refactor: apply proper migrations on existing utilities in the system

Instead of creating an ad-hoc static utility for the candidate, we
simplify the candidate to it's base form, then perform the change and
then go back.

E.g.:

1. Incoming string of `hover:blur!`
2. Is turned into the candidate AST for `hover:blur!`
3. Is simplified such that variants and important is stripped
4. Is printed as a string `blur`
5. Find replacement `blur-sm`
6. Parse into candidate AST
7. Re-apply the variants and important
8. Stringified `hover:blur-sm!`

packages/@tailwindcss-upgrade/src/template/codemods/legacy-classes.ts

@@ -11,43 +11,43 @@ import { isSafeMigration } from '../is-safe-migration'
 const __filename = url.fileURLToPath(import.meta.url)
 const __dirname = path.dirname(__filename)
 
-const LEGACY_CLASS_MAP = {
-  shadow: 'shadow-sm',
-  'shadow-sm': 'shadow-xs',
-  'shadow-xs': 'shadow-2xs',
+const LEGACY_CLASS_MAP = new Map([
+  ['shadow', 'shadow-sm'],
+  ['shadow-sm', 'shadow-xs'],
+  ['shadow-xs', 'shadow-2xs'],
 
-  'inset-shadow': 'inset-shadow-sm',
-  'inset-shadow-sm': 'inset-shadow-xs',
-  'inset-shadow-xs': 'inset-shadow-2xs',
+  ['inset-shadow', 'inset-shadow-sm'],
+  ['inset-shadow-sm', 'inset-shadow-xs'],
+  ['inset-shadow-xs', 'inset-shadow-2xs'],
 
-  'drop-shadow': 'drop-shadow-sm',
-  'drop-shadow-sm': 'drop-shadow-xs',
+  ['drop-shadow', 'drop-shadow-sm'],
+  ['drop-shadow-sm', 'drop-shadow-xs'],
 
-  rounded: 'rounded-sm',
-  'rounded-sm': 'rounded-xs',
+  ['rounded', 'rounded-sm'],
+  ['rounded-sm', 'rounded-xs'],
 
-  blur: 'blur-sm',
-  'blur-sm': 'blur-xs',
-}
+  ['blur', 'blur-sm'],
+  ['blur-sm', 'blur-xs'],
+])
 
-const THEME_KEYS = {
-  shadow: '--shadow',
-  'shadow-sm': '--shadow-sm',
-  'shadow-xs': '--shadow-xs',
-  'shadow-2xs': '--shadow-2xs',
+const THEME_KEYS = new Map([
+  ['shadow', '--shadow'],
+  ['shadow-sm', '--shadow-sm'],
+  ['shadow-xs', '--shadow-xs'],
+  ['shadow-2xs', '--shadow-2xs'],
 
-  'drop-shadow': '--drop-shadow',
-  'drop-shadow-sm': '--drop-shadow-sm',
-  'drop-shadow-xs': '--drop-shadow-xs',
+  ['drop-shadow', '--drop-shadow'],
+  ['drop-shadow-sm', '--drop-shadow-sm'],
+  ['drop-shadow-xs', '--drop-shadow-xs'],
 
-  rounded: '--radius',
-  'rounded-sm': '--radius-sm',
-  'rounded-xs': '--radius-xs',
+  ['rounded', '--radius'],
+  ['rounded-sm', '--radius-sm'],
+  ['rounded-xs', '--radius-xs'],
 
-  blur: '--blur',
-  'blur-sm': '--blur-sm',
-  'blur-xs': '--blur-xs',
-}
+  ['blur', '--blur'],
+  ['blur-sm', '--blur-sm'],
+  ['blur-xs', '--blur-xs'],
+])
 
 const DESIGN_SYSTEMS = new DefaultMap((base) => {
   return __unstable__loadDesignSystem('@import "tailwindcss";', { base })
@@ -65,58 +65,91 @@ export async function legacyClasses(
 ): Promise<string> {
   let defaultDesignSystem = await DESIGN_SYSTEMS.get(__dirname)
 
