analysis/report: better handling of sourcers with no node

Some IR values do not have a valid Source, most prominently constants.
Don't panic when we can avoid it: when trying to emit related
information, it makes more sense to drop the related information on
the floor than to panic. We still panic for the toplevel diagnostic,
because it makes no sense to print a diagnostic for an unknown
location. It is an explicit panic now, however, so we get a more
helpful message.

Closes gh-972

Author: dominikh

analysis/report/report.go

@@ -2,6 +2,7 @@ package report
 
 import (
 	"bytes"
+	"fmt"
 	"go/ast"
 	"go/printer"
 	"go/token"
@@ -44,7 +45,10 @@ func Fixes(fixes ...analysis.SuggestedFix) Option {
 
 func Related(node Positioner, message string) Option {
 	return func(opts *Options) {
-		pos, end := getRange(node, opts.ShortRange)
+		pos, end, ok := getRange(node, opts.ShortRange)
+		if !ok {
+			return
+		}
 		r := analysis.RelatedInformation{
 			Pos:     pos,
 			End:     end,
@@ -126,21 +130,26 @@ func shortRange(node ast.Node) (pos, end token.Pos) {
 	}
 }
 
-func getRange(node Positioner, short bool) (pos, end token.Pos) {
-	switch node := node.(type) {
+func getRange(node Positioner, short bool) (pos, end token.Pos, ok bool) {
+	switch n := node.(type) {
 	case sourcer:
-		s := node.Source()
+		s := n.Source()
+		if s == nil {
+			return 0, 0, false
+		}
 		if short {
-			return shortRange(s)
+			p, e := shortRange(s)
+			return p, e, true
 		}
-		return s.Pos(), s.End()
+		return s.Pos(), s.End(), true
 	case fullPositioner:
 		if short {
-			return shortRange(node)
+			p, e := shortRange(n)
+			return p, e, true
 		}
-		return node.Pos(), node.End()
+		return n.Pos(), n.End(), true
 	default:
-		return node.Pos(), token.NoPos
+		return n.Pos(), token.NoPos, true
 	}
 }
 
@@ -158,7 +167,10 @@ func Report(pass *analysis.Pass, node Positioner, message string, opts ...Option
 		}
 	}
 
-	pos, end := getRange(node, cfg.ShortRange)
+	pos, end, ok := getRange(node, cfg.ShortRange)
+	if !ok {
+		panic(fmt.Sprintf("no valid position for reporting node %v", node))
+	}
 	d := analysis.Diagnostic{
 		Pos:            pos,
 		End:            end,