fix(jsii): Prohibit illegal uses of structs (aka data types)

packages/jsii/lib/assembler.ts

@@ -446,6 +446,17 @@ export class Assembler implements Emitter {
             }
 
             jsiiType.interfaces = await this._processBaseInterfaces(fqn, clause.types.map(t => this._typeChecker.getTypeFromTypeNode(t)));
+            if (jsiiType.interfaces) {
+                this._deferUntilTypesAvailable(jsiiType.fqn, jsiiType.interfaces, type.symbol.valueDeclaration, (...ifaces) => {
+                    for (const iface of ifaces) {
+                        if (spec.isInterfaceType(iface) && iface.datatype) {
+                            this._diagnostic(type.symbol.valueDeclaration,
+                                ts.DiagnosticCategory.Error,
+                                `Attempted to implement struct ${iface.fqn} from class ${jsiiType.fqn}`);
+                        }
+                    }
+                 });
+            }
         }
 
         if (!type.isClass()) {
@@ -723,6 +734,19 @@ export class Assembler implements Emitter {
                         prop.immutable = true;
                     }
                 }
+            } else {
+                // This is *NOT* a data type, so it may not extend something that is one.
+                for (const base of bases) {
+                    if (!spec.isInterfaceType(base)) {
+                        // Invalid type we already warned about earlier, just ignoring it here...
+                        continue;
+                    }
+                    if (base.datatype) {
+                        this._diagnostic(type.symbol.valueDeclaration,
+                            ts.DiagnosticCategory.Error,
+                            `Attempted to extend struct ${base.fqn} from regular interface ${jsiiType.fqn}`);
+                    }
+                }
             }
         });
 

packages/jsii/test/negatives/neg.extend-struct.ts

@@ -0,0 +1,10 @@
+///!MATCH_ERROR: Attempted to extend struct jsii.Struct from regular interface jsii.IIllegal
+
+// Attempt to extend a Struct (aka data type) from a regular interface will fail.
+export interface Struct {
+  readonly field: string;
+}
+
+export interface IIllegal extends Struct {
+  method(): void;
+}

packages/jsii/test/negatives/neg.implement-struct.ts

@@ -0,0 +1,14 @@
+///!MATCH_ERROR: Attempted to implement struct jsii.Struct from class jsii.Illegal
+
+// Attempt to implement a Struct (aka data type) will fail.
+export interface Struct {
+  readonly field: string;
+}
+
+export class Illegal implements Struct {
+  public readonly field: string = 'foo';
+
+  public method(): void {
+    return;
+  }
+}

packages/jsii/test/negatives/neg.struct-extends-interface.ts

@@ -0,0 +1,10 @@
+///!MATCH_ERROR: Interface contains behavior: name should be "IStruct"
+
+// Attempt to extend an interface from a struct (aka data type)
+export interface IInterface {
+  readonly field: string;
+}
+
+export interface Struct extends IInterface {
+  readonly another: number;
+}