Adds underlines to differences in strings (#36409)

src/harness/fourslashImpl.ts

@@ -4205,10 +4205,57 @@ namespace FourSlash {
         const actualMsg = "\x1b[1mActual\x1b[0m\x1b[31m";
         const expectedString = quoted ? "\"" + expected + "\"" : expected;
         const actualString = quoted ? "\"" + actual + "\"" : actual;
-        return `\n${expectMsg}:\n${expectedString}\n\n${actualMsg}:\n${actualString}`;
+        return `\n${expectMsg}:\n${expectedString}\n\n${actualMsg}:\n${highlightDifferenceBetweenStrings(expected, actualString)}`;
     }
 
     function templateToRegExp(template: string) {
         return new RegExp(`^${ts.regExpEscape(template).replace(/\\\{\d+\\\}/g, ".*?")}$`);
     }
+
+    function rangesOfDiffBetweenTwoStrings(source: string, target: string) {
+        const ranges = [] as { start: number; length: number }[];
+
+        const addToIndex = (index: number) => {
+            const closestIndex = ranges[ranges.length - 1];
+            if (closestIndex) {
+                const doesAddToIndex = closestIndex.start + closestIndex.length === index - 1;
+                if (doesAddToIndex) {
+                    closestIndex.length = closestIndex.length + 1;
+                }
+            else {
+                ranges.push({ start: index - 1, length: 1 });
+            }
+          }
+          else {
+                ranges.push({ start: index - 1, length: 1 });
+          }
+        };
+
+        for (let index = 0; index < Math.max(source.length, target.length); index++) {
+            const srcChar = source[index];
+            const targetChar = target[index];
+            if (srcChar !== targetChar) addToIndex(index);
+        }
+
+        return ranges;
+      }
+
+      // Adds an _ when the source string and the target string have a whitespace difference
+      function highlightDifferenceBetweenStrings(source: string, target: string) {
+        const ranges = rangesOfDiffBetweenTwoStrings(source, target);
+        let emTarget = target;
+        ranges.forEach((range, index) => {
+            const lhs = `\x1b[4m`;
+            const rhs = `\x1b[0m\x1b[31m`;
+            const additionalOffset = index * lhs.length + index * rhs.length;
+            const before = emTarget.slice(0, range.start + 1 + additionalOffset);
+            const between = emTarget.slice(
+                range.start + 1 + additionalOffset,
+                range.start + range.length + 1 + additionalOffset
+            );
+             const after = emTarget.slice(range.start + range.length + 1 + additionalOffset, emTarget.length);
+            emTarget = before + lhs + between + rhs + after;
+        });
+        return emTarget;
+      }
 }