Fix infinite loop when using @variant inside @custom-variant that points to another @custom-variant (#19633)

This PR fixes an infinite loop when you use a `@variant` inside of a
`@custom-variant`, where the `@variant` used is another
`@custom-variant`.

The issue stems from the fact that a `@custom-variant` can use a `@slot`
that we have to replace with the proper AST nodes. However in this
setup, the AST nodes will include a `@slot` node as well, which causes
us to replace the `@slot` again, and so on, causing an infinite loop.

```css
@custom-variant a {
  @slot;
}

@custom-variant b {
  @variant a {
    @slot;
  }
}
```

The solution here is to replace the `@slot` nodes and then skip walking
the nodes that were just inserted. This does mean that we end up with a
`@slot` node in the final AST but that's not a real issue because that
will get replaced later when handling the next `@custom-variant`.

## Test plan

1. Existing tests still pass
2. Added a regression test to ensure that the infinite loop does not
happen anymore
3. Added additional tests to ensure that the behavior is correct

Thanks @wongjn for your initial debugging help and providing a test case
as well!

Fixes: #19618

CHANGELOG.md

@@ -24,6 +24,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Detect utilities when containing capital letters followed by numbers ([#19465](https://github.com/tailwindlabs/tailwindcss/pull/19465))
 - Fix class extraction for Rails' strict locals ([#19525](https://github.com/tailwindlabs/tailwindcss/pull/19525))
 - Align `@utility` name validation with Oxide scanner rules ([#19524](https://github.com/tailwindlabs/tailwindcss/pull/19524))
+- Fix infinite loop when using `@variant` inside `@custom-variant` ([#19633](https://github.com/tailwindlabs/tailwindcss/pull/19633))
 
 ### Deprecated
 

packages/tailwindcss/src/index.test.ts

@@ -4558,6 +4558,97 @@ describe('@custom-variant', () => {
     `)
   })
 
+  // https://github.com/tailwindlabs/tailwindcss/issues/19618
+  test('@custom-variant can use a @variant that eventually uses another @custom-variant', async () => {
+    expect(
+      await compileCss(
+        css`
+          @custom-variant a {
+            @slot;
+          }
+
+          @custom-variant b {
+            @variant a {
+              @slot;
+            }
+          }
+
+          @tailwind utilities;
+        `,
+        ['a:flex', 'b:flex', 'a:b:flex', 'b:a:flex'],
+      ),
+    ).toMatchInlineSnapshot(`
+      ".a\\:flex, .b\\:flex, .a\\:b\\:flex, .b\\:a\\:flex {
+        display: flex;
+      }"
+    `)
+  })
+
+  test('@custom-variant can use a @variant that eventually uses another @custom-variant (2)', async () => {
+    expect(
+      await compileCss(
+        css`
+          @custom-variant a {
+            .a {
+              @slot;
+            }
+          }
+
+          @custom-variant b {
+            .b {
+              @variant a {
+                .a-inside-b {
+                  @slot;
+                }
+              }
+            }
+          }
+
+          @tailwind utilities;
+        `,
+        ['a:flex', 'b:flex', 'a:b:flex', 'b:a:flex'],
+      ),
+    ).toMatchInlineSnapshot(`
+      ".a\\:flex .a, .b\\:flex .b .a .a-inside-b, .a\\:b\\:flex .a .b .a .a-inside-b, .b\\:a\\:flex .b .a .a-inside-b .a {
+        display: flex;
+      }"
+    `)
+  })
+
+  // https://github.com/tailwindlabs/tailwindcss/issues/19618#issuecomment-3830775912
+  test('@custom-variant can use existing @slot @variants', async () => {
+    expect(
+      await compileCss(
+        css`
+          @custom-variant hocus {
+            @variant hover {
+              @variant focus {
+                @slot;
+              }
+            }
+          }
+
+          @custom-variant hover {
+            &:hover {
+              @slot;
+            }
+
+            &[data-hover] {
+              @slot;
+            }
+          }
+
+          @tailwind utilities;
+        `,
+        ['hocus:flex'],
+      ),
+    ).toMatchInlineSnapshot(`
+      ".hocus\\:flex:hover:focus, .hocus\\:flex[data-hover]:focus {
+        display: flex;
+      }"
+    `)
+  })
+
   test('@custom-variant setup that results in a circular dependency error can be solved', async () => {
     expect(
       await compileCss(

packages/tailwindcss/src/variants.ts

@@ -1196,7 +1196,7 @@ export function substituteAtSlot(ast: AstNode[], nodes: AstNode[]) {
   walk(ast, (node) => {
     // Replace `@slot` with rule nodes
     if (node.kind === 'at-rule' && node.name === '@slot') {
-      return WalkAction.Replace(nodes)
+      return WalkAction.ReplaceSkip(nodes)
     }
 
     // Wrap `@keyframes` and `@property` in `AtRoot` nodes