diff --git a/scripts/find-unused-diganostic-messages.mjs b/scripts/find-unused-diganostic-messages.mjs
new file mode 100644
index 0000000000000..6ecff6642d3db
--- /dev/null
+++ b/scripts/find-unused-diganostic-messages.mjs
@@ -0,0 +1,30 @@
+// @ts-check
+// This file requires a modern version of node 14+, and grep to be available.
+
+// node scripts/find-unused-diagnostic-messages.mjs
+import { readFileSync } from "fs";
+import {EOL} from "os";
+import { execSync } from "child_process";
+
+const diags = readFileSync("src/compiler/diagnosticInformationMap.generated.ts", "utf8");
+const startOfDiags = diags.split("export const Diagnostics")[1];
+
+const missingNames = [];
+startOfDiags.split(EOL).forEach(line => {
+    if (!line.includes(":")) return;
+    const diagName = line.split(":")[0].trim();
+
+    try {
+        execSync(`grep -rnw 'src' -e 'Diagnostics.${diagName}'`).toString();
+        process.stdout.write(".");
+    } catch (error) {
+        missingNames.push(diagName);
+        process.stdout.write("x");
+    }
+});
+
+if (missingNames.length) {
+    process.exitCode = 1;
+    console.log("Could not find usage of these diagnostics in the codebase:");
+    console.log(missingNames);
+}
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index c71967b2eaffe..1c9bb931c5af1 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -127,26 +127,14 @@
         "category": "Error",
         "code": 1040
     },
-    "'{0}' modifier cannot be used with a class declaration.": {
-        "category": "Error",
-        "code": 1041
-    },
     "'{0}' modifier cannot be used here.": {
         "category": "Error",
         "code": 1042
     },
-    "'{0}' modifier cannot appear on a data property.": {
-        "category": "Error",
-        "code": 1043
-    },
     "'{0}' modifier cannot appear on a module or namespace element.": {
         "category": "Error",
         "code": 1044
     },
-    "A '{0}' modifier cannot be used with an interface declaration.": {
-        "category": "Error",
-        "code": 1045
-    },
     "Top-level declarations in .d.ts files must start with either a 'declare' or 'export' modifier.": {
         "category": "Error",
         "code": 1046
@@ -187,10 +175,6 @@
         "category": "Error",
         "code": 1056
     },
-    "An async function or method must have a valid awaitable return type.": {
-        "category": "Error",
-        "code": 1057
-    },
     "The return type of an async function must either be a valid promise or must not contain a callable 'then' member.": {
         "category": "Error",
         "code": 1058
@@ -703,10 +687,6 @@
         "category": "Error",
         "code": 1219
     },
-    "Generators are only available when targeting ECMAScript 2015 or higher.": {
-        "category": "Error",
-        "code": 1220
-    },
     "Generators are not allowed in an ambient context.": {
         "category": "Error",
         "code": 1221
@@ -835,10 +815,6 @@
         "category": "Error",
         "code": 1252
     },
-    "'{0}' tag cannot be used independently as a top level JSDoc tag.": {
-        "category": "Error",
-        "code": 1253
-    },
     "A 'const' initializer in an ambient context must be a string or numeric literal or literal enum reference.": {
         "category": "Error",
         "code": 1254
@@ -1088,10 +1064,6 @@
         "category": "Error",
         "code": 1359
     },
-    "Did you mean to parenthesize this function type?": {
-        "category": "Error",
-        "code": 1360
-    },
     "'{0}' cannot be used as a value because it was imported using 'import type'.": {
         "category": "Error",
         "code": 1361
@@ -1200,14 +1172,6 @@
         "category": "Error",
         "code": 1389
     },
-    "Provides a root package name when using outFile with declarations.": {
-        "category": "Message",
-        "code": 1390
-    },
-    "The 'bundledPackageName' option must be provided when using outFile and node module resolution with declaration emit.": {
-        "category": "Error",
-        "code": 1391
-    },
     "An import alias cannot use 'import type'": {
         "category": "Error",
         "code": 1392
@@ -1450,10 +1414,6 @@
         "category": "Error",
         "code": 2310
     },
-    "A class may only extend another class.": {
-        "category": "Error",
-        "code": 2311
-    },
     "An interface can only extend an object type or intersection of object types with statically known members.": {
         "category": "Error",
         "code": 2312
@@ -1574,10 +1534,6 @@
         "category": "Error",
         "code": 2341
     },
-    "An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.": {
-        "category": "Error",
-        "code": 2342
-    },
     "This syntax requires an imported helper named '{1}' which does not exist in '{0}'. Consider upgrading your version of '{0}'.": {
         "category": "Error",
         "code": 2343
@@ -1726,14 +1682,6 @@
         "category": "Error",
         "code": 2380
     },
-    "A signature with an implementation cannot use a string literal type.": {
-        "category": "Error",
-        "code": 2381
-    },
