fix(script): drop AST after script execution

[ashnaaseth2325-oss]

Context

This PR extends the idea introduced in #4855.

PR #4855 fixed a memory retention issue for modules, where the parsed AST and source text were kept in memory even after module initialization.

This PR applies the same idea to scripts, ensuring that the parsed AST is released once script execution has completed.

Summary

This PR prevents scripts from keeping their parsed AST after compilation.

Right now Script::Inner stores the full AST in the source field:

struct Inner {
    realm: Realm,
    source: boa_ast::Script,
    source_text: SourceText,
    codeblock: GcRefCell<Option<Gc<CodeBlock>>>,
}

The AST is only needed while generating bytecode in Script::codeblock().
After compilation it is not used anymore, but it still stays in Script::Inner.

Function objects created from the script keep a reference to the script (OrdinaryFunction::script_or_module). Because of this, the script can stay alive for a long time, which also keeps the whole AST in memory.

Steps to Reproduce

In a long-running program (for example a REPL or server), repeatedly run scripts that define functions:

let script = Script::parse(Source::from_bytes(src), None, &mut context)?;
script.evaluate(&mut context)?;

If those functions are stored on the global object, they keep the script alive.
Since the script holds the AST, memory usage keeps growing as more scripts run.

Impact

This mostly affects long-running programs that embed Boa.

Every script that defines functions can keep its AST in memory forever.
AST structures are usually much bigger than the source code, so memory usage can grow over time.

Modules already drop their AST after linking, but scripts did not.

Fix

Script::Inner::source is changed to:

source: RefCell<Option<boa_ast::Script>>

The AST is used during compilation and then removed once the CodeBlock is created:

*self.inner.source.borrow_mut() = None;

Result

After this change, the AST is dropped once compilation finishes.

Scripts now behave consistently with modules after the change introduced in #4855.

The AST is only required during parsing and compilation. Once the script finishes execution and the result is consumed, it can be safely released to reduce memory usage in long-running runtimes.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,666	49,666	0
Ignored	2,284	2,284	0
Failed	1,013	1,013	0
Panics	0	0	0
Conformance	93.77%	93.77%	0.00%

Tested main commit: 4acbac37250fbf3a0461fbff37b1b3019c7f1482
Tested PR commit: e1b71c05c9c9845dd8328963bd1f91fe9aa25aba
Compare commits: 4acbac3...e1b71c0

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.45%. Comparing base (6ddc2b4) to head (e1b71c0).
⚠️ Report is 765 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/script.rs	92.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4876       +/-   ##
===========================================
+ Coverage   47.24%   57.45%   +10.21%     
===========================================
  Files         476      556       +80     
  Lines       46892    60714    +13822     
===========================================
+ Hits        22154    34886    +12732     
- Misses      24738    25828     +1090     

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[ashnaaseth2325-oss]

Hello @zhuzhu81998
#4855 handled the module path (SourceTextModule::initialize_environment).
This PR is trying to apply the same idea to the script execution path (Script::evaluate_async_with_budget), since scripts can also retain their parsed AST after execution.
So the goal here is basically to mirror the module fix but for regular scripts.

[ashnaaseth2325-oss]

Hello @hansl @jedel1043
This PR was trying to apply the same idea to scripts. It looks like dropping the AST here causes some test262 regressions (+97 failures), so the placement is probably too early.
I'm looking into where it could be safely released instead.

[jedel1043]

I think a better approach would be to make this item an enum like

enum ScriptPhase {
    Ast(boa_ast::Script),
    Codeblock(Gc<CodeBlock>)
}

Then we transition through the phases of compilation of the script in codeblock.
This saves some nice bytes of memory representation.

[ashnaaseth2325-oss]

Hello @jedel1043
Thanks for the suggestion! I refactored the implementation to use an enum-based phase (ScriptPhase) as you described, transitioning from Ast → Codeblock during compilation. The project builds and tests pass locally.

[jedel1043]

Let's make use of Rust features :)

let phase = self.inner.phase.borrow();
let source = match &*phase {
    ScriptPhase::Codeblock(codeblock) => return Ok(codeblock.clone()),
    ScriptPhase::Ast(source) => source
}

[jedel1043]

I edited this btw, at first I used a clone but I realized that would clone the whole AST, which we don't want

[ashnaaseth2325-oss]

Hello @jedel1043
Thanks for the suggestion! I updated the code to follow the pattern you suggested (using borrow() and avoiding cloning the AST). I’m checking the CI failures now to see what still needs fixing.

[jedel1043]

Looks good!