cmd/compile, cmd/compile/internal/syntax: print relative column info

This change enables printing of relative column information if a
prior line directive specified a valid column. If there was no
line directive, or the line directive didn't specify a column
(or the -C flag is specified), no column information is shown in
file positions.

Implementation: Column values (and line values, for that matter)
that are zero are interpreted as "unknown". A line directive that
doesn't specify a column records that as a zero column in the
respective PosBase data structure. When computing relative columns,
a relative value is zero of the base's column value is zero.
When formatting a position, a zero column value is not printed.

To make this work without special cases, the PosBase for a file
is given a concrete (non-0:0) position 1:1 with the PosBase's
line and column also being 1:1. In other words, at the position
1:1 of a file, it's relative positions are starting with 1:1 as
one would expect.

In the package syntax, this requires self-recursive PosBases for
file bases, matching what cmd/internal/src.PosBase was already
doing. In src.PosBase, file and inlining bases also need to be
based at 1:1 to indicate "known" positions.

This change completes the cmd/compiler part of the issue below.

Fixes #22662.

Change-Id: I6c3d2dee26709581fba0d0261b1d12e93f1cba1a
Reviewed-on: https://go-review.googlesource.com/97375
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/gc/noder.go b/src/cmd/compile/internal/gc/noder.go
index 7865550293..e911ac6e42 100644
--- a/src/cmd/compile/internal/gc/noder.go
+++ b/src/cmd/compile/internal/gc/noder.go
@@ -78,13 +78,13 @@ func (p *noder) makeSrcPosBase(b0 *syntax.PosBase) *src.PosBase {
 	b1, ok := p.basemap[b0]
 	if !ok {
 		fn := b0.Filename()
-		if p0 := b0.Pos(); p0.IsKnown() {
+		if b0.IsFileBase() {
+			b1 = src.NewFileBase(fn, absFilename(fn))
+		} else {
 			// line directive base
+			p0 := b0.Pos()
 			p1 := src.MakePos(p.makeSrcPosBase(p0.Base()), p0.Line(), p0.Col())
 			b1 = src.NewLinePragmaBase(p1, fn, fileh(fn), b0.Line(), b0.Col())
-		} else {
-			// file base
-			b1 = src.NewFileBase(fn, absFilename(fn))
 		}
 		p.basemap[b0] = b1
 	}