Optimisations no longer applied to `_buildManifest.js` when using SWC

What version of Next.js are you using?

12.0.4-canary.4

What version of Node.js are you using?

16.13.0

What browser are you using?

Chrome

What operating system are you using?

macOS

How are you deploying your application?

Vercel

Describe the Bug

After enabling SWC I noticed that buildManfest.json’s output grew significantly (20kb⇢80kb) because it is no longer using optimisations for chunk names, e.g: self.__BUILD_MANIFEST=function(e,s,a,t,c,o,n,i,r,g,l,b,d,u,p,f,h,m,k)

// With SWC off:
ls -lh .next/static/JdI1kHMMIQ4b_kwr2NZiG/_buildManifest.js

-rw-r--r--  1 benschwarz  staff    26K 11 Nov 18:58 .next/static/JdI1kHMMIQ4b_kwr2NZiG/_buildManifest.js


// With SWC on: 
ls -lh .next/static/dtDJS-Od0qktpqIrQBB_S/_buildManifest.js

-rw-r--r--  1 benschwarz  staff    80K 11 Nov 18:51 .next/static/dtDJS-Od0qktpqIrQBB_S/_buildManifest.js

Over the wire (with Gzip compression) the file size difference isn’t anywhere near as dramatic (Repeated strings are of course optimised by Gzip, so they both end up around 6kb), but there may be an impact to JS parse & compile times, which could compromise main thread performance for sites with many pages.

Expected Behavior

Optimisations should be applied to _buildManifest.js to avoid a potential perf regression.

To Reproduce

• Run rm -rf .next
• Run next build
• Observe size of .next/static/<generatedHash>/_buildManifest.js (small)
• Run rm -rf .next
• Turn on SWC using swcMinify: true in next.config.json
• Run next build
• Observe size of .next/static/<generatedHash>/_buildManifest.js (large)

[benschwarz]

Confirmed, LGTM on a new build with 12.0.5-canary.6 👍

[kdy1]

Okay, I found the cause. The inliner inlines strings because those are literal, which is ‘simple enough’. I allowed it because the description of the google closure compiler said (in time of porting passes from it) they inline all strings because they focus on the gzipped size, but I can’t find it currently.

I’ll see if google closure compiler still inlines all string.