embedding: reimplement 373578bc properly.

The core of the problem that 373578bc was attempting to solve is
that diagnostics sometimes should be chained; one way of chaining
is the loc.expanded_from feature, which handles macro-like expansion,
but another is providing context.

Before this commit, context was provided using an ad-hoc override
of a diagnostic engine, which did not work in cases where diagnostic
engine was not threaded through the call stack. This commit uses
the newly added pythonparser context feature to elegantly handle
the problem.

artiq/compiler/embedding.py

@@ -556,23 +556,17 @@ class StitchingInferencer(Inferencer):
                 attr_value_type = builtins.TList(builtins.TInt64())
 
         if attr_value_type is None:
-            # Slow path. We don't know what exactly is the attribute value,
-            # so we quote it only for the error message that may possibly result.
-            ast = self.quote(attr_value, loc)
-
-            def proxy_diagnostic(diag):
             note = diagnostic.Diagnostic("note",
                 "while inferring a type for an attribute '{attr}' of a host object",
                 {"attr": attr_name},
                 loc)
-                diag.notes.append(note)
 
-                self.engine.process(diag)
-
-            proxy_engine = diagnostic.Engine()
-            proxy_engine.process = proxy_diagnostic
-            Inferencer(engine=proxy_engine).visit(ast)
-            IntMonomorphizer(engine=proxy_engine).visit(ast)
+            with self.engine.context(note):
+                # Slow path. We don't know what exactly is the attribute value,
+                # so we quote it only for the error message that may possibly result.
+                ast = self.quote(attr_value, object_loc.expanded_from)
+                Inferencer(engine=self.engine).visit(ast)
+                IntMonomorphizer(engine=self.engine).visit(ast)
                 attr_value_type = ast.type
 
         return attributes, attr_value_type
@@ -840,31 +834,23 @@ class Stitcher:
                 notes=self._call_site_note(loc, is_syscall))
             self.engine.process(diag)
         elif param.default is not inspect.Parameter.empty:
+            notes = []
+            notes.append(diagnostic.Diagnostic("note",
+                "expanded from here while trying to infer a type for an"
+                " unannotated optional argument '{argument}' from its default value",
+                {"argument": param.name},
+                self._function_loc(function)))
+            if loc is not None:
+                notes.append(self._call_site_note(loc, is_syscall))
+
+            with self.engine.context(*notes):
                 # Try and infer the type from the default value.
                 # This is tricky, because the default value might not have
                 # a well-defined type in APython.
                 # In this case, we bail out, but mention why we do it.
                 ast = self._quote(param.default, None)
-
-            def proxy_diagnostic(diag):
-                note = diagnostic.Diagnostic("note",
-                    "expanded from here while trying to infer a type for an"
-                    " unannotated optional argument '{argument}' from its default value",
-                    {"argument": param.name},
-                    self._function_loc(function))
-                diag.notes.append(note)
-
-                note = self._call_site_note(loc, is_syscall)
-                if note:
-                    diag.notes += note
-
-                self.engine.process(diag)
-
-            proxy_engine = diagnostic.Engine()
-            proxy_engine.process = proxy_diagnostic
-            Inferencer(engine=proxy_engine).visit(ast)
-            IntMonomorphizer(engine=proxy_engine).visit(ast)
-
+                Inferencer(engine=self.engine).visit(ast)
+                IntMonomorphizer(engine=self.engine).visit(ast)
                 return ast.type
         else:
             # Let the rest of the program decide.

artiq/test/lit/embedding/error_host_only.py

@@ -0,0 +1,19 @@
+# RUN: %python -m artiq.compiler.testbench.embedding +diag %s 2>%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+from artiq.language.core import *
+from artiq.language.types import *
+
+class foo:
+    # CHECK-L: ${LINE:+2}: fatal: this function cannot be called as an RPC
+    @host_only
+    def pause(self):
+        pass
+
+x = foo()
+
+@kernel
+def entrypoint():
+    # CHECK-L: ${LINE:+2}: note: in function called remotely here
+    # CHECK-L: ${LINE:+1}: note: while inferring a type for an attribute 'pause' of a host object
+    x.pause()