-    "Specialized overload signature is not assignable to any non-specialized signature.": {
-        "category": "Error",
-        "code": 2382
-    },
     "Overload signatures must all be exported or non-exported.": {
         "category": "Error",
         "code": 2383
@@ -1806,10 +1754,6 @@
         "category": "Error",
         "code": 2400
     },
-    "Duplicate identifier '_super'. Compiler uses '_super' to capture base class reference.": {
-        "category": "Error",
-        "code": 2401
-    },
     "Expression resolves to '_super' that compiler uses to capture base class reference.": {
         "category": "Error",
         "code": 2402
@@ -2002,18 +1946,10 @@
         "category": "Error",
         "code": 2452
     },
-    "The type argument for type parameter '{0}' cannot be inferred from the usage. Consider specifying the type arguments explicitly.": {
-        "category": "Error",
-        "code": 2453
-    },
     "Variable '{0}' is used before being assigned.": {
         "category": "Error",
         "code": 2454
     },
-    "Type argument candidate '{1}' is not a valid type argument because it is not a supertype of candidate '{0}'.": {
-        "category": "Error",
-        "code": 2455
-    },
     "Type alias '{0}' circularly references itself.": {
         "category": "Error",
         "code": 2456
@@ -2070,14 +2006,6 @@
         "category": "Error",
         "code": 2469
     },
-    "'Symbol' reference does not refer to the global Symbol constructor object.": {
-        "category": "Error",
-        "code": 2470
-    },
-    "A computed property name of the form '{0}' must be of type 'symbol'.": {
-        "category": "Error",
-        "code": 2471
-    },
     "Spread operator in 'new' expressions is only available when targeting ECMAScript 5 and higher.": {
         "category": "Error",
         "code": 2472
@@ -2106,10 +2034,6 @@
         "category": "Error",
         "code": 2478
     },
-    "Property '{0}' does not exist on 'const' enum '{1}'.": {
-        "category": "Error",
-        "code": 2479
-    },
     "'let' is not allowed to be used as a name in 'let' or 'const' declarations.": {
         "category": "Error",
         "code": 2480
@@ -2234,10 +2158,6 @@
         "category": "Error",
         "code": 2513
     },
-    "Classes containing abstract methods must be marked abstract.": {
-        "category": "Error",
-        "code": 2514
-    },
     "Non-abstract class '{0}' does not implement inherited abstract member '{1}' from class '{2}'.": {
         "category": "Error",
         "code": 2515
@@ -2262,10 +2182,6 @@
         "category": "Error",
         "code": 2520
     },
-    "Expression resolves to variable declaration '{0}' that compiler uses to support async functions.": {
-        "category": "Error",
-        "code": 2521
-    },
     "The 'arguments' object cannot be referenced in an async function or method in ES3 and ES5. Consider using a standard function or method.": {
         "category": "Error",
         "code": 2522
@@ -2342,10 +2258,6 @@
         "category": "Error",
         "code": 2540
     },
-    "The target of an assignment must be a variable or a property access.": {
-        "category": "Error",
-        "code": 2541
-    },
     "Index signature in type '{0}' only permits reading.": {
         "category": "Error",
         "code": 2542
@@ -2450,14 +2362,6 @@
         "category": "Error",
         "code": 2571
     },
-    "Rest signatures are incompatible.": {
-        "category": "Error",
-        "code": 2572
-    },
-    "Property '{0}' is incompatible with rest element type.": {
-        "category": "Error",
-        "code": 2573
-    },
     "A rest element type must be an array type.": {
         "category": "Error",
         "code": 2574
@@ -2502,14 +2406,6 @@
         "category": "Error",
         "code": 2585
     },
-    "Enum type '{0}' circularly references itself.": {
-        "category": "Error",
-        "code": 2586
-    },
-    "JSDoc type '{0}' circularly references itself.": {
-        "category": "Error",
-        "code": 2587
-    },
     "Cannot assign to '{0}' because it is a constant.": {
         "category": "Error",
         "code": 2588
@@ -2554,14 +2450,6 @@
         "category": "Error",
         "code": 2598
     },
-    "JSX element attributes type '{0}' may not be a union type.": {
-        "category": "Error",
-        "code": 2600
-    },
-    "The return type of a JSX element constructor must return an object type.": {
-        "category": "Error",
-        "code": 2601
-    },
     "JSX element implicitly has type 'any' because the global type 'JSX.Element' does not exist.": {
         "category": "Error",
         "code": 2602
@@ -2574,10 +2462,6 @@
         "category": "Error",
         "code": 2604
     },
-    "JSX element type '{0}' is not a constructor function for JSX elements.": {
-        "category": "Error",
-        "code": 2605
-    },
     "Property '{0}' of JSX spread attribute is not assignable to target property.": {
         "category": "Error",
         "code": 2606
@@ -2703,14 +2587,6 @@
         "category": "Error",
         "code": 2653
     },
