Allow keep to be explained

README.rst

@@ -979,6 +979,9 @@ In addition to directives, Gazelle supports ``# keep`` comments that protect
 parts of build files from being modified. ``# keep`` may be written before
 a rule, before an attribute, or after a string within a list.
 
+``# keep`` comments might take one of 2 forms; the ``# keep`` literal or a
+description prefixed by ``# keep: ``.
+
 Example
 ^^^^^^^
 
@@ -1003,6 +1006,7 @@ know what imports to resolve, so you may need to add dependencies manually with
       visibility = ["//visibility:public"],
       deps = [
           "@com_github_example_gen//:go_default_library",  # keep
+          "@com_github_example_gen//a/b/c:go_default_library",  # keep: this is also important
       ],
   )
 

rule/rule.go

@@ -996,7 +996,7 @@ func (r *Rule) sync() {
 func ShouldKeep(e bzl.Expr) bool {
 	for _, c := range append(e.Comment().Before, e.Comment().Suffix...) {
 		text := strings.TrimSpace(strings.TrimPrefix(c.Token, "#"))
-		if text == "keep" {
+		if text == "keep" || strings.HasPrefix(text, "keep: ") {
 			return true
 		}
 	}

rule/rule_test.go

@@ -302,6 +302,13 @@ func TestKeepRule(t *testing.T) {
 			src: `
 # keep
 x_library(name = "x")
+`,
+			want: true,
+		}, {
+			desc: "prefix with description",
+			src: `
+# keep: hack, see more in ticket #42
+x_library(name = "x")
 `,
 			want: true,
 		}, {
@@ -360,6 +367,27 @@ func TestShouldKeepExpr(t *testing.T) {
 "s"
 `,
 			want: true,
+		}, {
+			desc: "before with description",
+			src: `
+# keep: we need it for the ninja feature
+"s"
+`,
+			want: true,
+		}, {
+			desc: "before but not the correct prefix (keeping)",
+			src: `
+			# keeping this for now
+"s"
+`,
+			want: false,
+		}, {
+			desc: "before but not the correct prefix (no colon)",
+			src: `
+			# keep this around for the time being
+"s"
+`,
+			want: false,
 		}, {
 			desc: "suffix",
 			src: `