Error on invalid @plugin usage (#14134)

This PR just adds two minor errors to guard against invalid `@plugin`
usage similarly to what we do with `@source` in
#14078.

Author: philipp-spiess

packages/tailwindcss/src/index.test.ts

@@ -1258,6 +1258,44 @@ describe('Parsing themes values from CSS', () => {
 })
 
 describe('plugins', () => {
+  test('@plugin can not have a body.', () => {
+    expect(() =>
+      compile(
+        css`
+          @plugin {
+            color: red;
+          }
+        `,
+        {
+          loadPlugin: () => {
+            return ({ addVariant }) => {
+              addVariant('hocus', '&:hover, &:focus')
+            }
+          },
+        },
+      ).build(['hocus:underline']),
+    ).toThrowErrorMatchingInlineSnapshot(`[Error: \`@plugin\` cannot have a body.]`)
+  })
+
+  test('@plugin cannot be nested.', () => {
+    expect(() =>
+      compile(
+        css`
+          div {
+            @plugin "my-plugin";
+          }
+        `,
+        {
+          loadPlugin: () => {
+            return ({ addVariant }) => {
+              addVariant('hocus', '&:hover, &:focus')
+            }
+          },
+        },
+      ).build(['hocus:underline']),
+    ).toThrowErrorMatchingInlineSnapshot(`[Error: \`@plugin\` cannot be nested.]`)
+  })
+
   test('addVariant with string selector', () => {
     let compiled = compile(
       css`

packages/tailwindcss/src/index.ts

@@ -78,7 +78,15 @@ export function compile(
     if (node.kind !== 'rule') return
 
     // Collect paths from `@plugin` at-rules
-    if (node.selector.startsWith('@plugin ')) {
+    if (node.selector === '@plugin' || node.selector.startsWith('@plugin ')) {
+      if (node.nodes.length > 0) {
+        throw new Error('`@plugin` cannot have a body.')
+      }
+
+      if (parent !== null) {
+        throw new Error('`@plugin` cannot be nested.')
+      }
+
       plugins.push(loadPlugin(node.selector.slice(9, -1)))
       replaceWith([])
       return