Allow newlines and tabs in the argument list of the theme() function (#14917)

We noticed an issue that the `theme()` function wourld not properly
parse in CSS if you split the argument list over multiple lines. This is
fixed by treating `\n` and `\t` the same as space:

```css
.custom-font {
  font-family: theme(
    fontFamily.unknown,
    Helvetica Neue,
    Helvetica,
    sans-serif
  );
}
```

## Test plan

Added tests, but also tried it in the Vite example:

<img width="1995" alt="Screenshot 2024-11-08 at 13 46 09"
src="https://github.com/user-attachments/assets/f9bf94b0-3f9b-4334-8911-9190987e2df5">

CHANGELOG.md

@@ -31,6 +31,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure adjacent rules are merged together after handling nesting when generating optimized CSS ([#14873](https://github.com/tailwindlabs/tailwindcss/pull/14873))
 - Rebase `url()` inside imported CSS files when using Vite ([#14877](https://github.com/tailwindlabs/tailwindcss/pull/14877))
 - Ensure that CSS transforms from other Vite plugins correctly work in full builds (e.g. `:deep()` in Vue) ([#14871](https://github.com/tailwindlabs/tailwindcss/pull/14871))
+- Ensure the CSS `theme()` function handles newlines and tabs in its arguments list ([#14917](https://github.com/tailwindlabs/tailwindcss/pull/14917))
 - Don't unset keys like `--inset-shadow-*` when unsetting keys like `--inset-*` ([#14906](https://github.com/tailwindlabs/tailwindcss/pull/14906))
 - Ensure spacing utilities with no value (e.g. `px` or `translate-y`) don't generate CSS ([#14911](https://github.com/tailwindlabs/tailwindcss/pull/14911))
 - _Upgrade (experimental)_: Install `@tailwindcss/postcss` next to `tailwindcss` ([#14830](https://github.com/tailwindlabs/tailwindcss/pull/14830))

packages/@tailwindcss-upgrade/src/codemods/migrate-border-compatibility.ts

@@ -134,6 +134,11 @@ function substituteFunctionsInValue(
     if (node.kind === 'function' && node.value === 'theme') {
       if (node.nodes.length < 1) return
 
+      // Ignore whitespace before the first argument
+      if (node.nodes[0].kind === 'separator' && node.nodes[0].value.trim() === '') {
+        node.nodes.shift()
+      }
+
       let pathNode = node.nodes[0]
       if (pathNode.kind !== 'word') return
 

packages/@tailwindcss-upgrade/src/template/codemods/theme-to-var.ts

@@ -236,6 +236,11 @@ function substituteFunctionsInValue(
     if (node.kind === 'function' && node.value === 'theme') {
       if (node.nodes.length < 1) return
 
+      // Ignore whitespace before the first argument
+      if (node.nodes[0].kind === 'separator' && node.nodes[0].value.trim() === '') {
+        node.nodes.shift()
+      }
+
       let pathNode = node.nodes[0]
       if (pathNode.kind !== 'word') return
 

packages/tailwindcss/src/css-functions.test.ts

@@ -473,6 +473,26 @@ describe('theme function', () => {
           }"
         `)
       })
+
+      test('theme(\n\tfontFamily.unknown,\n\tHelvetica Neue,\n\tHelvetica,\n\tsans-serif\n)', async () => {
+        expect(
+          // prettier-ignore
+          await compileCss(css`
+            .fam {
+              font-family: theme(
+                fontFamily.unknown,
+                Helvetica Neue,
+                Helvetica,
+                sans-serif
+              );
+            }
+          `),
+        ).toMatchInlineSnapshot(`
+          ".fam {
+            font-family: Helvetica Neue, Helvetica, sans-serif;
+          }"
+        `)
+      })
     })
 
     describe('recursive theme()', () => {

packages/tailwindcss/src/css-functions.ts

@@ -42,6 +42,11 @@ export function substituteFunctionsInValue(
         )
       }
 
+      // Ignore whitespace before the first argument
+      if (node.nodes[0].kind === 'separator' && node.nodes[0].value.trim() === '') {
+        node.nodes.shift()
+      }
+
       let pathNode = node.nodes[0]
       if (pathNode.kind !== 'word') {
         throw new Error(

packages/tailwindcss/src/value-parser.test.ts

@@ -52,6 +52,22 @@ describe('parse', () => {
     ])
   })
 
+  it('should parse a function with multiple arguments across lines', () => {
+    expect(parse('theme(\n\tfoo,\n\tbar\n)')).toEqual([
+      {
+        kind: 'function',
+        value: 'theme',
+        nodes: [
+          { kind: 'separator', value: '\n\t' },
+          { kind: 'word', value: 'foo' },
+          { kind: 'separator', value: ',\n\t' },
+          { kind: 'word', value: 'bar' },
+          { kind: 'separator', value: '\n' },
+        ],
+      },
+    ])
+  })
+
   it('should parse a function with nested arguments', () => {
     expect(parse('theme(foo, theme(bar))')).toEqual([
       {

packages/tailwindcss/src/value-parser.ts

@@ -109,13 +109,15 @@ const CLOSE_PAREN = 0x29
 const COLON = 0x3a
 const COMMA = 0x2c
 const DOUBLE_QUOTE = 0x22
+const EQUALS = 0x3d
+const GREATER_THAN = 0x3e
+const LESS_THAN = 0x3c
+const NEWLINE = 0x0a
 const OPEN_PAREN = 0x28
 const SINGLE_QUOTE = 0x27
-const SPACE = 0x20
-const LESS_THAN = 0x3c
-const GREATER_THAN = 0x3e
-const EQUALS = 0x3d
 const SLASH = 0x2f
+const SPACE = 0x20
+const TAB = 0x09
 
 export function parse(input: string) {
   input = input.replaceAll('\r\n', '\n')
@@ -144,11 +146,13 @@ export function parse(input: string) {
       // ```
       case COLON:
       case COMMA:
-      case SPACE:
-      case SLASH:
-      case LESS_THAN:
+      case EQUALS:
       case GREATER_THAN:
-      case EQUALS: {
+      case LESS_THAN:
+      case NEWLINE:
+      case SLASH:
+      case SPACE:
+      case TAB: {
         // 1. Handle everything before the separator as a word
         // Handle everything before the closing paren as a word
         if (buffer.length > 0) {
@@ -169,11 +173,13 @@ export function parse(input: string) {
           if (
             peekChar !== COLON &&
             peekChar !== COMMA &&
-            peekChar !== SPACE &&
-            peekChar !== SLASH &&
-            peekChar !== LESS_THAN &&
+            peekChar !== EQUALS &&
             peekChar !== GREATER_THAN &&
-            peekChar !== EQUALS
+            peekChar !== LESS_THAN &&
+            peekChar !== NEWLINE &&
+            peekChar !== SLASH &&
+            peekChar !== SPACE &&
+            peekChar !== TAB
           ) {
             break
           }