Warn when using windows style path separator (microsoft#4023)

fix [microsoft#1261](microsoft#1261)

Using windows style separator can cause issue when trying to run a spec
on a non windows platform on top of being a pain to work with(escaping)

This PR adds validation to config element that are already validated as
`absolute-path` to also check they unix a unix style of path

.chronus/changes/path-unix-style-2024-6-25-15-13-16.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Warn when using `\` in config file field that expect a path.

packages/compiler/src/config/config-loader.ts

@@ -260,6 +260,13 @@ function validatePathAbsolute(
       target: target === NoTarget ? target : getLocationInYamlScript(target.file, target.path),
     });
   }
+  if (path.includes("\\")) {
+    return createDiagnostic({
+      code: "path-unix-style",
+      format: { path },
+      target: target === NoTarget ? target : getLocationInYamlScript(target.file, target.path),
+    });
+  }
 
   return undefined;
 }

packages/compiler/src/core/messages.ts

@@ -606,6 +606,12 @@ const diagnostics = {
       default: paramMessage`Path "${"path"}" cannot be relative. Use {cwd} or {project-root} to specify what the path should be relative to.`,
     },
   },
+  "path-unix-style": {
+    severity: "warning",
+    messages: {
+      default: paramMessage`Path should use unix style separators. Use "/" instead of "\\".`,
+    },
+  },
   "config-path-not-found": {
     severity: "error",
     messages: {

packages/compiler/src/core/schema-validator.ts

@@ -18,6 +18,16 @@ export interface JSONSchemaValidatorOptions {
   strict?: boolean;
 }
 
+function absolutePathStatus(path: string): "valid" | "not-absolute" | "windows-style" {
+  if (path.startsWith(".") || !isPathAbsolute(path)) {
+    return "not-absolute";
+  }
+  if (path.includes("\\")) {
+    return "windows-style";
+  }
+  return "valid";
+}
+
 export function createJSONSchemaValidator<T>(
   schema: JSONSchemaType<T>,
   options: JSONSchemaValidatorOptions = { strict: true }
@@ -30,9 +40,7 @@ export function createJSONSchemaValidator<T>(
 
   ajv.addFormat("absolute-path", {
     type: "string",
-    validate: (path) => {
-      return !path.startsWith(".") && isPathAbsolute(path);
-    },
+    validate: (path) => absolutePathStatus(path) === "valid",
   });
   return { validate };
 
@@ -66,11 +74,21 @@ function ajvErrorToDiagnostic(
 ): Diagnostic {
   const tspTarget = resolveTarget(error, target);
   if (error.params.format === "absolute-path") {
-    return createDiagnostic({
-      code: "config-path-absolute",
-      format: { path: getErrorValue(obj, error) as any },
-      target: tspTarget,
-    });
+    const value = getErrorValue(obj, error) as any;
+    const status = absolutePathStatus(value);
+    if (status === "windows-style") {
+      return createDiagnostic({
+        code: "path-unix-style",
+        format: { path: value },
+        target: tspTarget,
+      });
+    } else {
+      return createDiagnostic({
+        code: "config-path-absolute",
+        format: { path: value },
+        target: tspTarget,
+      });
+    }
   }
 
   const messageLines = [`Schema violation: ${error.message} (${error.instancePath || "/"})`];

packages/compiler/test/core/emitter-options.test.ts

@@ -117,5 +117,15 @@ describe("compiler: emitter options", () => {
         message: `Path "assets" cannot be relative. Use {cwd} or {project-root} to specify what the path should be relative to.`,
       });
     });
+
+    it("emit diagnostic if passing windows style path", async () => {
+      const diagnostics = await diagnoseEmitterOptions({
+        "asset-dir": "C:\\abc\\def",
+      });
+      expectDiagnostics(diagnostics, {
+        code: "path-unix-style",
+        message: `Path should use unix style separators. Use "/" instead of "\\".`,
+      });
+    });
   });
 });