Enable conditional styling for the css prop

.changeset/red-rats-double.md

@@ -0,0 +1,6 @@
+---
+"next-yak": patch
+"yak-swc": patch
+---
+
+Enable conditional styling for the css prop

packages/example/app/page.tsx

@@ -142,6 +142,18 @@ export default function Home() {
         >
           CSS Prop works if this is green
         </p>
+        <p
+          css={css`
+            color: red;
+            ${() =>
+              true &&
+              css`
+                color: green;
+              `}
+          `}
+        >
+          Conditional CSS Prop works if this is green
+        </p>
         <Inputs />
       </main>
     </YakThemeProvider>

packages/next-yak/runtime/__tests__/cssPropTest.tsx

@@ -128,3 +128,61 @@ const StyledComponentWithCSSProp = () => {
     </Text>
   </div>;
 };
+
+const ComponentWithConditionalCSSButWithoutOwnProps = () => {
+  const x = Math.random() > 0.5;
+  return (
+    <div
+      css={css`
+        ${() =>
+          x &&
+          css`
+            padding: 20px;
+          `}
+      `}
+    />
+  );
+};
+
+const ComponentWithConditionalCSSVarsButWithoutOwnProps = () => {
+  const x = Math.random() > 0.5;
+  return (
+    <div
+      css={css`
+        padding: ${() => x && "20px"};
+      `}
+    />
+  );
+};
+
+const ComponentWithDynamicCSSShouldGenerateTypeError = () => {
+  return (
+    <div
+      // @ts-expect-error - properties not supported
+      css={css<{ $primary: boolean }>`
+        padding: ${({ $primary }) => $primary && "20px"};
+      `}
+    />
+  );
+};
+
+const dynamicMixin = css<{ $primary: boolean }>`
+  ${({ $primary }) =>
+    $primary &&
+    css`
+      font-size: 1.7rem;
+    `}
+`;
+
+const ComponentWithCSSThatUsesDynamicMixinShouldGenerateTypeError = () => {
+  return (
+    <div
+      css={css`
+        ${
+          // @ts-expect-error - properties not supported
+          dynamicMixin
+        }
+      `}
+    />
+  );
+};

packages/next-yak/runtime/cssLiteral.tsx

@@ -58,14 +58,13 @@ export type PropsToClassNameFn = (props: unknown) =>
  * Therefore this is only an internal function only and it must be cast to any
  * before exported to the user.
  */
-export function css(styles: TemplateStringsArray, ...values: []): StaticCSSProp;
-export function css<TProps = {}>(
+export function css<TProps>(
   styles: TemplateStringsArray,
   ...values: CSSInterpolation<NoInfer<TProps> & { theme: YakTheme }>[]
-): ComponentStyles<TProps>;
+): TProps extends object ? ComponentStyles<TProps> : StaticCSSProp;
 export function css<TProps>(
   ...args: Array<any>
-): StaticCSSProp | ComponentStyles<TProps> {
+): TProps extends object ? ComponentStyles<TProps> : StaticCSSProp {
   const classNames: string[] = [];
   const dynamicCssFunctions: PropsToClassNameFn[] = [];
   const style: Record<string, string> = {};
@@ -110,9 +109,11 @@ export function css<TProps>(
   // Non Dynamic CSS
   if (dynamicCssFunctions.length === 0) {
     const className = classNames.join(" ");
+    // @ts-expect-error - Conditional return types are tricky in the implementation and generate false positives
     return () => ({ className, style });
   }
 
+  // @ts-expect-error - Conditional return types are tricky in the implementation and generate false positives
   return (props: unknown) => {
     const allClassNames: string[] = [...classNames];
     const allStyles: Record<string, string> = { ...style };

packages/next-yak/runtime/styled.tsx

@@ -129,7 +129,7 @@ const yakStyled = <
       CSSInterpolation<T & NoInfer<TCSSProps> & { theme: YakTheme }>
     >
   ) => {
-    const getRuntimeStyles = css(styles, ...(values as any));
+    const getRuntimeStyles = css<object>(styles, ...(values as any));
     const yak = (props: Substitute<TCSSProps & T, TAttrsIn>) => {
       // if the css component does not require arguments
       // it can be called without arguments and we skip calling useTheme()

packages/yak-swc/yak_swc/src/yak_transforms.rs

@@ -191,17 +191,16 @@ impl YakTransform for TransformCssMixin {
   ) -> YakTransformResult {
     let has_dynamic_content = !runtime_expressions.is_empty() || !runtime_css_variables.is_empty();
 
