+        if (first) {
+          output.write('\n')
+        } else {
+          output.write(',\n')
+        }
+
+        output.write(indent(JSON.stringify(result, null, 2), 2))
+        first = false
+        break
+      }
+    }
+  }
+
+  if (format === 'json') {
+    output.write(first ? ']' : '\n]')
+  }
+}
+
 export function readCandidateGroups({
   stdin,
   stdinIsTTY,
@@ -155,24 +232,7 @@ export async function processCandidateGroups({
 }): Promise<CandidateGroupResult[]> {
   let designSystem = await loadDesignSystem(css, cwd)
 
-  return inputs.map((input) => {
-    let originalCandidates = splitCandidates(input)
-    let outputCandidates = sortCandidates(
-      designSystem,
-      designSystem.canonicalizeCandidates(originalCandidates, {
-        collapse: true,
-        logicalToPhysical: true,
-      }),
-    )
-
-    let output = outputCandidates.join(' ')
-
-    return {
-      input,
-      output,
-      changed: output !== input,
-    }
-  })
+  return inputs.map((input) => createCandidateGroupResult(designSystem, input))
 }
 
 export function formatCandidateResults(results: CandidateGroupResult[], format: OutputFormat) {
@@ -189,7 +249,7 @@ export function formatCandidateResults(results: CandidateGroupResult[], format:
 function helpMessage() {
   return help({
     render: false,
-    usage: [usage(), usageWithCss()],
+    usage: [usage(), usageWithCss(), usageWithStream()],
     options: {
       ...options(),
       ...sharedOptions,
@@ -232,7 +292,7 @@ function parseFormat(input: string): OutputFormat {
 function usageError(message: string): RunCommandLineResult {
   return {
     exitCode: 1,
-    stdout: helpMessage(),
+    stdout: helpMessage() ?? '',
     stderr: message,
   }
 }
@@ -246,11 +306,37 @@ function splitCandidates(input: string) {
     .filter((candidate) => candidate.length > 0)
 }
 
-function sortCandidates(
+function canonicalize(
   designSystem: Awaited<ReturnType<typeof __unstable__loadDesignSystem>>,
-  candidates: string[],
+  input: string,
 ) {
-  return defaultSort(designSystem.getClassOrder(candidates))
+  let candidates = splitCandidates(input)
+  candidates = designSystem.canonicalizeCandidates(candidates, {
+    collapse: true,
+    logicalToPhysical: true,
+  })
+  return defaultSort(designSystem.getClassOrder(candidates)).join(' ')
+}
+
+function createCandidateGroupResult(
+  designSystem: Awaited<ReturnType<typeof __unstable__loadDesignSystem>>,
+  input: string,
+): CandidateGroupResult {
+  let output = canonicalize(designSystem, input)
+
+  return {
+    input,
+    output,
+    changed: output !== input,
+  }
+}
+
+function indent(input: string, size: number) {
+  let prefix = ' '.repeat(size)
+  return input
+    .split('\n')
+    .map((line) => prefix + line)
+    .join('\n')
 }
 
 function defaultSort(entries: [string, bigint | null][]) {