go/printer: implement SourcePos mode

If a printer is configured with the SourcePos mode
set, it will emit //-line comments as necessary to
ensure that the result - if reparsed - reflects the
original source position information.

This change required a bit of reworking of the
output section in printer.go. Specifically:

- Introduced new Config mode 'SourcePos'.

- Introduced new position 'out' which tracks the
position of the generated output if it were read
in again. If there is a discrepancy between out
and the current AST/source position, a //line
comment is emitted to correct for it.

- Lazy emission of indentation so that //line
comments can be placed correctly. As a result,
the trimmer will have to do less work.

- Merged writeItem into writeString.

- Merged writeByteN into writeByte.

- Use a []byte instead of a byte.Buffer both in the
printer and in the trimmer (eliminates dependency).

Also: introduced explicit printer.Mode type (in
sync w/ parser.Mode, scanner.Mode, etc.)

Runs all tests. Applied gofmt to src, misc w/o changes.

Fixes issue 1047.
Fixes issue 2697.

R=rsc, rsc
CC=golang-dev
http://codereview.appspot.com/5643066

1 files changed, 1 insertions, 1 deletions

diff --git a/src/cmd/gofmt/gofmt.go b/src/cmd/gofmt/gofmt.go
index 6d610adc0..55c01beb5 100644
--- a/src/cmd/gofmt/gofmt.go
+++ b/src/cmd/gofmt/gofmt.go
@@ -45,7 +45,7 @@ var (
 	exitCode    = 0
 	rewrite     func(*ast.File) *ast.File
 	parserMode  parser.Mode
-	printerMode uint
+	printerMode printer.Mode
 )
 
 func report(err error) {