fix(transformer_plugins): unwrap ChainExpression after define replacement removes optional markers

[IWANABETHATGUY]

Summary

• Fix panic when ReplaceGlobalDefines removes all optional markers from a ChainExpression, causing the optional chaining lowering transformer to hit unreachable!()
• Fix incorrect test expectation for process?.env[0] with process.env -> {}

Root Cause

When ReplaceGlobalDefines replaces a member expression inside a ChainExpression (e.g., process?.env[0] with define process.env -> {}), the inner StaticMemberExpression that carried optional: true is replaced by {}. This leaves a ChainExpression wrapper with no optional elements. When the optional chaining transformer later tries to lower this (target < ES2020), it panics at transform_chain_expression_impl in crates/oxc_transformer/src/es2020/optional_chaining.rs because it expects at least one optional element in the chain.

This affects the playground pipeline where define runs before the transformer.

Fix

After walk_expression in visit_expression, check if the current expression is a ChainExpression with no remaining optional markers. If so, unwrap it to a plain expression (e.g., ChainExpression(ComputedMemberExpression(...)) becomes ComputedMemberExpression(...)), producing a valid AST.

Test plan

• Added integration test define_then_transform_optional_chain that reproduces the exact panic (define first, then transformer with ES2019 target)

• cargo test -p oxc_transformer_plugins — all 42 integration tests pass

• just lint passes clean

[IWANABETHATGUY]

playground reproduction:

https://playground.oxc.rs/?options=%7B%22run%22%3A%7B%22lint%22%3Atrue%2C%22formatter%22%3Afalse%2C%22transform%22%3Atrue%2C%22isolatedDeclarations%22%3Afalse%2C%22whitespace%22%3Afalse%2C%22mangle%22%3Afalse%2C%22compress%22%3Afalse%2C%22scope%22%3Atrue%2C%22symbol%22%3Atrue%2C%22cfg%22%3Afalse%7D%2C%22parser%22%3A%7B%22extension%22%3A%22tsx%22%2C%22allowReturnOutsideFunction%22%3Atrue%2C%22preserveParens%22%3Atrue%2C%22allowV8Intrinsics%22%3Atrue%2C%22semanticErrors%22%3Atrue%7D%2C%22linter%22%3A%7B%7D%2C%22formatter%22%3A%7B%22useTabs%22%3Afalse%2C%22tabWidth%22%3A2%2C%22endOfLine%22%3A%22lf%22%2C%22printWidth%22%3A80%2C%22singleQuote%22%3Afalse%2C%22jsxSingleQuote%22%3Afalse%2C%22quoteProps%22%3A%22as-needed%22%2C%22trailingComma%22%3A%22all%22%2C%22semi%22%3Atrue%2C%22arrowParens%22%3A%22always%22%2C%22bracketSpacing%22%3Atrue%2C%22bracketSameLine%22%3Afalse%2C%22objectWrap%22%3A%22preserve%22%2C%22singleAttributePerLine%22%3Afalse%7D%2C%22transformer%22%3A%7B%22target%22%3A%22es2015%22%2C%22useDefineForClassFields%22%3Atrue%2C%22experimentalDecorators%22%3Atrue%2C%22emitDecoratorMetadata%22%3Atrue%7D%2C%22isolatedDeclarations%22%3A%7B%22stripInternal%22%3Afalse%7D%2C%22codegen%22%3A%7B%22normal%22%3Atrue%2C%22jsdoc%22%3Atrue%2C%22annotation%22%3Atrue%2C%22legal%22%3Atrue%7D%2C%22compress%22%3A%7B%7D%2C%22mangle%22%3A%7B%22topLevel%22%3Atrue%2C%22keepNames%22%3Afalse%7D%2C%22controlFlow%22%3A%7B%22verbose%22%3Afalse%7D%2C%22inject%22%3A%7B%22inject%22%3A%7B%7D%7D%2C%22define%22%3A%7B%22define%22%3A%7B%22process.env%22%3A%22%7B%7D%22%7D%7D%7D&code=console.log%28process%3F.env%5B0%5D%29%3B

Note: It can only be reproduced when ReplaceGlobalDefines are running before the transformer.

[Copilot]

Pull request overview

Fixes a panic in the transformer pipeline when ReplaceGlobalDefines rewrites an optional chain in a way that leaves a ChainExpression wrapper without any remaining optional markers (notably in playground-like “define first, then lower” flows).

Changes:

• Unwrap Expression::ChainExpression nodes after define-walking when the chain no longer contains any optional: true markers.
• Add an integration test that reproduces the define-then-transform optional-chaining lowering scenario (target < ES2020).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
crates/oxc_transformer_plugins/src/replace_global_defines.rs	Adds post-walk normalization to unwrap “empty” ChainExpressions and helper logic to detect remaining optional markers.
crates/oxc_transformer_plugins/tests/integrations/replace_global_defines.rs	Adds an integration test that runs defines before transformer lowering to ensure the prior panic path is covered.