feat(eslint-plugin): [no-unnec-type-assertion] allow const assertions (#1741)

Author: a-tarasyuk

Diff for: packages/eslint-plugin/docs/rules/no-unnecessary-type-assertion.md

@@ -43,6 +43,10 @@ const foo = <number>3;
 const foo = 3 as number;
 ```
 
+```ts
+const foo = 'foo' as const;
+```
+
 ```ts
 function foo(x: number | undefined): number {
   return x!;

Diff for: packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts

@@ -1,4 +1,7 @@
-import { TSESTree } from '@typescript-eslint/experimental-utils';
+import {
+  TSESTree,
+  AST_NODE_TYPES,
+} from '@typescript-eslint/experimental-utils';
 import {
   isObjectType,
   isObjectFlagSet,
@@ -122,6 +125,14 @@ export default util.createRule<Options, MessageIds>({
       return false;
     }
 
+    function isConstAssertion(node: TSESTree.TypeNode): boolean {
+      return (
+        node.type === AST_NODE_TYPES.TSTypeReference &&
+        node.typeName.type === AST_NODE_TYPES.Identifier &&
+        node.typeName.name === 'const'
+      );
+    }
+
     return {
       TSNonNullExpression(node): void {
         const originalNode = parserServices.esTreeNodeToTSNodeMap.get(node);
@@ -201,7 +212,8 @@ export default util.createRule<Options, MessageIds>({
         if (
           options.typesToIgnore?.includes(
             sourceCode.getText(node.typeAnnotation),
-          )
+          ) ||
+          isConstAssertion(node.typeAnnotation)
         ) {
           return;
         }

Diff for: packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts

@@ -130,6 +130,38 @@ function Test(props: {
       `,
       filename: 'react.tsx',
     },
+    {
+      code: `
+const a = [1, 2];
+const b = [3, 4];
+const c = [...a, ...b] as const;
+      `,
+    },
+    {
+      code: `const a = [1, 2] as const;`,
+    },
+    {
+      code: `const a = 'a' as const;`,
+    },
+    {
+      code: `const a = { foo: 'foo' } as const`,
+    },
+    {
+      code: `
+const a = [1, 2];
+const b = [3, 4];
+const c = <const>[...a, ...b];
+      `,
+    },
+    {
+      code: `const a = <const>[1, 2];`,
+    },
+    {
+      code: `const a = <const>'a';`,
+    },
+    {
+      code: `const a = <const>{ foo: 'foo' };`,
+    },
   ],
 
   invalid: [