fix(directives): report server functions that cannot work (#2318)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Brenley Dueck <brenleydueck@gmail.com>

Author: 3 people

.changeset/server-function-diagnostics.md

@@ -0,0 +1,15 @@
+---
+"@solidjs/start": patch
+---
+
+Report server functions that cannot work instead of compiling them into broken output.
+
+- A `"use server"` function that reads a variable from an enclosing function now fails the build. The function is moved to the top level of its module, so the variable is not in scope when it runs.
+- The same check covers `this` and `arguments` in an arrow function, `super`, and private class members.
+- A `"use server"` directive in an object or class method now fails the build. It was ignored before, which shipped the method body and the modules it imports to the browser.
+- A `"use server"` string that is not the first statement of a module or a function body now logs a warning. It has no effect there.
+- An export a `"use server"` module cannot serve now logs a warning that names it. These exports are still left out of the client build.
+- A `"use server"` module can now export an anonymous default function. Both `export default async () => {}` and `export default async function () {}` work.
+- Server function ids are now built from the names a function is nested under, such as `Page.load`, instead of the order the functions appear in. An id no longer changes when another server function is added to the same file, and two functions that share a name are told apart by the names around them. Production ids stay opaque.
+- Server functions are now compiled in `.mts` and `.cts` files.
+- Build errors now point at the full path of the file, not just its name.