-    "Exported external package typings file cannot contain tripleslash references. Please contact the package author to update the package definition.": {
-        "category": "Error",
-        "code": 2654
-    },
-    "Exported external package typings file '{0}' is not a module. Please contact the package author to update the package definition.": {
-        "category": "Error",
-        "code": 2656
-    },
     "JSX expressions must have one parent element.": {
         "category": "Error",
         "code": 2657
@@ -2811,10 +2687,6 @@
         "category": "Error",
         "code": 2681
     },
-    "'get' and 'set' accessor must have the same 'this' type.": {
-        "category": "Error",
-        "code": 2682
-    },
     "'this' implicitly has type 'any' because it does not have a type annotation.": {
         "category": "Error",
         "code": 2683
@@ -4123,10 +3995,6 @@
         "category": "Error",
         "code": 6048
     },
-    "Unsupported locale '{0}'.": {
-        "category": "Error",
-        "code": 6049
-    },
     "Unable to open file '{0}'.": {
         "category": "Error",
         "code": 6050
@@ -4183,10 +4051,6 @@
         "category": "Message",
         "code": 6066
     },
-    "Enables experimental support for ES7 async functions.": {
-        "category": "Message",
-        "code": 6068
-    },
     "Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6).": {
         "category": "Message",
         "code": 6069
@@ -4323,10 +4187,6 @@
         "category": "Message",
         "code": 6102
     },
-    "Option '{0}' should have array of strings as a value.": {
-        "category": "Error",
-        "code": 6103
-    },
     "Checking if '{0}' is the longest matching prefix for '{1}' - '{2}'.": {
         "category": "Message",
         "code": 6104
@@ -4379,14 +4239,6 @@
         "category": "Message",
         "code": 6116
     },
-    "Resolving using primary search paths...": {
-        "category": "Message",
-        "code": 6117
-    },
-    "Resolving from node_modules folder...": {
-        "category": "Message",
-        "code": 6118
-    },
     "======== Type reference directive '{0}' was successfully resolved to '{1}', primary: {2}. ========": {
         "category": "Message",
         "code": 6119
@@ -4878,10 +4730,6 @@
         "code": 6242
     },
 
-    "Projects to reference": {
-        "category": "Message",
-        "code": 6300
-    },
     "Enable project compilation": {
         "category": "Message",
         "code": 6302
@@ -4955,10 +4803,6 @@
         "category": "Message",
         "code": 6359
     },
-    "delete this - Project '{0}' is up to date because it was previously built": {
-        "category": "Message",
-        "code": 6360
-    },
     "Project '{0}' is up to date": {
         "category": "Message",
         "code": 6361
@@ -5483,18 +5327,6 @@
         "category": "Error",
         "code": 8034
     },
-    "Only identifiers/qualified-names with optional type arguments are currently supported in a class 'extends' clause.": {
-        "category": "Error",
-        "code": 9002
-    },
-    "'class' expressions are not currently supported.": {
-        "category": "Error",
-        "code": 9003
-    },
-    "Language service is disabled.": {
-        "category": "Error",
-        "code": 9004
-    },
     "Declaration emit for this file requires using private name '{0}'. An explicit type annotation may unblock declaration emit.": {
         "category": "Error",
         "code": 9005
@@ -5515,10 +5347,6 @@
         "category": "Error",
         "code": 17002
     },
-    "JSX attribute expected.": {
-        "category": "Error",
-        "code": 17003
-    },
     "Cannot use JSX unless the '--jsx' flag is provided.": {
         "category": "Error",
         "code": 17004
@@ -5583,10 +5411,6 @@
         "category": "Error",
         "code": 18000
     },
-    "A path in an 'extends' option must be relative or rooted, but '{0}' is not.": {
-        "category": "Error",
-        "code": 18001
-    },
     "The 'files' list in config file '{0}' is empty.": {
         "category": "Error",
         "code": 18002
@@ -5793,10 +5617,6 @@
         "category": "Message",
         "code": 95001
     },
-    "Convert function '{0}' to class": {
-        "category": "Message",
-        "code": 95002
-    },
     "Convert '{0}' to '{1} in {0}'": {
         "category": "Message",
         "code": 95003
@@ -5825,10 +5645,6 @@
         "category": "Message",
         "code": 95009
     },
-    "Annotate with types from JSDoc": {
-        "category": "Message",
-        "code": 95010
-    },
     "Infer type of '{0}' from usage": {
         "category": "Message",
         "code": 95011
@@ -6193,18 +6009,6 @@
         "category": "Message",
         "code": 95101
     },
-    "Add '@class' tag": {
-        "category": "Message",
-        "code": 95102
-    },
-    "Add '@this' tag": {
-        "category": "Message",
-        "code": 95103
-    },
-    "Add 'this' parameter.": {
-        "category": "Message",
-        "code": 95104
-    },
     "Convert function expression '{0}' to arrow function": {
         "category": "Message",
         "code": 95105