-    if (self.is_exported || self.is_within_jsx_attribute) && has_dynamic_content {
+    if self.is_exported && has_dynamic_content && !self.is_within_jsx_attribute {
       // For now dynamic mixins are not supported cross file
       // as the scope handling is quite complicated
-      let error_msg = if self.is_exported {
-        "Dynamic mixins must not be exported. Please ensure that this mixin requires no props."
-      } else {
-        "Dynamic mixins must not be used within JSX attributes. Please ensure that this mixin requires no props."
-      };
-
       HANDLER.with(|handler| {
-        handler.struct_span_err(expression.span, error_msg).emit();
+        handler
+          .struct_span_err(
+            expression.span,
+            "Dynamic mixins must not be exported. Please ensure that this mixin requires no props.",
+          )
+          .emit();
       });
     }
 

packages/yak-swc/yak_swc/tests/fixture/css-prop-conditional/input.tsx

@@ -0,0 +1,207 @@
+import { css } from "next-yak";
+
+const Elem = () => {
+  const show = Math.random() > 0.5;
+  return (
+    <div
+      css={css`
+        ${() =>
+          show &&
+          css`
+            color: red;
+          `}
+      `}
+    />
+  );
+};
+
+const Elem2 = () => {
+  const show = Math.random() > 0.5;
+  return (
+    <div
+      css={css`
+        ${() =>
+          show &&
+          css`
+            color: red;
+          `}
+      `}
+      className="test-class"
+    />
+  );
+};
+
+const Elem3 = () => {
+  const show = Math.random() > 0.5;
+  return (
+    <div
+      style={{ padding: "5px" }}
+      css={css`
+        ${() =>
+          show &&
+          css`
+            padding: 10px;
+          `}
+      `}
+    />
+  );
+};
+
+const Elem4 = (props: any) => {
+  const show = Math.random() > 0.5;
+  return (
+    <div
+      css={css`
+        ${() =>
+          show &&
+          css`
+            color: green;
+          `}
+      `}
+      {...props}
+    />
+  );
+};
+
+const Elem5 = (props: any) => {
+  return (
+    <div
+      css={css`
+        ${() =>
+          props.show &&
+          css`
+            color: purple;
+          `}
+      `}
+      {...props.a}
+      {...props.b}
+    />
+  );
+};
+
+const Elem6 = (props: any) => {
+  return (
+    <div
+      css={css`
+        ${() =>
+          props.show &&
+          css`
+            font-size: 16px;
+          `}
+      `}
+      className="main"
+      style={{ fontWeight: "bold" }}
+    />
+  );
+};
+
+const Elem7 = (props: any) => {
+  return (
+    <div
+      css={css`
+        ${() => props.show && css``}
+      `}
+      className="empty-css"
+    />
+  );
+};
+
+const Elem8 = () => {
+  const show = Math.random() > 0.5;
+  return (
+    <div
+      css={css`
+        color: ${() => show && "red"};
+      `}
+    />
+  );
+};
+
+const Elem9 = () => {
+  const show = Math.random() > 0.5;
+  return (
+    <div
+      css={css`
+        color: ${() => show && "red"};
+      `}
+      className="test-class"
+    />
+  );
+};
+
+const Elem10 = () => {
+  const show = Math.random() > 0.5;
+  return (
+    <div
+      style={{ padding: "5px" }}
+      css={css`
+        padding: ${() => show && "10px"};
+      `}
+    />
+  );
+};
+
+const Elem11 = (props: any) => {
+  const show = Math.random() > 0.5;
+  return (
+    <div
+      css={css`
+        color: ${() => show && "green"};
+      `}
+      {...props}
+    />
+  );
+};
+
+const Elem12 = (props: any) => {
+  return (
+    <div
+      css={css`
+        color: ${() => props.show && "purple"};
+      `}
+      {...props.a}
+      {...props.b}
+    />
+  );
+};
+
+const Elem13 = (props: any) => {
+  return (
+    <div
+      css={css`
+        font-size: ${() => props.show && "16px"};
+      `}
+      className="main"
+      style={{ fontWeight: "bold" }}
+    />
+  );
+};
+
+const Elem14 = (props: any) => {
+  return (
+    <div
+      css={css`
+        display: ${() => props.show && "block"};
+      `}
+      className="empty-css"
+    />
+  );
+};
+
+const Elem15 = (props: any) => {
+  return (
+    <div
+      css={css`
+        ${() =>
+          props.a &&
+          css`
+            ${() =>
+              props.b &&
+              css`
+                color: ${() => props.c && "orange"};
+              `}
+          `}
+      `}
+    />
+  );
+};