-  for (let candidate of designSystem.parseCandidate(rawCandidate)) {
-    if (candidate.kind === 'functional') {
-      let parts = [candidate.root]
-      if (candidate.value?.kind === 'named') {
-        parts.push(candidate.value.value)
+  function* migrate(rawCandidate: string) {
+    for (let candidate of designSystem.parseCandidate(rawCandidate)) {
+      // Create a base candidate string from the candidate.
+      // E.g.: `hover:blur!` -> `blur`
+      let baseCandidate = structuredClone(candidate) as Candidate
+      baseCandidate.variants = []
+      baseCandidate.important = false
+      let baseCandidateString = printCandidate(designSystem, baseCandidate)
+
+      // Find the new base candidate string. `blur` -> `blur-sm`
+      let newBaseCandidateString = LEGACY_CLASS_MAP.get(baseCandidateString)
+      if (!newBaseCandidateString) continue
+
+      // Parse the new base candidate string into an actual candidate AST.
+      let [newBaseCandidate] = designSystem.parseCandidate(newBaseCandidateString)
+      if (!newBaseCandidate) continue
+
+      // Re-apply the variants and important flag from the original candidate.
+      // E.g.: `hover:blur!` -> `blur` -> `blur-sm` -> `hover:blur-sm!`
+      let newCandidate = structuredClone(newBaseCandidate) as Candidate
+      newCandidate.variants = candidate.variants
+      newCandidate.important = candidate.important
+
+      yield [
+        candidate,
+        newCandidate,
+        THEME_KEYS.get(baseCandidateString),
+        THEME_KEYS.get(newBaseCandidateString),
+      ] as const
+    }
+  }
+
+  for (let [fromCandidate, toCandidate, fromThemeKey, toThemeKey] of migrate(rawCandidate)) {
+    // Every utility that has a simple representation (e.g.: `blur`, `radius`,
+    // etc.`) without variants or special characters _could_ be a potential
+    // problem during the migration.
+    let isPotentialProblematicClass = (() => {
+      if (fromCandidate.variants.length > 0) {
+        return false
       }
 
-      let root = parts.join('-')
-      if (Object.hasOwn(LEGACY_CLASS_MAP, root)) {
-        let newRoot = LEGACY_CLASS_MAP[root as keyof typeof LEGACY_CLASS_MAP]
-
-        if (location && !root.includes('-') && !isSafeMigration(location)) {
-          continue
-        }
-
-        let fromThemeKey = THEME_KEYS[root as keyof typeof THEME_KEYS]
-        let toThemeKey = THEME_KEYS[newRoot as keyof typeof THEME_KEYS]
-
-        if (fromThemeKey && toThemeKey) {
-          // Migrating something that resolves to a value in the theme.
-          let customFrom = designSystem.resolveThemeValue(fromThemeKey)
-          let defaultFrom = defaultDesignSystem.resolveThemeValue(fromThemeKey)
-          let customTo = designSystem.resolveThemeValue(toThemeKey)
-          let defaultTo = defaultDesignSystem.resolveThemeValue(toThemeKey)
-
-          // The new theme value is not defined, which means we can't safely
-          // migrate the utility.
-          if (customTo === undefined) {
-            continue
-          }
-
-          // The "from" theme value changed compared to the default theme value.
-          if (customFrom !== defaultFrom) {
-            continue
-          }
-
-          // The "to" theme value changed compared to the default theme value.
-          if (customTo !== defaultTo) {
-            continue
-          }
-        }
-
-        for (let newCandidate of designSystem.parseCandidate(newRoot)) {
-          let clone = structuredClone(newCandidate) as Candidate
-
-          clone.important = candidate.important
-          clone.variants = candidate.variants
-
-          return printCandidate(designSystem, clone)
-        }
+      if (fromCandidate.kind === 'arbitrary') {
+        return false
       }
+
+      if (fromCandidate.kind === 'static') {
+        return !fromCandidate.root.includes('-')
+      }
+
+      if (fromCandidate.kind === 'functional') {
+        return fromCandidate.value === null || !fromCandidate.root.includes('-')
+      }
+
+      return false
+    })()
+
+    if (location && isPotentialProblematicClass && !isSafeMigration(location)) {
+      continue
     }
+
+    if (fromThemeKey && toThemeKey) {
+      // Migrating something that resolves to a value in the theme.
+      let customFrom = designSystem.resolveThemeValue(fromThemeKey)
+      let defaultFrom = defaultDesignSystem.resolveThemeValue(fromThemeKey)
+      let customTo = designSystem.resolveThemeValue(toThemeKey)
+      let defaultTo = defaultDesignSystem.resolveThemeValue(toThemeKey)
+
+      // The new theme value is not defined, which means we can't safely
+      // migrate the utility.
+      if (customTo === undefined) {
+        continue
+      }
+
+      // The "from" theme value changed compared to the default theme value.
+      if (customFrom !== defaultFrom) {
+        continue
+      }
+
+      // The "to" theme value changed compared to the default theme value.
+      if (customTo !== defaultTo) {
+        continue
+      }
+    }
+
+    return printCandidate(designSystem, toCandidate)
   }
 
   return rawCandidate