babel-traverse: Mark appropriate template literals as pure (#5914)

* Identify pure template literals

* Mark template literals as pure where possible

* Changes based on code review

* nit

packages/babel-traverse/src/scope/index.js

@@ -615,6 +615,17 @@ export default class Scope {
       return this.isPure(node.value, constantsOnly);
     } else if (t.isUnaryExpression(node)) {
       return this.isPure(node.argument, constantsOnly);
+    } else if (t.isTaggedTemplateExpression(node)) {
+      return (
+        t.matchesPattern(node.tag, "String.raw") &&
+        !this.hasBinding("String", true) &&
+        this.isPure(node.quasi, constantsOnly)
+      );
+    } else if (t.isTemplateLiteral(node)) {
+      for (const expression of (node.expressions: Array<Object>)) {
+        if (!this.isPure(expression, constantsOnly)) return false;
+      }
+      return true;
     } else {
       return t.isPureish(node);
     }

packages/babel-traverse/test/scope.js

@@ -77,6 +77,22 @@ describe("scope", function() {
       assert.ok(
         getPath("({ x: 1 })").get("body")[0].get("expression").isPure(),
       );
+      assert.ok(!getPath("`${a}`").get("body")[0].get("expression").isPure());
+      assert.ok(
+        getPath("let a = 1; `${a}`").get("body")[1].get("expression").isPure(),
+      );
+      assert.ok(
+        !getPath("let a = 1; `${a++}`")
+          .get("body")[1]
+          .get("expression")
+          .isPure(),
+      );
+      assert.ok(
+        !getPath("tagged`foo`").get("body")[0].get("expression").isPure(),
+      );
+      assert.ok(
+        getPath("String.raw`foo`").get("body")[0].get("expression").isPure(),
+      );
     });
 
     test("label", function() {