fix: replace unreachable!/todo! with proper error handling and no-op

[tkshsbcue]

fix: replace unreachable!/todo! with proper error handling and no-op

Summary

Two stability improvements: (1) Replace unreachable!() in the hoistable and lexical declaration parsers with proper Error::general so unexpected tokens produce parse errors instead of panics. (2) Implement access_set for Access::This in the bytecompiler—in sloppy mode, assignment to this is a no-op per ECMAScript; previously todo!() would panic.

Motivation

Parser unreachable! — The hoistable and lexical declaration parsers used unreachable!("unknown token found: {:?}", tok) in their catch-all arms. In theory the caller ensures only valid tokens reach these parsers, but if a bug or edge case produces an unexpected token, the parser would panic. For embedded use and tooling, panics are unacceptable. Returning a parse error lets the host handle the failure gracefully.

Bytecompiler todo! — Assigning to this in sloppy mode (e.g. this = 5) is valid JavaScript; the assignment is a no-op (the RHS is evaluated for side effects only). The bytecompiler had todo!("access_set this"), so executing such code would panic. Implementing the no-op path removes this panic and aligns with the spec.

Changes

Category	Description
Parser	core/parser/src/parser/statement/declaration/hoistable/mod.rs: _ => unreachable!(...) → Err(Error::general("expected 'function', 'async', or 'class' in declaration", tok.span().start()))
Parser	core/parser/src/parser/statement/declaration/lexical.rs: _ => unreachable!(...) → Err(Error::general("expected 'let' or 'const' in lexical declaration", tok.span().start()))
Bytecompiler	core/engine/src/bytecompiler/mod.rs: Access::This => todo!(...) → evaluate RHS via expr_fn(self) for side effects, then no-op (no assignment)

Technical Details

• Files modified: core/parser/.../hoistable/mod.rs, core/parser/.../lexical.rs, core/engine/src/bytecompiler/mod.rs
• Behavioral impact: Parser: unexpected tokens in declaration context now yield parse errors. Bytecompiler: this = expr in sloppy mode now executes without panicking (RHS evaluated, assignment ignored).
• Strict mode: Assignment to this in strict mode is a SyntaxError and is rejected by the parser, so the bytecompiler path is only reached in sloppy mode.

Testing

• cargo test -p boa_parser — all 296 tests pass
• cargo test -p boa_engine --lib — all 921 tests pass
• cargo clippy — no warnings

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,687	49,687	0
Ignored	2,262	2,262	0
Failed	1,014	1,014	0
Panics	0	0	0
Conformance	93.81%	93.81%	0.00%

Tested main commit: 28648a3207b482955f3f93935de1b181ab72c477
Tested PR commit: 9f3b70d6ea1b58550faadc2c215e0a2e2e0dd7e8
Compare commits: 28648a3...9f3b70d

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.93%. Comparing base (6ddc2b4) to head (9f3b70d).
⚠️ Report is 776 commits behind head on main.

Files with missing lines	Patch %	Lines
...parser/src/parser/statement/declaration/lexical.rs	0.00%	4 Missing ⚠️
.../src/parser/statement/declaration/hoistable/mod.rs	0.00%	3 Missing ⚠️
core/engine/src/bytecompiler/mod.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4940       +/-   ##
===========================================
+ Coverage   47.24%   57.93%   +10.69%     
===========================================
  Files         476      557       +81     
  Lines       46892    60944    +14052     
===========================================
+ Hits        22154    35309    +13155     
- Misses      24738    25635      +897     

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[tkshsbcue]

@jedel1043 i hope you can review my PR
Thanks!