Allow const assertions in no-object-literal-type-assertion (#4681)

* Ignore const assertions

These are not normal type assertions and asserting that literal objects
are immutable is a normal usage pattern.

* Bump typescript to get utility function for const assert

* Make sure tests pass on older typescripts

* Clean up tests and doc

Author: asmundg

package.json

@@ -73,7 +73,7 @@
         "tslint-config-prettier": "^1.18.0",
         "tslint-plugin-prettier": "^2.0.1",
         "tslint-test-config-non-relative": "file:test/external/tslint-test-config-non-relative",
-        "typescript": "~3.1.6",
+        "typescript": "~3.4.0",
         "yarn-deduplicate": "^1.1.1"
     },
     "engines": {

src/rules/noObjectLiteralTypeAssertionRule.ts

@@ -48,7 +48,9 @@ export class Rule extends Lint.Rules.AbstractRule {
             The type assertion in the latter case is either unnecessary or hides an error.
             The compiler will warn for excess properties with this syntax, but not missing required fields.
             For example: \`const x: { foo: number } = {}\` will fail to compile, but
-            \`const x = {} as { foo: number }\` will succeed.`,
+            \`const x = {} as { foo: number }\` will succeed.
+            Additionally, the const assertion \`const x = { foo: 1 } as const\`,
+            introduced in TypeScript 3.4, is considered beneficial and is ignored by this rule.`,
         optionsDescription: Lint.Utils.dedent`
             One option may be configured:
 
@@ -86,6 +88,8 @@ function walk(ctx: Lint.WalkContext<Options>): void {
         if (
             isAssertionExpression(node) &&
             node.type.kind !== ts.SyntaxKind.AnyKeyword &&
+            // Allow const assertions, introduced in TS 3.4
+            !(ts.isConstTypeReference !== undefined && ts.isConstTypeReference(node.type)) &&
             // Compare with UnknownKeyword if using TS 3.0 or above
             (!!(ts.SyntaxKind as any).UnknownKeyword
                 ? node.type.kind !== (ts.SyntaxKind as any).UnknownKeyword

test/rules/no-object-literal-type-assertion/default/test.ts.lint

@@ -16,6 +16,12 @@ x as T;
 {} as unknown;
 <unknown> {};
 
+#if typescript >= 3.4.0
+// Allow const assertion
+({}) as const;
+<const> ({});
+#endif
+
 foo({} as T);
     ~~~~~~~ [0]
 

yarn.lock

@@ -1851,10 +1851,10 @@ type-detect@^1.0.0:
   resolved "https://registry.yarnpkg.com/type-detect/-/type-detect-1.0.0.tgz#762217cc06db258ec48908a1298e8b95121e8ea2"
   integrity sha1-diIXzAbbJY7EiQihKY6LlRIejqI=
 
-typescript@~3.1.6:
-  version "3.1.6"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.1.6.tgz#b6543a83cfc8c2befb3f4c8fba6896f5b0c9be68"
-  integrity sha512-tDMYfVtvpb96msS1lDX9MEdHrW4yOuZ4Kdc4Him9oU796XldPYF/t2+uKoX0BBa0hXXwDlqYQbXY5Rzjzc5hBA==
+typescript@~3.4.0:
+  version "3.4.5"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.4.5.tgz#2d2618d10bb566572b8d7aad5180d84257d70a99"
+  integrity sha512-YycBxUb49UUhdNMU5aJ7z5Ej2XGmaIBL0x34vZ82fn3hGvD+bgrMrVDpatgz2f7YxUMJxMkbWxJZeAvDxVe7Vw==
 
 uglify-js@^3.1.4:
   version "3.5.4"