tfdiags: SourceRange.StartString

This helper provides a concise string showing the filename, start line and
start column for a range, for easy inclusion in error messages.

tfdiags/source_range.go

@@ -1,5 +1,11 @@
 package tfdiags
 
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
 type SourceRange struct {
 	Filename   string
 	Start, End SourcePos
@@ -8,3 +14,22 @@ type SourceRange struct {
 type SourcePos struct {
 	Line, Column, Byte int
 }
+
+// StartString returns a string representation of the start of the range,
+// including the filename and the line and column numbers.
+func (r SourceRange) StartString() string {
+	filename := r.Filename
+
+	// We'll try to relative-ize our filename here so it's less verbose
+	// in the common case of being in the current working directory. If not,
+	// we'll just show the full path.
+	wd, err := os.Getwd()
+	if err == nil {
+		relFn, err := filepath.Rel(wd, filename)
+		if err == nil {
+			filename = relFn
+		}
+	}
+
+	return fmt.Sprintf("%s:%d,%d", filename, r.Start.Line, r.Start.Column)
+}