swc minifier can hit exponential memory: 40 modules ballooned past 40GB in Next.js build

Vjeux · x · 2026-10-13

Christopher Chedeau (Vjeux) used AI to find and fix an swc minifier bug: minifying a chain of constants where each uses the two previous ones takes Fibonacci-like time and memory.
Bundlers produce this shape when scope-hoisting modules; a chain of 40 such modules in a Next.js production build grew past 40GB.
Root cause: replaceseqassignment copies a multi-use variable's initializer into each use when istriviallit accepts it, and istriviallit accepted any tree of binary operators over literals regardless of size, so copies cascade within one mergesequencesinexprs call.
Fix PR #12521 bounds literal expressions copied by the sequential inliner.

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