Fix PostCSS crash when using toJSON() (#18083)

When I added source maps to PostCSS I mistakenly assumed that `.source`
on a node could be `undefined`. The comment above the property in
PostCSS says that `source` can be `undefined` but this is a commentary
on the value upon **access** not its expected value on **write**:

```ts
declare abstract class Node_ {
  /**
   * …
   *
   * The nodes that are created manually using the public APIs
   * provided by PostCSS will have `source` undefined and
   * will be absent in the source map.
   *
   * …
   */
  source?: Node.Source
}
```

Rather, what these types mean is that *if the property exists* it must
be defined. But otherwise the property can be missing if a node has no
source location metadata. This generally wasn't a problem with the
string-returning APIs but the `toJSON()` API in PostCSS expects that
`source` is defined if present. This caused a crash because our license
comment doesn't have a source location.

I've addressed this by deleting the `source` property from the node if
source location data is not available.

Fixes #18082

ref parcel-bundler/parcel#10161

Author: thecrypticace

packages/@tailwindcss-postcss/src/ast.ts

@@ -49,6 +49,20 @@ export function cssAstToPostCssAst(ast: AstNode[], source: PostcssSource | undef
     }
   }
 
+  function updateSource(astNode: PostCssChildNode, loc: SourceLocation | undefined) {
+    let source = toSource(loc)
+
+    // The `source` property on PostCSS nodes must be defined if present because
+    // `toJSON()` reads each property and tries to read from source.input if it
+    // sees a `source` property. This means for a missing or otherwise absent
+    // source it must be *missing* from the object rather than just `undefined`
+    if (source) {
+      astNode.source = source
+    } else {
+      delete astNode.source
+    }
+  }
+
   function transform(node: AstNode, parent: PostCssContainerNode) {
     // Declaration
     if (node.kind === 'declaration') {
@@ -57,14 +71,14 @@ export function cssAstToPostCssAst(ast: AstNode[], source: PostcssSource | undef
         value: node.value ?? '',
         important: node.important,
       })
-      astNode.source = toSource(node.src)
+      updateSource(astNode, node.src)
       parent.append(astNode)
     }
 
     // Rule
     else if (node.kind === 'rule') {
       let astNode = postcss.rule({ selector: node.selector })
-      astNode.source = toSource(node.src)
+      updateSource(astNode, node.src)
       astNode.raws.semicolon = true
       parent.append(astNode)
       for (let child of node.nodes) {
@@ -75,7 +89,7 @@ export function cssAstToPostCssAst(ast: AstNode[], source: PostcssSource | undef
     // AtRule
     else if (node.kind === 'at-rule') {
       let astNode = postcss.atRule({ name: node.name.slice(1), params: node.params })
-      astNode.source = toSource(node.src)
+      updateSource(astNode, node.src)
       astNode.raws.semicolon = true
       parent.append(astNode)
       for (let child of node.nodes) {
@@ -90,7 +104,7 @@ export function cssAstToPostCssAst(ast: AstNode[], source: PostcssSource | undef
       // spaces.
       astNode.raws.left = ''
       astNode.raws.right = ''
-      astNode.source = toSource(node.src)
+      updateSource(astNode, node.src)
       parent.append(astNode)
     }