Implement mutableTemplateObject and ignoreToPrimitiveHint assumpt…

…ions

packages/babel-core/src/config/validation/options.js

@@ -332,6 +332,7 @@ type EnvPath = $ReadOnly<{
 export type NestingPath = RootPath | OverridesPath | EnvPath;
 
 export const assumptionsNames = new Set<string>([
+  "ignoreToPrimitiveHint",
   "mutableTemplateObject",
   "newableArrowFunctions",
   "setPublicClassFields",

packages/babel-plugin-transform-template-literals/src/index.js

@@ -3,10 +3,14 @@ import { template, types as t } from "@babel/core";
 
 export default declare((api, options) => {
   api.assertVersion(7);
-  const { loose } = options;
+
+  const ignoreToPrimitiveHint =
+    options.loose || api.assumption("ignoreToPrimitiveHint");
+  const mutableTemplateObject =
+    options.loose || api.assumption("mutableTemplateObject");
 
   let helperName = "taggedTemplateLiteral";
-  if (loose) helperName += "Loose";
+  if (mutableTemplateObject) helperName += "Loose";
 
   /**
    * This function groups the objects into multiple calls to `.concat()` in
@@ -122,13 +126,15 @@ export default declare((api, options) => {
 
         // since `+` is left-to-right associative
         // ensure the first node is a string if first/second isn't
-        const considerSecondNode = !loose || !t.isStringLiteral(nodes[1]);
-        if (!t.isStringLiteral(nodes[0]) && considerSecondNode) {
+        if (
+          !t.isStringLiteral(nodes[0]) &&
+          !(ignoreToPrimitiveHint && t.isStringLiteral(nodes[1]))
+        ) {
           nodes.unshift(t.stringLiteral(""));
         }
         let root = nodes[0];
 
-        if (loose) {
+        if (ignoreToPrimitiveHint) {
           for (let i = 1; i < nodes.length; i++) {
             root = t.binaryExpression("+", root, nodes[i]);
           }

packages/babel-plugin-transform-template-literals/test/fixtures/assumption-ignoreToPrimitiveHint/options.json

@@ -0,0 +1,6 @@
+{
+  "plugins": ["transform-template-literals"],
+  "assumptions": {
+    "ignoreToPrimitiveHint": true
+  }
+}

packages/babel-plugin-transform-template-literals/test/fixtures/assumption-ignoreToPrimitiveHint/tag/input.js

@@ -0,0 +1 @@
+tag`foo ${bar} baz`;

packages/babel-plugin-transform-template-literals/test/fixtures/assumption-ignoreToPrimitiveHint/tag/output.js

@@ -0,0 +1,13 @@
+function _templateObject() {
+  const data = _taggedTemplateLiteral(["foo ", " baz"]);
+
+  _templateObject = function () {
+    return data;
+  };
+
+  return data;
+}
+
+function _taggedTemplateLiteral(strings, raw) { if (!raw) { raw = strings.slice(0); } return Object.freeze(Object.defineProperties(strings, { raw: { value: Object.freeze(raw) } })); }
+
+tag(_templateObject(), bar);

packages/babel-plugin-transform-template-literals/test/fixtures/assumption-mutableTemplateObject/no-tag/input.js

@@ -0,0 +1 @@
+`foo ${bar} baz`;

packages/babel-plugin-transform-template-literals/test/fixtures/assumption-mutableTemplateObject/no-tag/output.js

@@ -0,0 +1 @@
+"foo ".concat(bar, " baz");

packages/babel-plugin-transform-template-literals/test/fixtures/assumption-mutableTemplateObject/options.json

@@ -0,0 +1,6 @@
+{
+  "plugins": ["transform-template-literals"],
+  "assumptions": {
+    "mutableTemplateObject": true
+  }
+}

packages/babel-plugin-transform-template-literals/test/fixtures/loose/ignoreToPrimitiveHint/input.js

@@ -0,0 +1 @@
+var o = `foo ${bar} baz`;

packages/babel-plugin-transform-template-literals/test/fixtures/loose/ignoreToPrimitiveHint/output.js

@@ -0,0 +1 @@
+var o = "foo " + bar + " baz";

packages/babel-plugin-transform-template-literals/test/fixtures/loose/mutableTemplateObject/input.js

@@ -0,0 +1 @@
+var o = tag`foo ${bar} baz`;

packages/babel-plugin-transform-template-literals/test/fixtures/loose/mutableTemplateObject/output.js

@@ -0,0 +1,13 @@
+function _templateObject() {
+  const data = _taggedTemplateLiteralLoose(["foo ", " baz"]);
+
+  _templateObject = function () {
+    return data;
+  };
+
+  return data;
+}
+
+function _taggedTemplateLiteralLoose(strings, raw) { if (!raw) { raw = strings.slice(0); } strings.raw = raw; return strings; }
+
+var o = tag(_templateObject(), bar);