feat: parse parameter decorators for transforms

cli/index.d.ts

@@ -276,7 +276,10 @@ export abstract class Transform {
   /** Called when parsing is complete, before a program is instantiated from the AST. */
   afterParse?(parser: Parser): void | Promise<void>;
 
-  /** Called after the program is instantiated. */
+  /**
+   * Called after the program is instantiated and before compilation-time validation runs.
+   * This is the last hook where transforms can rewrite preserved AST-only syntax before it is rejected.
+   */
   afterInitialize?(program: Program): void | Promise<void>;
 
   /** Called when compilation is complete, before the module is being validated. */

cli/index.js

@@ -718,7 +718,7 @@ export async function main(argv, options) {
     stats.initializeTime += stats.end(begin);
   }
 
-  // Call afterInitialize transform hook
+  // Call afterInitialize transform hook, the last AST rewrite point before compilation-time validation.
   {
     let error = await applyTransform("afterInitialize", program);
     if (error) return prepareResult(error);

eslint.config.js

@@ -25,6 +25,10 @@ export default defineConfig([
 
     // FIXME: Tagged template literal tests with invalid escapes
     "tests/compiler/templateliteral.ts",
+
+    // Decorators on `this` are not allowed typically in TypeScript, but this
+    // fixture exercises that AS-only syntax and is validated by transform tests.
+    "tests/transform/parameter-decorators.ts",
   ]),
 
   js.configs.recommended,

package.json

@@ -85,8 +85,8 @@
     "test:browser": "node --enable-source-maps tests/browser",
     "test:asconfig": "cd tests/asconfig && npm run test",
     "test:transform": "npm run test:transform:esm && npm run test:transform:cjs",
-    "test:transform:esm": "node bin/asc tests/compiler/empty --transform ./tests/transform/index.js --noEmit && node bin/asc tests/compiler/empty --transform ./tests/transform/simple.js --noEmit",
-    "test:transform:cjs": "node bin/asc tests/compiler/empty --transform ./tests/transform/cjs/index.js --noEmit && node bin/asc tests/compiler/empty --transform ./tests/transform/cjs/simple.js --noEmit",
+    "test:transform:esm": "node bin/asc tests/compiler/empty --transform ./tests/transform/index.js --noEmit && node bin/asc tests/compiler/empty --transform ./tests/transform/simple.js --noEmit && node bin/asc tests/transform/parameter-decorators.ts --transform ./tests/transform/remove-parameter-decorators.js --noEmit",
+    "test:transform:cjs": "node bin/asc tests/compiler/empty --transform ./tests/transform/cjs/index.js --noEmit && node bin/asc tests/compiler/empty --transform ./tests/transform/cjs/simple.js --noEmit && node bin/asc tests/transform/parameter-decorators.ts --transform ./tests/transform/cjs/remove-parameter-decorators.js --noEmit",
     "test:cli": "node tests/cli/options.js",
     "asbuild": "npm run asbuild:debug && npm run asbuild:release",
     "asbuild:debug": "node bin/asc --config src/asconfig.json --target debug",

src/ast.ts

@@ -181,9 +181,12 @@ export abstract class Node {
     name: IdentifierExpression,
     type: TypeNode,
     initializer: Expression | null,
-    range: Range
+    range: Range,
+    decorators: DecoratorNode[] | null = null
   ): ParameterNode {
-    return new ParameterNode(parameterKind, name, type, initializer, range);
+    let parameter = new ParameterNode(parameterKind, name, type, initializer, range);
+    parameter.decorators = decorators;
+    return parameter;
   }
 
   // special
@@ -926,6 +929,9 @@ export class FunctionTypeNode extends TypeNode {
   ) {
     super(NodeKind.FunctionType, isNullable, range);
   }
+
+  /** Decorators on an explicit `this` parameter, if any, preserved for transforms. */
+  explicitThisDecorators: DecoratorNode[] | null = null;
 }
 
 /** Represents a type parameter. */
@@ -971,6 +977,8 @@ export class ParameterNode extends Node {
     super(NodeKind.Parameter, range);
   }
 
+  /** Decorators, if any, preserved so transforms can rewrite them before validation. */
+  decorators: DecoratorNode[] | null = null;
   /** Implicit field declaration, if applicable. */
   implicitFieldDeclaration: FieldDeclaration | null = null;
   /** Common flags indicating specific traits. */

src/compiler.ts

@@ -536,6 +536,8 @@ export class Compiler extends DiagnosticEmitter {
 
     // initialize lookup maps, built-ins, imports, exports, etc.
     this.program.initialize();
+    // Reject any parameter decorators that transforms left on the AST.
+    this.program.validateParameterDecorators();
 
 
     // Binaryen treats all function references as being leaked to the outside world when

src/extra/ast.ts

@@ -427,6 +427,7 @@ export class ASTBuilder {
     sb.push(isNullable ? "((" : "(");
     let explicitThisType = node.explicitThisType;
     if (explicitThisType) {
+      this.serializeParameterDecorators(node.explicitThisDecorators);
       sb.push("this: ");
       this.visitTypeNode(explicitThisType);
     }
@@ -1153,6 +1154,7 @@ export class ASTBuilder {
       let numParameters = parameters.length;
       let explicitThisType = signature.explicitThisType;
       if (explicitThisType) {
+        this.serializeParameterDecorators(signature.explicitThisDecorators);
         sb.push("this: ");
         this.visitTypeNode(explicitThisType);
       }
@@ -1563,9 +1565,38 @@ export class ASTBuilder {
     indent(sb, this.indentLevel);
   }
 
+  serializeParameterDecorators(decorators: DecoratorNode[] | null): void {
+    if (decorators) {
+      for (let i = 0, k = decorators.length; i < k; ++i) {
+        this.visitParameterDecorator(decorators[i]);
+      }
+    }
+  }
+
+  private visitParameterDecorator(node: DecoratorNode): void {
+    let sb = this.sb;
+    sb.push("@");
+    this.visitNode(node.name);
+    let args = node.args;
+    if (args) {
+      sb.push("(");
+      let numArgs = args.length;
+      if (numArgs) {
+        this.visitNode(args[0]);
+        for (let i = 1; i < numArgs; ++i) {
+          sb.push(", ");
+          this.visitNode(args[i]);
+        }
+      }
+      sb.push(")");
+    }
+    sb.push(" ");
+  }
+
   serializeParameter(node: ParameterNode): void {
     let sb = this.sb;
     let kind = node.parameterKind;
+    this.serializeParameterDecorators(node.decorators);
     let implicitFieldDeclaration = node.implicitFieldDeclaration;
     if (implicitFieldDeclaration) {
       this.serializeAccessModifiers(implicitFieldDeclaration);

src/index-wasm.ts

@@ -347,7 +347,7 @@ export function getDependee(program: Program, file: string): string | null {
 
 // Compiler
 
-/** Initializes the program pre-emptively for transform hooks. */
+/** Initializes the program pre-emptively so `afterInitialize` transforms can rewrite the AST before compilation. */
 export function initializeProgram(program: Program): void {
   program.initialize();
 }