tools/bump: say which gates actually ran

The body claimed the nix checks had run regardless of --gate, which is
exactly the sort of thing a reviewer takes at face value.

tools/bump/main.go

@@ -29,7 +29,7 @@ import (
 )
 
 type runFlags struct {
-	DryRun bool   `flag:"dry-run,default=false,Apply and report, but run no final gate and touch no remote"`
+	DryRun bool   `flag:"dry-run,default=false,Rebuild the branch locally and report, but run no final gate and touch no remote"`
 	NoPR   bool   `flag:"no-pr,default=false,Push nothing and open no pull request"`
 	Areas  string `flag:"areas,Comma-separated areas to run (default: all)"`
 	Skip   string `flag:"skip,Comma-separated areas to skip"`
@@ -136,7 +136,7 @@ func cmdRun(ctx context.Context) error {
 		return err
 	}
 
-	body := renderBody(results, markerOf(results, tree, head))
+	body := renderBody(results, markerOf(results, tree, head), gate)
 
 	if err := writeStepSummary(body); err != nil { //nolint:noinlineerr
 		return err
@@ -165,6 +165,7 @@ func cmdRun(ctx context.Context) error {
 		Branch: runCfg.Branch,
 		Base:   runCfg.Base,
 		Title:  "all: bump pinned versions",
+		Gate:   gate,
 		Force:  runCfg.Force,
 	}, results)
 }

tools/bump/pr.go

@@ -81,6 +81,7 @@ type publishOptions struct {
 	Branch string
 	Base   string
 	Title  string
+	Gate   string
 	Force  bool
 }
 
@@ -99,7 +100,7 @@ func publish(ctx context.Context, r *repo, opt publishOptions, results []result)
 	}
 
 	now := markerOf(results, tree, head)
-	body := renderBody(results, now)
+	body := renderBody(results, now, opt.Gate)
 
 	pr, err := lookupPR(ctx, r, opt.Slug, opt.Branch)
 	if err != nil {

tools/bump/report.go

@@ -63,7 +63,7 @@ func markerOf(results []result, tree, head string) marker {
 // renderBody writes the pull request body. It leads with what landed and what
 // did not, because the point of the bot is that the reader can decide from the
 // body plus the checks without reproducing the run.
-func renderBody(results []result, m marker) string {
+func renderBody(results []result, m marker, gate string) string {
 	var sb strings.Builder
 
 	sb.WriteString("Automated version bump.\n\n")
@@ -81,8 +81,8 @@ func renderBody(results []result, m marker) string {
 	writeDetails(&sb, results)
 	writeDropped(&sb, results)
 
-	sb.WriteString("\nThe integration matrix is left to this pull request's own CI; ")
-	sb.WriteString("the nix checks and the tailscale builder images were already run in the bump job.\n")
+	sb.WriteString("\n")
+	sb.WriteString(gateNote(gate))
 	sb.WriteString("\n")
 	sb.WriteString(m.render())
 	sb.WriteString("\n")
@@ -158,14 +158,31 @@ func writeLog(sb *strings.Builder, log string) {
 	sb.WriteString("\n  ```\n")
 }
 
+// gateNote says what the bump job did and did not already run, so the reader
+// knows how much of the green tick below is new information.
+func gateNote(gate string) string {
+	switch gate {
+	case gateFull:
+		return "The nix checks and the tailscale builder images were already run in the bump job; " +
+			"the integration matrix is left to this pull request's own CI.\n"
+	case gateQuick:
+		return "Only `nix build .#checks.<system>.build` was run in the bump job; " +
+			"everything else is left to this pull request's own CI.\n"
+	default:
+		return "No gate was run in the bump job; every check below is the first one.\n"
+	}
+}
+
 // cell keeps a markdown table cell from breaking the table.
 func cell(s string) string {
 	s = strings.ReplaceAll(s, "|", "\\|")
 	s = strings.ReplaceAll(s, "\n", " ")
 
 	const maxCell = 160
-	if len(s) > maxCell {
-		s = s[:maxCell] + "…"
+
+	runes := []rune(s)
+	if len(runes) > maxCell {
+		s = string(runes[:maxCell]) + "…"
 	}
 
 	if s == "" {

tools/bump/report_test.go

@@ -26,7 +26,7 @@ func TestMarkerRoundTrip(t *testing.T) {
 	}
 
 	want := markerOf(results, "tree-sha", "head-sha")
-	body := renderBody(results, want)
+	body := renderBody(results, want, gateFull)
 
 	got, ok := parseMarker(body)
 	if !ok {