pkg/compiler: check for unused resources

If a resource is never used as an input, it is not useful.
It's effectively the same as using an integer.
Detect such cases, they are quite confusing.
Fix all existing errors in descriptions.
This uncovered some interesting bugs as well,
e.g. use of a completely unrelated fd subtype after copy-paste
(while the resource that was supposed to be used there is completely unused).

1 files changed, 23 insertions, 9 deletions

diff --git a/pkg/compiler/check.go b/pkg/compiler/check.go
index e8abc574e..9bb8c1c0c 100644
--- a/pkg/compiler/check.go
+++ b/pkg/compiler/check.go
@@ -547,16 +547,17 @@ type structDir struct {
 }
 
 func (comp *compiler) checkConstructors() {
-	ctors := make(map[string]bool) // resources for which we have ctors
+	ctors := make(map[string]bool)  // resources for which we have ctors
+	inputs := make(map[string]bool) // resources which are used as inputs
 	checked := make(map[structDir]bool)
 	for _, decl := range comp.desc.Nodes {
 		switch n := decl.(type) {
 		case *ast.Call:
 			for _, arg := range n.Args {
-				comp.checkTypeCtors(arg.Type, prog.DirIn, true, ctors, checked)
+				comp.checkTypeCtors(arg.Type, prog.DirIn, true, ctors, inputs, checked)
 			}
 			if n.Ret != nil {
-				comp.checkTypeCtors(n.Ret, prog.DirOut, true, ctors, checked)
+				comp.checkTypeCtors(n.Ret, prog.DirOut, true, ctors, inputs, checked)
 			}
 		}
 	}
@@ -564,17 +565,23 @@ func (comp *compiler) checkConstructors() {
 		switch n := decl.(type) {
 		case *ast.Resource:
 			name := n.Name.Name
-			if !ctors[name] && comp.used[name] {
+			if !comp.used[name] {
+				continue
+			}
+			if !ctors[name] {
 				comp.error(n.Pos, "resource %v can't be created"+
-					" (never mentioned as a syscall return value or output argument/field)",
-					name)
+					" (never mentioned as a syscall return value or output argument/field)", name)
+			}
+			if !inputs[name] {
+				comp.error(n.Pos, "resource %v is never used as an input"+
+					"(such resources are not useful)", name)
 			}
 		}
 	}
 }
 
 func (comp *compiler) checkTypeCtors(t *ast.Type, dir prog.Dir, isArg bool,
-	ctors map[string]bool, checked map[structDir]bool) {
+	ctors, inputs map[string]bool, checked map[structDir]bool) {
 	desc := comp.getTypeDesc(t)
 	if desc == typeResource {
 		// TODO(dvyukov): consider changing this to "dir == prog.DirOut".
@@ -589,6 +596,13 @@ func (comp *compiler) checkTypeCtors(t *ast.Type, dir prog.Dir, isArg bool,
 				r = comp.resources[r.Base.Ident]
 			}
 		}
+		if dir != prog.DirOut {
+			r := comp.resources[t.Ident]
+			for r != nil && !inputs[r.Name.Name] {
+				inputs[r.Name.Name] = true
+				r = comp.resources[r.Base.Ident]
+			}
+		}
 		return
 	}
 	if desc == typeStruct {
@@ -600,7 +614,7 @@ func (comp *compiler) checkTypeCtors(t *ast.Type, dir prog.Dir, isArg bool,
 		}
 		checked[key] = true
 		for _, fld := range s.Fields {
-			comp.checkTypeCtors(fld.Type, dir, false, ctors, checked)
+			comp.checkTypeCtors(fld.Type, dir, false, ctors, inputs, checked)
 		}
 		return
 	}
@@ -610,7 +624,7 @@ func (comp *compiler) checkTypeCtors(t *ast.Type, dir prog.Dir, isArg bool,
 	_, args, _ := comp.getArgsBase(t, isArg)
 	for i, arg := range args {
 		if desc.Args[i].Type == typeArgType {
-			comp.checkTypeCtors(arg, dir, desc.Args[i].IsArg, ctors, checked)
+			comp.checkTypeCtors(arg, dir, desc.Args[i].IsArg, ctors, inputs, checked)
 		}
 	}
 }