Print directive literals verbatim (#1560)

* Print directive literals verbatim

This addresses https://github.com/prettier/prettier/issues/1555,
but doesn't seem to pass the AST_COMPARE=1 tests:

    AST_COMPARE=1 npm test -- tests/quotes -t strings

However, running `prettier --debug-check` on the relevant file *does*
work:

    prettier tests/quotes/strings.js --debug-check

* Change directive literal quotes if it doesn't contain quotes

This addresses https://github.com/prettier/prettier/pull/1560#discussion_r115396257

From https://github.com/prettier/prettier/issues/1555#issue-227206837:

> It's okay to change the type of quotation marks used, but only if
doing so does not require changing any characters within the directive.

* Don't change directive literal quotes if it contains a backslash

This passes the `--debug-check` tests again:

    prettier tests/quotes/strings.js --debug-check

* Try to add regression test for escaped directive literals

This seems not to work, despite the following command having the correct
output:

    echo "'\''" | prettier

You can use the following to get an idea of how flow/typescript parse
this:

    node -p "JSON.stringify(require('./src/parser').parse('\\'\\\\\'\\'', {parser: 'flow'}), null, 2)"
    node -p "JSON.stringify(require('./src/parser').parse('\\'\\\\\'\\'', {parser: 'typescript'}), null, 2)"

* WIP Disable Flow/Typescript for ./tests/directives

We don't yet handle escaped directives for them, but Babylon works.

(similar to 90bf93713c (diff-0de18284f37da79ab8af4e4690919abaR1))

* Revert "WIP Disable Flow/Typescript for ./tests/directives"

This reverts commit 2aba6231271f6985a395c31e3df9323e8f3da115.

* Prevent test strings from being parsed as directives

See https://github.com/prettier/prettier/pull/1560#issue-227225960

* Add more escaped directive tests

* Infer DirectiveLiterals from Flow parser

* Don't test TypeScript on directives

See https://github.com/prettier/prettier/pull/1560#issuecomment-300296221

* fixup! Infer DirectiveLiterals from Flow parser

src/printer.js

@@ -181,6 +181,20 @@ function genericPrintNoParens(path, options, print, args) {
     case "EmptyStatement":
       return "";
     case "ExpressionStatement":
+      // Detect Flow-parsed directives
+      if (n.directive) {
+        return concat([
+          nodeStr(
+            {
+              type: 'DirectiveLiteral',
+              value: n.expression.value,
+              raw: n.expression.raw
+            },
+            options
+          ),
+          semi
+        ]);
+      }
       return concat([path.call(print, "expression"), semi]); // Babel extension.
     case "ParenthesizedExpression":
       return concat(["(", path.call(print, "expression"), ")"]);
@@ -3831,6 +3845,7 @@ function nodeStr(node, options) {
   const alternate = preferred === single ? double : single;
 
   let shouldUseAlternateQuote = false;
+  let canChangeDirectiveQuotes = !rawContent.includes('\\');
 
   // If `rawContent` contains at least one of the quote preferred for enclosing
   // the string, we might want to enclose with the alternate quote instead, to
@@ -3840,12 +3855,23 @@ function nodeStr(node, options) {
     const numAlternateQuotes = (rawContent.match(alternate.regex) || []).length;
 
     shouldUseAlternateQuote = numPreferredQuotes > numAlternateQuotes;
+    canChangeDirectiveQuotes = canChangeDirectiveQuotes &&
+      numPreferredQuotes === 0 &&
+      numAlternateQuotes === 0;
   }
 
   const enclosingQuote = shouldUseAlternateQuote
     ? alternate.quote
     : preferred.quote;
 
+  // Directives are exact code unit sequences, which means that you can't
+  // change the escape sequences they use.
+  // See https://github.com/prettier/prettier/issues/1555
+  // and https://tc39.github.io/ecma262/#directive-prologue
+  if (node.type === 'DirectiveLiteral' && !canChangeDirectiveQuotes) {
+    return raw;
+  }
+
   // It might sound unnecessary to use `makeString` even if `node.raw` already
   // is enclosed with `enclosingQuote`, but it isn't. `node.raw` could contain
   // unnecessary escapes (such as in `"\'"`). Always using `makeString` makes

tests/directives/__snapshots__/jsfmt.spec.js.snap

@@ -1,5 +1,18 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`escaped.js 1`] = `
+'\\'';
+'\\"';
+"\\'";
+"\\"";
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+'\\'';
+'\\"';
+"\\'";
+"\\"";
+
+`;
+
 exports[`last-line-0.js 1`] = `
 'use strict';~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 "use strict";

tests/directives/escaped.js

@@ -0,0 +1,4 @@
+'\'';
+'\"';
+"\'";
+"\"";

tests/directives/jsfmt.spec.js

@@ -1 +1 @@
-run_spec(__dirname, null, ["typescript", "babylon"]);
+run_spec(__dirname, null, ["babylon"]);

tests/quotes/__snapshots__/jsfmt.spec.js.snap

@@ -31,6 +31,10 @@ function b(object, key) {
 `;
 
 exports[`strings.js 1`] = `
+// Prevent strings from being parsed as directives
+// See https://github.com/prettier/prettier/pull/1560#issue-227225960
+0;
+
 // Every string will be changed to double quotes, unless we end up with fewer
 // escaped quotes by using single quotes. (Vice versa if the "singleQuote"
 // option is true).
@@ -101,6 +105,10 @@ exports[`strings.js 1`] = `
 "var backslash = \\"\\\\\\", doubleQuote = '\\"';"
 'var backslash = "\\\\", doubleQuote = \\'"\\';'
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// Prevent strings from being parsed as directives
+// See https://github.com/prettier/prettier/pull/1560#issue-227225960
+0;
+
 // Every string will be changed to double quotes, unless we end up with fewer
 // escaped quotes by using single quotes. (Vice versa if the "singleQuote"
 // option is true).
@@ -161,6 +169,10 @@ exports[`strings.js 1`] = `
 `;
 
 exports[`strings.js 2`] = `
+// Prevent strings from being parsed as directives
+// See https://github.com/prettier/prettier/pull/1560#issue-227225960
+0;
+
 // Every string will be changed to double quotes, unless we end up with fewer
 // escaped quotes by using single quotes. (Vice versa if the "singleQuote"
 // option is true).
@@ -231,6 +243,10 @@ exports[`strings.js 2`] = `
 "var backslash = \\"\\\\\\", doubleQuote = '\\"';"
 'var backslash = "\\\\", doubleQuote = \\'"\\';'
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// Prevent strings from being parsed as directives
+// See https://github.com/prettier/prettier/pull/1560#issue-227225960
+0;
+
 // Every string will be changed to double quotes, unless we end up with fewer
 // escaped quotes by using single quotes. (Vice versa if the "singleQuote"
 // option is true).

tests/quotes/strings.js

@@ -1,3 +1,7 @@
+// Prevent strings from being parsed as directives
+// See https://github.com/prettier/prettier/pull/1560#issue-227225960
+0;
+
 // Every string will be changed to double quotes, unless we end up with fewer
 // escaped quotes by using single quotes. (Vice versa if the "singleQuote"
 // option is true).