go/printer: leave indentation alone when printing nodes from different files

ASTs may be created by various tools and built from nodes of
different files. An incorrectly constructed AST will likely
not print at all, but a (structurally) correct AST with bad
position information should still print structurally correct.

One heuristic used was to reset indentation when the filename
in the position information of nodes changed. However, this
can lead to wrong indentation for structurally correct ASTs.

Fix: Don't change the indentation in this case.

Related to issue 4300.

R=r
CC=golang-dev
https://golang.org/cl/6849066

src/pkg/go/printer/printer.go

@@ -225,7 +225,14 @@ func (p *printer) writeString(pos token.Position, s string, isLit bool) {
 		// (used when printing merged ASTs of different files
 		// e.g., the result of ast.MergePackageFiles)
 		if p.last.IsValid() && p.last.Filename != pos.Filename {
-			p.indent = 0
+			// Note: Do not set p.indent to 0 - this seems to be a bad heuristic.
+			//       ASTs may be created by various tools and built from nodes of
+			//       different files. An incorrectly constructed AST will likely
+			//       not print at all, but a (structurally) correct AST with bad
+			//       position information should still print structurally correct.
+			//       If p.indent is reset, indentation may be off, and likely lead
+			//       to indentation underflow (to detect set: debug = true).
+			//       See also issue 4300 (11/16/2012).
 			p.mode = 0
 			p.wsbuf = p.wsbuf[0:0]
 		}