compiler: Report non-kernel __enter__/__exit__ methods

Signed-off-by: Jonathan Coates <jonathan.coates@oxionics.com>

artiq/compiler/transforms/inferencer.py

@@ -1674,7 +1674,14 @@ class Inferencer(algorithm.Visitor):
                 else:
                     typ = typ.find()
 
-                if not (len(typ.args) == arity and len(typ.optargs) == 0):
+                if not types.is_function(typ):
+                    diag = diagnostic.Diagnostic("error",
+                        "function '{attr}{attr_type}' must be a @kernel",
+                        {"attr": attr_name,
+                         "attr_type": printer.name(typ)},
+                        node.loc)
+                    self.engine.process(diag)
+                elif not (len(typ.args) == arity and len(typ.optargs) == 0):
                     diag = diagnostic.Diagnostic("error",
                         "function '{attr}{attr_type}' must accept "
                         "{arity} positional argument{s} and no optional arguments",

artiq/test/lit/embedding/error_with_rpc.py

@@ -0,0 +1,20 @@
+# RUN: %python -m artiq.compiler.testbench.embedding +diag %s 2>%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+from artiq.experiment import kernel
+
+class contextmgr:
+    def __enter__(self):
+        pass
+
+    @kernel
+    def __exit__(self, n1, n2, n3):
+        pass
+
+c = contextmgr()
+
+@kernel
+def entrypoint():
+    # CHECK-L: ${LINE:+1}: error: function '__enter__[rpc2 #](...)->NoneType' must be a @kernel function
+    with c:
+        pass