Add implicit prereq attribute

docs/knit.md

@@ -106,6 +106,10 @@ bar:V: foo[Q]
 The `foo` rule will be quiet only when used as a prerequisite to the `bar`
 rule.
 
+Some attributes can only be applied in this way:
+
+* `I` (implicit): this prereq does not appear in `$input`.
+
 ### Recipes
 
 A recipe is a list of commands to execute. They are executed within the `sh`

rules/graph.go

@@ -323,7 +323,7 @@ func (g *Graph) resolveTargetForRuleSet(rs *RuleSet, dir string, target string,
 			if len(r.recipe) != 0 {
 				// recipe exists -- overwrite prereqs
 				prereqs = r.prereqs
-				expprereqs = r.prereqs
+				expprereqs = explicits(r.prereqs)
 			} else {
 				// recipe is empty -- only add the prereqs
 				prereqs = append(prereqs, r.prereqs...)
@@ -405,7 +405,7 @@ func (g *Graph) resolveTargetForRuleSet(rs *RuleSet, dir string, target string,
 
 				// success -- add the prereqs
 				rule.prereqs = append(rule.prereqs, metarule.prereqs...)
-				expprereqs = metarule.prereqs
+				expprereqs = explicits(metarule.prereqs)
 				// overwrite the recipe/attrs/targets if the matched rule has a
 				// recipe, or we don't yet have a recipe
 				if len(mr.recipe) > 0 || len(rule.recipe) == 0 {
@@ -506,6 +506,17 @@ func (g *Graph) resolveTargetForRuleSet(rs *RuleSet, dir string, target string,
 	return n, nil
 }
 
+func explicits(prereqs []string) []string {
+	exp := make([]string, 0, len(prereqs))
+	for _, p := range prereqs {
+		parsed, _ := parsePrereq(p)
+		if !parsed.attrs.Implicit {
+			exp = append(exp, parsed.name)
+		}
+	}
+	return exp
+}
+
 func (n *node) inputs() []string {
 	ins := make([]string, 0, len(n.myPrereqs))
 	for i, prereq := range n.myPrereqs {

rules/rules.go

@@ -96,14 +96,15 @@ func (r *MetaRule) String() string {
 }
 
 type AttrSet struct {
-	Regex   bool // regular expression meta-rule
-	Virtual bool // targets are not files
-	Quiet   bool // is not displayed as part of the build process
-	NoMeta  bool // cannot be matched by meta rules
-	NonStop bool // does not stop if the recipe fails
-	Rebuild bool // this rule is always out-of-date
-	Linked  bool // only run this rule if a sub-rule that requires it needs to run
-	Order   bool
+	Regex    bool // regular expression meta-rule
+	Virtual  bool // targets are not files
+	Quiet    bool // is not displayed as part of the build process
+	NoMeta   bool // cannot be matched by meta rules
+	NonStop  bool // does not stop if the recipe fails
+	Rebuild  bool // this rule is always out-of-date
+	Linked   bool // only run this rule if a sub-rule that requires it needs to run
+	Implicit bool // not listed in $input
+	Order    bool
 }
 
 func (a *AttrSet) UpdateFrom(other AttrSet) {
@@ -211,6 +212,8 @@ func ParseAttribs(input string) (AttrSet, error) {
 			attrs.Linked = true
 		case 'O':
 			attrs.Order = true
+		case 'I':
+			attrs.Implicit = true
 		default:
 			return attrs, attrError{c}
 		}

test/implicit/Knitfile

@@ -0,0 +1,6 @@
+return b{
+    $ a: foo[I] bar
+        echo $input
+    $ foo:V:
+    $ bar:V:
+}

test/implicit/test.toml

@@ -0,0 +1,12 @@
+name = "Check implicit attributes"
+
+[flags]
+
+knitfile = "Knitfile"
+ncpu = 1
+dryrun = true
+
+[[builds]]
+
+args = []
+output = "echo bar"