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).

2 files changed, 27 insertions, 10 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)
 		}
 	}
 }
diff --git a/pkg/compiler/testdata/errors2.txt b/pkg/compiler/testdata/errors2.txt
index fb69c8c5f..68b8a66df 100644
--- a/pkg/compiler/testdata/errors2.txt
+++ b/pkg/compiler/testdata/errors2.txt
@@ -11,6 +11,7 @@ resource r2[r1]			### recursive resource r2->r1->r2
 resource r3[int32]		### unused resource r3
 
 foo$0(a0 ptr[out, r0], a1 ptr[out, r1], a2 ptr[out, r2])
+foo$1(a0 r0, a1 r1, a2 r2)
 
 # Recursive structs/unions.
 
@@ -175,9 +176,11 @@ resource r104[int8]
 resource r105[int8]
 resource r106[int8]		### resource r106 can't be created (never mentioned as a syscall return value or output argument/field)
 resource r107[int8]		### resource r107 can't be created (never mentioned as a syscall return value or output argument/field)
+resource r108[int8]		### resource r108 is never used as an input(such resources are not useful)
 
-foo$300(a r100)
+foo$300(a0 r100, a1 r101, a2 r102, a3 r103, a4 r104)
 foo$301(a ptr[out, array[r103]], b ptr[in, s300], c r107) r104
+foo$302() r108
 
 s300 {
 	f1	ptr[inout, s301]