Fix @unused for last argument of function (#1518)

Regressed in d2a20dc9d54727597d66fd091cb16711375cac2b

---------

Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>

warn/warn_control_flow.go

diff --git a/warn/warn_control_flow.go b/warn/warn_control_flow.go
index 2c0a17a..e1ca26a 100644
--- a/warn/warn_control_flow.go
+++ b/warn/warn_control_flow.go
@@ -473,13 +473,16 @@
 				}
 			}
 			// Collect its (normal) parameters as defined in the current scope.
-			for _, param := range expr.Params {
+			finalParameterMarkedUnused := expr.ColonPos != nil && edit.ContainsComments(expr.ColonPos, "@unused")
+			for i, param := range expr.Params {
 				// Function parameters are defined in the current scope.
 				if ident, _ := build.GetParamIdent(param); ident != nil {
 					definedSymbols[ident.Name] = ident
-					if ident.Name == "name" || strings.HasPrefix(ident.Name, "_") || edit.ContainsComments(param, "@unused") {
+					if ident.Name == "name" || strings.HasPrefix(ident.Name, "_") || edit.ContainsComments(param, "@unused") ||
+						i == len(expr.Params)-1 && finalParameterMarkedUnused {
 						// Don't warn about function arguments if they start with "_"
-						// or explicitly marked with @unused.
+						// or explicitly marked with @unused. An @unused comment at the
+						// end of the function header applies to its final argument.
 						// Also don't warn about unused "name" arguments, it could be a
 						// macro where such argument is encouraged (by `unnamed-macro`)
 						// even if not used.

warn/warn_control_flow_test.go

diff --git a/warn/warn_control_flow_test.go b/warn/warn_control_flow_test.go
index adfe679..a721ced 100644
--- a/warn/warn_control_flow_test.go
+++ b/warn/warn_control_flow_test.go
@@ -655,6 +655,40 @@
 		scopeEverywhere)
 
 	checkFindings(t, "unused-variable", `
+def foo(x):  # @unused
+  pass
+
+def bar(
+    x,
+    y):  # @unused
+  pass
+
+foo()
+bar()
+`,
+		[]string{
+			":5: Variable \"x\" is unused.",
+		},
+		scopeEverywhere)
+
+	checkFindings(t, "unused-variable", `
+def foo(x) -> int:  # @unused
+  pass
+
+def bar(
+    x,
+    y) -> int:  # @unused
+  pass
+
+foo()
+bar()
+`,
+		[]string{
+			":5: Variable \"x\" is unused.",
+		},
+		scopeEverywhere)
+
+	checkFindings(t, "unused-variable", `
 def foo(
     name,
     x):