Merge pull request #95 from Microsoft/no-single-element-tuple-type

Add 'no-single-element-tuple-type' rule

Author: Andy

docs/no-single-element-tuple-type.md

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+# no-single-element-tuple-type
+
+Some users mistakenly write `[T]` when then intend to write an array type `T[]`.
+
+**Bad**:
+
+```ts
+export const x: [T];
+```
+
+**Good**:
+
+```ts
+export const x: T[];
+```

src/rules/noSingleElementTupleTypeRule.ts

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+import * as Lint from "tslint";
+import * as ts from "typescript";
+
+import { failure } from "../util";
+
+export class Rule extends Lint.Rules.AbstractRule {
+	static metadata: Lint.IRuleMetadata = {
+		ruleName: "no-single-element-tuple-type",
+		description: "Forbids `[T]`, which should be `T[]`.",
+		optionsDescription: "Not configurable.",
+		options: null,
+		type: "functionality",
+		typescriptOnly: true,
+	};
+
+	apply(sourceFile: ts.SourceFile): Lint.RuleFailure[] {
+		return this.applyWithFunction(sourceFile, walk);
+	}
+}
+
+function walk(ctx: Lint.WalkContext<void>): void {
+	const { sourceFile } = ctx;
+	sourceFile.forEachChild(function cb(node) {
+		if (ts.isTupleTypeNode(node) && node.elementTypes.length === 1) {
+			ctx.addFailureAtNode(node, failure(
+				Rule.metadata.ruleName,
+				"Type [T] is a single-element tuple type. You probably meant T[]."));
+		}
+		node.forEachChild(cb);
+	});
+}

test/no-single-element-tuple-type/test.ts.lint

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+const x: [string];
+         ~~~~~~~~
+const y: [string, number];
+
+[0]: [Type [T] is a single-element tuple type. You probably meant T[]. See: https://github.com/Microsoft/dtslint/blob/master/docs/no-padding.md]

test/no-single-element-tuple-type/tslint.json

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+{
+    "rulesDirectory": ["../../bin/rules"],
+    "rules": {
+      "no-single-element-tuple-type": true
+    }
+  }