fix(es/minifier): Preserve pure IIFE annotations - #12167
fix(es/minifier): Preserve pure IIFE annotations#12167mrlonely (mameikagou) wants to merge 3 commits into
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Merging this PR will not alter performance
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| let dont_invoke_pure_iife = | ||
| !self.mode.preserve_vars() && self.can_drop_unused_pat(&var.name); |
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For an unused binding such as let {...rest} = /*#__PURE__*/ function (v) { /* large body */ return v }({}), can_drop_unused_pat returns true because the rest identifier has no usages, so this disables IIFE invocation and adds the persistent no-inline mark. However, take_pat_if_unused immediately preserves every object pattern containing a rest property, meaning the declaration cannot be removed; the condition is recomputed on every pass, so the previously eligible IIFE remains uninlined and retains its full body in minified output. Only enable this protection when unused-pattern cleanup can actually remove the pattern.
AGENTS.md reference: AGENTS.md:L7-L7
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| let dont_invoke_pure_iife = | ||
| !self.mode.preserve_vars() && self.can_drop_unused_pat(&var.name); |
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This computes can_drop_unused_pat for every variable declarator, including declarations without an initializer and initializers that are not annotated IIFEs. That helper calls find_pat_ids once through can_drop_unused_vars and then again itself, allocating and walking every binding pattern twice on each optimizer pass even though the result is only needed for the rare pure-IIFE case. Check that var.init is an annotated function/arrow call first, then perform the usage scan only for that case to avoid adding repeated hot-path work to ordinary minification.
AGENTS.md reference: AGENTS.md:L7-L7
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| if self.ctx.bit_ctx.contains(BitCtx::DontInvokePureIife) | ||
| && call.ctxt.has_mark(self.marks.pure) | ||
| { | ||
| log_abort!("iife: Has pure annotation"); | ||
| return false; |
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With an unused binding such as let {x} = /*#__PURE__*/ function () { return null }(), this guard preserves the pure call until take_pat_if_unused, which treats the annotation as permission to remove the whole pattern and call. The annotation only says invoking the function has no side effects; destructuring its null result still throws a TypeError, so the optimized program incorrectly continues instead of throwing. Preserve the destructuring operation unless its returned value is known to be safely destructurable.
AGENTS.md reference: crates/swc_ecma_minifier/AGENTS.md:L34-L36
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| if !matches!( | ||
| expr, | ||
| Expr::Call(..) | Expr::New(..) | Expr::TaggedTpl(..) | Expr::Seq(..) |
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When comments are retained and an annotation is accompanied by another leading comment, such as let dead = /*! license */ /*#__PURE__*/ (() => {})(console.log("effect")), this branch keeps the surviving call's argument span instead of the removed IIFE span. The code generator only takes leading comments at emitted node spans, so both comments remain attached to the now-unreferenced IIFE position and the license/preserve comment disappears. Move or selectively remove the PURE annotation while transferring other comments to the replacement expression.
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It seems fine if we don't invoke any IIFE with pure annotation at all. It's very rare to have pure annotations and Terser behaves the same. |
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Description:
Pure annotations on unused IIFEs could be lost when the IIFE was inlined before unused-variable cleanup, leaving assignments that dead-code elimination could not remove.
This preserves annotated IIFEs while optimizing initializers whose bindings are already known to be removable. Later cleanup can then remove the call while retaining argument and sequence side effects. Live bindings remain eligible for the existing optimizations.
Related issue (if exists):
Closes #12165
Tests:
cargo test -p swc_ecma_minifier./scripts/exec.sh terser_reduce_vars_iifecargo clippy -p swc_ecma_minifier --features concurrent --tests -- -D warningscargo fmt --all -- --check