feat(compiler-cli): lower some exported expressions (#30038)

The compiler uses metadata to represent what it statically knows about
various expressions in a program. Occasionally, expressions in the program
for which metadata is extracted may contain sub-expressions which are not
representable in metadata. One such construct is an arrow function.

The compiler does not always need to understand such expressions completely.
For example, for a provider defined with `useValue`, the compiler does not
need to understand the value at all, only the outer provider definition. In
this case, the compiler employs a technique known as "expression lowering",
where it rewrites the provider expression into one that can be represented
in metadata. Chiefly, this involves extracting out the dynamic part (the
`useValue` expression) into an exported constant.

Lowering is applied through a heuristic, which considers the containing
statement as well as the field name of the expression.

Previously, this heuristic was not completely accurate in the case of
route definitions and the `loadChildren` field, which is lowered. If the
route definition using `loadChildren` existed inside a decorator invocation,
lowering was performed correctly. However, if it existed inside a standalone
variable declaration with an export keyword, the heuristic would conclude
that lowering was unnecessary. For ordinary providers this is true; however
the compiler attempts to fully understand the ROUTES token and thus even if
an array of routes is declared in an exported variable, any `loadChildren`
expressions within still need to be lowered.

This commit enables lowering of already exported variables under a limited
set of conditions (where the initializer expression is of a specific form).
This should enable the use of `loadChildren` in route definitions.

PR Close #30038

packages/compiler-cli/src/transformers/lower_expressions.ts

@@ -223,9 +223,19 @@ function isEligibleForLowering(node: ts.Node | undefined): boolean {
         // Don't lower expressions in a declaration.
         return false;
       case ts.SyntaxKind.VariableDeclaration:
-        // Avoid lowering expressions already in an exported variable declaration
-        return (ts.getCombinedModifierFlags(node as ts.VariableDeclaration) &
-                ts.ModifierFlags.Export) == 0;
+        const isExported = (ts.getCombinedModifierFlags(node as ts.VariableDeclaration) &
+                            ts.ModifierFlags.Export) == 0;
+        // This might be unnecessary, as the variable might be exported and only used as a reference
+        // in another expression. However, the variable also might be involved in provider
+        // definitions. If that's the case, there is a specific token (`ROUTES`) which the compiler
+        // attempts to understand deeply. Sub-expressions within that token (`loadChildren` for
+        // example) might also require lowering even if the top-level declaration is already
+        // properly exported.
+        const varNode = node as ts.VariableDeclaration;
+        return isExported || (varNode.initializer !== undefined &&
+                              (ts.isObjectLiteralExpression(varNode.initializer) ||
+                               ts.isArrayLiteralExpression(varNode.initializer) ||
+                               ts.isCallExpression(varNode.initializer)));
     }
     return isEligibleForLowering(node.parent);
   }

packages/compiler-cli/test/ngc_spec.ts

@@ -875,6 +875,42 @@ describe('ngc transformer command-line', () => {
         expect(mymoduleSource).toMatch(/ɵ0 = .*foo\(\)/);
       });
 
+      it('should lower loadChildren in an exported variable expression', () => {
+        write('mymodule.ts', `
+          import {Component, NgModule} from '@angular/core';
+          import {RouterModule} from '@angular/router';
+
+          export function foo(): string {
+            console.log('side-effect');
+            return 'test';
+          }
+
+          @Component({
+            selector: 'route',
+            template: 'route',
+          })
+          export class Route {}
+
+          export const routes = [
+            {path: '', pathMatch: 'full', component: Route, loadChildren: foo()}
+          ];
+
+          @NgModule({
+            declarations: [Route],
+            imports: [
+              RouterModule.forRoot(routes),
+            ]
+          })
+          export class MyModule {}
+        `);
+        expect(compile()).toEqual(0);
+
+        const mymodulejs = path.resolve(outDir, 'mymodule.js');
+        const mymoduleSource = fs.readFileSync(mymodulejs, 'utf8');
+        expect(mymoduleSource).toContain('loadChildren: ɵ0');
+        expect(mymoduleSource).toMatch(/ɵ0 = .*foo\(\)/);
+      });
+
       it('should be able to lower supported expressions', () => {
         writeConfig(`{
           "extends": "./tsconfig-base.json",