ruleguard: better text truncation in the rendered message (#330)

Instead of using variable names like `$x` when substitution
string is too long, truncate it to the max len.

So "some very long text" becomes "some v<...> text".
This is more user-friendly, as pattern variables are very
non-intuitive in most cases.

analyzer/testdata/src/extra/file.go

@@ -328,7 +328,7 @@ func testYodaExpr() {
 	if nil != clusterContext.PostInstallData.CoreDNSUpdateFunction { // want `\Qsuggestion: clusterContext.PostInstallData.CoreDNSUpdateFunction != nil`
 	}
 	// This is far too long, so it's shortened in the output.
-	if nil != clusterContext.PostInstallData.AnotherNestedStruct.DeeplyNestedField { // want `\Qsuggestion: $s != nil`
+	if nil != clusterContext.PostInstallData.AnotherNestedStruct.DeeplyNestedField { // want `\QclusterContext.PostInstallD<...>stedStruct.DeeplyNestedField != nil`
 	}
 }
 

ruleguard/ruleguard_test.go

@@ -9,6 +9,42 @@ import (
 	"github.com/quasilyte/gogrep"
 )
 
+func TestTruncateText(t *testing.T) {
+	tests := []struct {
+		input  string
+		maxLen int
+		want   string
+	}{
+		{"hello world", 60, "hello world"},
+		{"hello world", 8, "h<...>ld"},
+		{"hello world", 7, "h<...>d"},
+		{"hello world", 6, "<...>d"},
+		{"hello world", 5, "<...>"},
+		{"have := truncateText(test.input, test.maxLen)", 20, "have :=<...>.maxLen)"},
+		{"have := truncateText(test.input, test.maxLen)", 30, "have := trun<...> test.maxLen)"},
+		{"have := truncateText(test.input, test.maxLen)", 40, "have := truncateT<...>nput, test.maxLen)"},
+		{"have := truncateText(test.input, test.maxLen)", 41, "have := truncateTe<...>nput, test.maxLen)"},
+		{"have := truncateText(test.input, test.maxLen)", 42, "have := truncateTe<...>input, test.maxLen)"},
+		{"have := truncateText(test.input, test.maxLen)", 50, "have := truncateText(test.input, test.maxLen)"},
+	}
+
+	for _, test := range tests {
+		have := truncateText(test.input, test.maxLen)
+		if len(have) > test.maxLen {
+			t.Errorf("truncateText(%q, %v): len %d exceeeds max len",
+				test.input, test.maxLen, len(have))
+		}
+		if len(test.input) > test.maxLen && len(have) != test.maxLen {
+			t.Errorf("truncateText(%q, %v): truncated more than necessary",
+				test.input, test.maxLen)
+		}
+		if have != test.want {
+			t.Fatalf("truncateText(%q, %v):\nhave: %q\nwant: %q",
+				test.input, test.maxLen, have, test.want)
+		}
+	}
+}
+
 func TestRenderMessage(t *testing.T) {
 	tests := []struct {
 		msg  string

ruleguard/runner.go

@@ -375,14 +375,24 @@ func (rr *rulesRunner) renderMessage(msg string, m matchData, truncate bool) str
 		}
 		buf.Reset()
 		buf.Write(rr.nodeText(n))
-		// Don't interpolate strings that are too long.
-		var replacement string
-		if truncate && buf.Len() > 60 {
-			replacement = key
-		} else {
-			replacement = buf.String()
+		replacement := buf.String()
+		if truncate {
+			replacement = truncateText(replacement, 60)
 		}
 		msg = strings.ReplaceAll(msg, key, replacement)
 	}
 	return msg
 }
+
+func truncateText(s string, maxLen int) string {
+	const placeholder = "<...>"
+	if len(s) <= maxLen-len(placeholder) {
+		return s
+	}
+	maxLen -= len(placeholder)
+	leftLen := maxLen / 2
+	rightLen := (maxLen % 2) + leftLen
+	left := s[:leftLen]
+	right := s[len(s)-rightLen:]
+	return left + placeholder + right
+}