next.js/packages/next/src/build/webpack/plugins/optional-peer-dependency-resolve-plugin.ts

import type { Resolver } from 'webpack'

const pluginSymbol = Symbol('OptionalPeerDependencyResolverPlugin')

export class OptionalPeerDependencyResolverPlugin {
  apply(resolver: Resolver) {
    const target = resolver.ensureHook('raw-module')
    target.tapAsync(
      'OptionalPeerDependencyResolverPlugin',
      (request, resolveContext, callback) => {
        // if we've already recursed into this plugin, we want to skip it
        if ((request as any)[pluginSymbol]) {
          return callback()
        }

        // popping the stack to prevent the recursion check
        resolveContext.stack?.delete(Array.from(resolveContext.stack).pop()!)

        resolver.doResolve(
          target,
          // when we call doResolve again, we need to make sure we don't
          // recurse into this plugin again
          { ...request, [pluginSymbol]: true } as any,
          null,
          resolveContext,
          (err, result) => {
            if (
              !result &&
              request?.descriptionFileData?.peerDependenciesMeta &&
              request.request
            ) {
              const peerDependenciesMeta = request.descriptionFileData
                .peerDependenciesMeta as Record<string, { optional?: boolean }>

              const isOptional =
                peerDependenciesMeta &&
                peerDependenciesMeta[request.request] &&
                peerDependenciesMeta[request.request].optional

              if (isOptional) {
                return callback(null, {
                  path: false,
                })
              }
            }

            return callback(err, result)
          }
        )
      }
    )
  }
}