Add WASM Backend Support with File Imports

[FetBoba]

This PR enables compilation of .mls files to WASM modules with support for WASM-WASM file imports. What was done:

Config Parsing, WASM Compilation (generating wat and glue mjs files), WASM compiled mls files can import files of the same type. Import between JS and WASM compiled files is currently rejected and will be implemented after this PR.

[LPTK]

Thanks. Please fix the merge conflicts and review your own diff. There should be no needless whitespace changes.

[Copilot]

Pull request overview

This PR adds a WASM compilation path for .mls modules, including emitting WAT and generated JS glue, and enables WASM-to-WASM file imports (while explicitly rejecting JS↔WASM imports for now).

Changes:

• Add CompilationTarget.Wasm configuration parsing and enforce where target can be set.
• Implement WASM compilation output (.wat + glue .mjs) with dependency compilation and wiring of imported module exports.
• Extend WASM session export/import plumbing to use a per-module name (instead of a single REPL module name) and add tests/fixtures for WASM artifact emission and imports.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
hkmc2DiffTests/src/test/scala/hkmc2/WasmDiffMaker.scala	Updates WAT builder invocation to pass a module name for session exports/imports.
hkmc2/shared/src/test/mlscript/wasm/ClassMethods.mls	Updates expected diagnostic text for missing struct fields.
hkmc2/shared/src/test/mlscript-compile/wasm/SimpleTarget.mls	Adds a wasm-targeted compile fixture.
hkmc2/shared/src/test/mlscript-compile/wasm/ImportValue.mls	Adds a wasm-targeted import fixture (importer).
hkmc2/shared/src/test/mlscript-compile/wasm/IVal.mls	Adds a wasm-targeted import fixture (dependency).
hkmc2/shared/src/test/mlscript-compile/.gitignore	Ignores generated .wat artifacts under compile fixtures.
hkmc2/shared/src/main/scala/hkmc2/semantics/Elaborator.scala	Disallows target overrides in @config(...) annotations (must be top-level #config).
hkmc2/shared/src/main/scala/hkmc2/codegen/wasm/text/WatBuilder.scala	Makes session exports use the current module name; improves field-select error messaging; updates program signature.
hkmc2/shared/src/main/scala/hkmc2/codegen/wasm/text/Ctx.scala	Extends SessionExportCtx to carry a module name and threads it through collectors.
hkmc2/shared/src/main/scala/hkmc2/MLsCompiler.scala	Adds a WASM compilation pipeline, dependency compilation, and glue generation.
hkmc2/shared/src/main/scala/hkmc2/Config.scala	Adds parsing support for target: CompilationTarget.{JS,Wasm} in config overrides.
hkmc2/jvm/src/test/scala/hkmc2/WasmModuleImportTest.scala	Adds an end-to-end JVM test validating wasm import wiring and instantiation.
hkmc2/js/src/test/scala/hkmc2/CompilerTest.scala	Adds a Scala.js test validating wasm artifact emission and refactors error detection helper.

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[Copilot]

Throwing IllegalStateException after emitting a diagnostic will crash compilation (including the JS-facing compiler wrapper, which expects diagnostics rather than exceptions). Instead, after raise(...), abort this Wasm compilation in a controlled way (e.g., return early/skip emission for this module, or use a dedicated non-throwing short-circuit similar to WatBuilder.program’s boundary + break pattern).

Suggested change

	throw new IllegalStateException(s"Expected Wasm-targeted imported module at ${sourceFile.toString}")
	dependencies += WasmDependency(
	importPath,
	emitWasm(importedModule, prelude, memo),
	)
	else
	dependencies += WasmDependency(
	importPath,
	emitWasm(importedModule, prelude, memo),
	)

[LPTK]

There are very extensive changes to MLsCompiler.scala, but it's not clear why you are doing them, and some of them are actually harmful (elaborating imported files yourself instead of using the existing caching mechanism). Please address this. If you want to make such extensive changes to the logic, you should justify them and document the intent in the code.

[LPTK]

Why are you changing the way imports work? This won't cache the imports, which is wrong. Furthermore, there is no reason not to share this logic between targets.

[Derppening]

I agree with @LPTK - The glue code looks okay but the changes to MLsCompiler.scala should be better explained and justified.

[LPTK]

Didn't we say that this should be generated into a .wat file alongside the .mjs?

[LPTK]

I see the file is there... You should obviously load the wat content from the file, not duplicate it here!

[LPTK]

Clearly we don't want to recompile these for on every single WASM module. It should be compiled as part of some RuntimeWASM.mls module (similar to Runtime.mls but used for WASM).

[LPTK]

The result is unexpected.