feat: Improve error message for shadowed inout arguments (#351)

Closes #338

guppylang/checker/linearity_checker.py

@@ -244,6 +244,14 @@ def _check_assign_targets(self, targets: list[ast.expr]) -> None:
         [target] = targets
         for tgt in find_nodes(lambda n: isinstance(n, PlaceNode), target):
             assert isinstance(tgt, PlaceNode)
+            # Special error message for shadowing of @inout vars
+            x = tgt.place.id
+            if x in self.scope.vars and is_inout_var(self.scope[x]):
+                raise GuppyError(
+                    f"Assignment shadows argument `{tgt.place}` annotated as `@inout`. "
+                    "Consider assigning to a different name.",
+                    tgt,
+                )
             for tgt_place in leaf_places(tgt.place):
                 x = tgt_place.id
                 if x in self.scope and not self.scope.used(x):
@@ -361,6 +369,25 @@ def check_cfg_linearity(cfg: "CheckedCFG", globals: Globals) -> None:
         for bb in cfg.bbs
     }
 
+    # Check that @inout vars are not being shadowed. This would also be caught by
+    # the dataflow analysis below, however we can give nicer error messages here.
+    for bb, scope in scopes.items():
+        if bb == cfg.entry_bb:
+            # Arguments are assigned in the entry BB, so would yield a false positive
+            # in the check below. Shadowing in the entry BB will be caught by the check
+            # in `_check_assign_targets`.
+            continue
+        entry_scope = scopes[cfg.entry_bb]
+        for x, place in scope.vars.items():
+            if x in entry_scope:
+                entry_place = entry_scope[x]
+                if is_inout_var(entry_place):
+                    raise GuppyError(
+                        f"Assignment shadows argument `{entry_place}` annotated as "
+                        "`@inout`. Consider assigning to a different name.",
+                        place.defined_at,
+                    )
+
     # Mark the @inout variables as implicitly used in the exit BB
     exit_scope = scopes[cfg.exit_bb]
     for var in cfg.entry_bb.sig.input_row:

tests/error/inout_errors/shadow.err

@@ -1,6 +1,7 @@
-Guppy compilation failed. Error in file $FILE:11
+Guppy compilation failed. Error in file $FILE:12
 
-9:     @guppy(module)
-10:    def test(q: qubit @inout) -> None:
-                ^^^^^^^^^^^^^^^
-GuppyError: Variable `q` with linear type `qubit` is not used
+10:    @guppy(module)
+11:    def test(q: qubit @inout) -> None:
+12:        q = qubit()
+           ^
+GuppyError: Assignment shadows argument `q` annotated as `@inout`. Consider assigning to a different name.

tests/error/inout_errors/shadow_if.err

@@ -1,6 +1,7 @@
-Guppy compilation failed. Error in file $FILE:11
+Guppy compilation failed. Error in file $FILE:13
 
-9:     @guppy(module)
-10:    def test(q: qubit @inout, b: bool) -> None:
-                ^^^^^^^^^^^^^^^
-GuppyError: Variable `q` with linear type `qubit` is not used on all control-flow paths
+11:    def test(q: qubit @inout, b: bool) -> None:
+12:        if b:
+13:            q = qubit()
+               ^
+GuppyError: Assignment shadows argument `q` annotated as `@inout`. Consider assigning to a different name.