feat: remove support for down-leveling of TypeScript declarations fil…

…es to ts3.9 to improve performance (#1474)

BREAKING CHANGE: This PR removes the TypeScript declaration file
down-leveling feature from the JSII compiler to address significant
performance issues. Our benchmarks show that >50% of the JSII compile
time is spent in the `downlevel-dts` dependency, causing JSII to be 2.7x
slower than standard TypeScript compilation. The down-leveling feature
was originally added to support TypeScript versions down to 3.9, but
analysis shows minimal usage - none of our AWS CDK codebase uses this
feature, and only about 12 non-fork repositories on GitHub would be
affected by its removal, additionally typescript 3.9 and earlier have
been EOL for quite a while on [definitively
typed](https://github.com/DefinitelyTyped/DefinitelyTyped?tab=readme-ov-file#support-window).

If you found this pr because removing this feature impacted your builds,
it is likely because you have [this
pattern](https://github.com/sandersn/downlevel-dts?tab=readme-ov-file#type-modifiers-on-importexport-names-45)
somewhere in your codebase. Simply refactor that pattern and rebuild.
Ensure you also are not seeing the below typesVersion in your app's
package.json
```json
  "typesVersions": {
    "<=3.9": {
      "*": [
        ".types-compat/ts3.9/*",
        ".types-compat/ts3.9/*/index.d.ts"
      ]
    }
  },
```


---

By submitting this pull request, I confirm that my contribution is made
under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

---------

Signed-off-by: github-actions <[email protected]>
Signed-off-by: Hogan Bobertz <[email protected]>
Co-authored-by: github-actions <[email protected]>
Co-authored-by: Momo Kornher <[email protected]>

.projenrc.ts

@@ -216,7 +216,6 @@ project.addDeps(
   '@jsii/spec',
   'case',
   'chalk@^4',
-  'downlevel-dts',
   'fast-deep-equal',
   'log4js',
   'semver',

fixtures/@scope/jsii-calc-base/package.json

@@ -23,14 +23,6 @@
   },
   "main": "lib/index.js",
   "types": "lib/index.d.ts",
-  "typesVersions": {
-    "<=3.9": {
-      "*": [
-        ".types-compat/ts3.9/*",
-        ".types-compat/ts3.9/*/index.d.ts"
-      ]
-    }
-  },
   "exports": {
     ".": {
       "types": "./lib/index.d.ts",

fixtures/jsii-calc/package.json

@@ -32,14 +32,6 @@
   },
   "main": "lib/index.js",
   "types": "lib/index.d.ts",
-  "typesVersions": {
-    "<=3.9": {
-      "*": [
-        ".types-compat/ts3.9/*",
-        ".types-compat/ts3.9/*/index.d.ts"
-      ]
-    }
-  },
   "dependencies": {
     "@fixtures/jsii-calc-bundled": "file:../@fixtures/jsii-calc-bundled",
     "@scope/jsii-calc-base": "^0.0.0",

src/compiler.ts

@@ -6,9 +6,7 @@ import * as ts from 'typescript';
 
 import { Assembler } from './assembler';
 import { findDependencyDirectory } from './common/find-utils';
-import { emitDownleveledDeclarations, TYPES_COMPAT } from './downlevel-dts';
 import { Emitter } from './emitter';
-import { normalizeConfigPath } from './helpers';
 import { JsiiDiagnostic } from './jsii-diagnostic';
 import { ProjectInfo } from './project-info';
 import { WARNINGSCODE_FILE_NAME } from './transforms/deprecation-warnings';
@@ -314,15 +312,6 @@ export class Compiler implements Emitter {
       LOG.error('Compilation errors prevented the JSII assembly from being created');
     }
 
-    if (!hasErrors) {
-      emitDownleveledDeclarations(
-        this.projectRoot,
-        this.options.projectInfo.packageJson,
-        // outDir might be absolute. Need to normalize it.
-        normalizeConfigPath(this.projectRoot, this.tsconfig.compilerOptions.outDir),
-      );
-    }
-
     // Some extra validation on the config.
     // Make sure that { "./.warnings.jsii.js": "./.warnings.jsii.js" } is in the set of
     // exports, if they are specified.
@@ -365,9 +354,6 @@ export class Compiler implements Emitter {
     }
 
     const pi = this.options.projectInfo;
-    const configDir = path.dirname(this.configPath);
-    const absoluteTypesCompat = path.resolve(configDir, pi.tsc?.outDir ?? '.', TYPES_COMPAT);
-    const relativeTypesCompat = path.relative(configDir, absoluteTypesCompat);
 
     return {
       compilerOptions: {
@@ -381,7 +367,6 @@ export class Compiler implements Emitter {
       include: [pi.tsc?.rootDir != null ? path.join(pi.tsc.rootDir, '**', '*.ts') : path.join('**', '*.ts')],
       exclude: [
         'node_modules',
-        relativeTypesCompat,
         ...(pi.excludeTypescript ?? []),
         ...(pi.tsc?.outDir != null &&
         (pi.tsc?.rootDir == null || path.resolve(pi.tsc.outDir).startsWith(path.resolve(pi.tsc.rootDir) + path.sep))