perf: support only attaching slot scope ids when necessary

This is done by adding the `slotted: false` option to:

- compiler-dom
- compiler-ssr
- compiler-sfc (forwarded to template compiler)

At runtime, only slotted component will render slot fragments with
slot scope Ids. For SSR, only slotted component will add slot scope Ids
to rendered slot content. This should improve both runtime performance
and reduce SSR rendered markup size.

Note: requires SFC tooling (e.g. `vue-loader` and `vite`) to pass on
the `slotted` option from the SFC descriptoer to the `compileTemplate`
call.

packages/compiler-sfc/src/compileTemplate.ts

@@ -45,6 +45,7 @@ export interface SFCTemplateCompileOptions {
   filename: string
   id: string
   scoped?: boolean
+  slotted?: boolean
   isProd?: boolean
   ssr?: boolean
   ssrCssVars?: string[]
@@ -158,6 +159,7 @@ function doCompileTemplate({
   filename,
   id,
   scoped,
+  slotted,
   inMap,
   source,
   ssr = false,
@@ -204,6 +206,7 @@ function doCompileTemplate({
         ? genCssVarsFromList(ssrCssVars, shortId, isProd)
         : '',
     scopeId: scoped ? longId : undefined,
+    slotted,
     ...compilerOptions,
     nodeTransforms: nodeTransforms.concat(compilerOptions.nodeTransforms || []),
     filename,

packages/compiler-sfc/src/parse.ts

@@ -59,6 +59,9 @@ export interface SFCDescriptor {
   styles: SFCStyleBlock[]
   customBlocks: SFCBlock[]
   cssVars: string[]
+  // whether the SFC uses :slotted() modifier.
+  // this is used as a compiler optimization hint.
+  slotted: boolean
 }
 
 export interface SFCParseResult {
@@ -100,7 +103,8 @@ export function parse(
     scriptSetup: null,
     styles: [],
     customBlocks: [],
-    cssVars: []
+    cssVars: [],
+    slotted: false
   }
 
   const errors: (CompilerError | SyntaxError)[] = []
@@ -231,6 +235,10 @@ export function parse(
     warnExperimental(`v-bind() CSS variable injection`, 231)
   }
 
+  // check if the SFC uses :slotted
+  const slottedRE = /(?:::v-|:)slotted\(/
+  descriptor.slotted = descriptor.styles.some(s => slottedRE.test(s.content))
+
   const result = {
     descriptor,
     errors