cmd/compile: extrapolate $GOROOT in unified IR

This ensures that diagnostics for files within $GOROOT continue to be
reported using their full filepath, rather than the abbreviated
filepath. Notably, this is necessary for test/run.go, which has tests
that expect to see the full filepath.

Updates #48247.

Change-Id: I440e2c6dd6109ca059d81cee49e476bba805d703
Reviewed-on: https://go-review.googlesource.com/c/go/+/348670
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/compile/internal/noder/reader.go

@@ -10,6 +10,7 @@ import (
 	"bytes"
 	"fmt"
 	"go/constant"
+	"internal/buildcfg"
 	"strings"
 
 	"cmd/compile/internal/base"
@@ -194,15 +195,32 @@ func (pr *pkgReader) posBaseIdx(idx int) *src.PosBase {
 	r := pr.newReader(relocPosBase, idx, syncPosBase)
 	var b *src.PosBase
 
-	filename := r.string()
+	absFilename := r.string()
+	filename := absFilename
+
+	// For build artifact stability, the export data format only
+	// contains the "absolute" filename as returned by objabi.AbsFile.
+	// However, some tests (e.g., test/run.go's asmcheck tests) expect
+	// to see the full, original filename printed out. Re-expanding
+	// "$GOROOT" to buildcfg.GOROOT is a close-enough approximation to
+	// satisfy this.
+	//
+	// TODO(mdempsky): De-duplicate this logic with similar logic in
+	// cmd/link/internal/ld's expandGoroot. However, this will probably
+	// require being more consistent about when we use native vs UNIX
+	// file paths.
+	const dollarGOROOT = "$GOROOT"
+	if strings.HasPrefix(filename, dollarGOROOT) {
+		filename = buildcfg.GOROOT + filename[len(dollarGOROOT):]
+	}
 
 	if r.bool() {
-		b = src.NewFileBase(filename, filename)
+		b = src.NewFileBase(filename, absFilename)
 	} else {
 		pos := r.pos0()
 		line := r.uint()
 		col := r.uint()
-		b = src.NewLinePragmaBase(pos, filename, filename, line, col)
+		b = src.NewLinePragmaBase(pos, filename, absFilename, line, col)
 	}
 
 	pr.posBases[idx] = b