fix(module): drop AST and source text after module initialization

[ashnaaseth2325-oss]

Summary

This PR fixes a memory retention issue in the module system where the parsed AST (boa_ast::Module) and the module source text were kept in memory even after compilation finished.

These structures are only required during the module initialization phase (initialize_environment). However, they were previously stored as permanent fields inside ModuleCode, meaning every loaded module kept its entire AST and source text allocated for the lifetime of the process.

In workloads that load many modules, this can lead to steady memory growth over time.

---

Where the Issue Occurred

The problem originates in:

core/engine/src/module/source.rs

ModuleCode previously stored the full AST and source text directly:

struct ModuleCode {
    has_tla: bool,
    requested_modules: IndexSet<ModuleRequest>,
    source: boa_ast::Module,
    source_text: SourceText,
}

Both values are only used inside SourceTextModule::initialize_environment(), but they remained allocated afterward.

Since modules are cached by SimpleModuleLoader, the AST and source text would stay in memory indefinitely.

---

Fix

The AST and source text are now stored as optional values so they can be consumed and dropped once compilation is complete.

source: RefCell<Option<boa_ast::Module>>,
source_text: RefCell<Option<SourceText>>,

They are extracted during initialization:

let source = self.code.source.borrow_mut().take()
    .expect("module source consumed before initialize_environment");

After this point, the AST and source text are no longer retained by the module.

---

Impact

This prevents modules from permanently holding large AST structures that are no longer needed after compilation.

In module-heavy workloads (such as large dependency graphs or repeated module loading), this keeps memory usage stable instead of growing with every loaded module.

---

Result

• AST memory is released immediately after module linking
• Modules retain only the data needed for execution
• Long-running Boa processes avoid unnecessary memory growth

[hansl]

You should be able to just call take() on the RefCell here. It is implemented when default is implemented which is the case for an Option.

[ashnaaseth2325-oss]

Hello @hansl @jedel1043
Updated the implementation to use RefCell::take() instead of borrow_mut().take(). Thanks for pointing that out.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,862	52,862	0
Passed	49,627	49,627	0
Ignored	2,263	2,263	0
Failed	972	972	0
Panics	0	0	0
Conformance	93.88%	93.88%	0.00%

Tested PR commit: de97da8d9a3f970a008b0ba866e5607d9c744a2e

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.19%. Comparing base (6ddc2b4) to head (de97da8).
⚠️ Report is 756 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4855      +/-   ##
==========================================
+ Coverage   47.24%   57.19%   +9.94%     
==========================================
  Files         476      554      +78     
  Lines       46892    60473   +13581     
==========================================
+ Hits        22154    34585   +12431     
- Misses      24738    25888    +1150     

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[jedel1043]

This kinda makes it impossible to re-evaluate a module again. Might be fine I guess