Repository: tailwindlabs/tailwindcss
File: packages/tailwindcss/src/compat/apply-compat-hooks.ts (TypeScript)

Code before:
    if (node.selector === '@config' || node.selector.startsWith('@config ')) {
      if (node.nodes.length > 0) {
        throw new Error('`@config` cannot have a body.')
      }

      if (parent !== null) {
        throw new Error('`@config` cannot be nested.')
      }

      configPaths.push({ id: node.selector.slice(9, -1), base: context.base })
      replaceWith([])
      return
    }
  })

  // Override `resolveThemeValue` with a version that is backwards compatible
  // with dot notation paths like `colors.red.500`. We could do this by default
  // in `resolveThemeValue` but handling it here keeps all backwards
  // compatibility concerns localized to our compatibility layer.
  let resolveThemeVariableValue = designSystem.resolveThemeValue

  designSystem.resolveThemeValue = function resolveThemeValue(path: string) {
    if (path.startsWith('--')) {
      return resolveThemeVariableValue(path)
    }

    // Extract an eventual modifier from the path. e.g.:
    // - "colors.red.500 / 50%" -> "50%"
    let lastSlash = path.lastIndexOf('/')
    let modifier: string | null = null
    if (lastSlash !== -1) {
      modifier = path.slice(lastSlash + 1).trim()
      path = path.slice(0, lastSlash).trim() as ThemeKey
    }

    let themeValue = lookupThemeValue(designSystem.theme, path)

    // Apply the opacity modifier if present
    if (modifier && themeValue) {
      return withAlpha(themeValue, modifier)
    }

    return themeValue
  }

  // If there are no plugins or configs registered, we don't need to register
  // any additional backwards compatibility hooks.
  if (!pluginPaths.length && !configPaths.length) return

  let [configs, pluginDetails] = await Promise.all([
    Promise.all(

Review comment (suggestion):
We should have a guard on the design system so we only do this one time. Right now for _any_ missed value this will do a good bit of additional work which ideally would only happen one time.

Also, right now there are some things that are not idempotent so you'd end up with multiple sets of the same keyframes in your theme depending on how many times you called the theme function with a non-CSS key path.

So this _definitely_ needs to run at most one time.

Code after:
      }

      if (parent !== null) {
        throw new Error('`@config` cannot be nested.')
      }

      configPaths.push({ id: node.selector.slice(9, -1), base: context.base })
      replaceWith([])
      return
    }
  })

  // Override `resolveThemeValue` with a version that is backwards compatible
  // with dot notation paths like `colors.red.500`. We could do this by default
  // in `resolveThemeValue` but handling it here keeps all backwards
  // compatibility concerns localized to our compatibility layer.
  let resolveThemeVariableValue = designSystem.resolveThemeValue

  designSystem.resolveThemeValue = function resolveThemeValue(path: string) {
    if (path.startsWith('--')) {
      return resolveThemeVariableValue(path)
    }

    // If the theme value is not found in the simple resolver, we upgrade to the full backward
    // compatibility support implementation of the `resolveThemeValue` function.
    upgradeToFullPluginSupport({
      designSystem,
      base,
      ast,
      globs,
      configs: [],
      pluginDetails: [],
    })
    return designSystem.resolveThemeValue(path)
  }

  // If there are no plugins or configs registered, we don't need to register
  // any additional backwards compatibility hooks.
  if (!pluginPaths.length && !configPaths.length) return

  let [configs, pluginDetails] = await Promise.all([
    Promise.all(
      configPaths.map(async ({ id, base }) => {
        let loaded = await loadModule(id, base, 'config')
        return {
          path: id,
          base: loaded.base,
          config: loaded.module as UserConfig,
        }
      }),
    ),