explain reports captures and the chain a file would get

1 files changed, 25 insertions, 7 deletions

diff --git a/internal/engine/match.go b/internal/engine/match.go
index 67f0c06..591f686 100644
--- a/internal/engine/match.go
+++ b/internal/engine/match.go
@@ -228,10 +228,11 @@ func noDelete(f *facts, scopes [][]string) string {
 
 // RuleTrace is one rule's outcome in an Explain call.
 type RuleTrace struct {
-	Rule    *Rule
-	Match   bool
-	Trace   *cond.Trace // nil when not evaluated
-	Stopped string      // "stopped by rule acme" when an earlier (stop) ended the search
+	Rule     *Rule
+	Match    bool
+	Captures []string    // of a matching rule, as its actions' {N} see them
+	Trace    *cond.Trace // nil when not evaluated
+	Stopped  string      // "stopped by rule acme" when an earlier (stop) ended the search
 }
 
 // ExcludeTrace is one (exclude ...) form's outcome in an Explain call.
@@ -250,6 +251,10 @@ type Explanation struct {
 	Excluded string // the first exclude that matches, which sets the file aside; "" when none does
 	Rules    []RuleTrace
 	NoDelete string // why a delete from the matching rules would be skipped (spec §5.5); "" when it would not
+	// Chain is what the matching rules would do to the file: the same steps
+	// the directory's plan builds for it, placeholders expanded and
+	// conflicts resolved (GUI design §1.3).
+	Chain []plan.Step
 }
 
 // Explain reports, for one file, whether krino's ordinary scan would ever
@@ -333,11 +338,16 @@ func (e *Engine) Explain(ctx context.Context, path string) (*Explanation, error)
 				continue
 			}
 		}
+		var caps []string
 		if match {
+			// Eval, on the same memoised facts and before matched becomes
+			// true, is what gives the captures its actions would use.
+			res := r.Cond.Eval(f)
+			caps = res.Captures
 			f.matched = true
-			matched = append(matched, RuleMatch{Rule: r})
+			matched = append(matched, RuleMatch{Rule: r, Captures: caps, Reasons: res.Reasons})
 		}
-		rules = append(rules, RuleTrace{Rule: r, Match: match, Trace: trace})
+		rules = append(rules, RuleTrace{Rule: r, Match: match, Captures: caps, Trace: trace})
 		if match && r.Conf.Stop {
 			stoppedBy = r.Name
 		}
@@ -347,7 +357,15 @@ func (e *Engine) Explain(ctx context.Context, path string) (*Explanation, error)
 		noDel = noDelete(f, d.DupScopes)
 	}
 
-	return &Explanation{Dir: d, File: sf, Skip: skip, Excludes: excludes, Excluded: excluded, Rules: rules, NoDelete: noDel}, nil
+	var chain []plan.Step
+	if len(matched) > 0 {
+		in := []plan.Input{{File: sf, Rules: planRules(matched), NoDelete: noDel}}
+		if built := plan.Build(d.Root, in, now, plan.OS{}, plan.NewClaims()); len(built) == 1 {
+			chain = built[0].Steps
+		}
+	}
+
+	return &Explanation{Dir: d, File: sf, Skip: skip, Excludes: excludes, Excluded: excluded, Rules: rules, NoDelete: noDel, Chain: chain}, nil
 }
 
 // excludesUseDuplicate reports whether any of d's excludes has a