cmd/compile/internal/syntax, types2: move cmpPos to pos.Cmp

Make position comparison generally available.

Change-Id: I94b6f658fa19a15b30574dbb2181879115c131a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/307215
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

2 files changed, 43 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/syntax/pos.go b/src/cmd/compile/internal/syntax/pos.go
index 99734d42d8..baebcc995c 100644
--- a/src/cmd/compile/internal/syntax/pos.go
+++ b/src/cmd/compile/internal/syntax/pos.go
@@ -59,6 +59,45 @@ func (pos Pos) RelCol() uint {
 	return pos.Col()
 }
 
+// Cmp compares the positions p and q and returns a result r as follows:
+//
+//	r <  0: p is before q
+//	r == 0: p and q are the same position (but may not be identical)
+//	r >  0: p is after q
+//
+// If p and q are in different files, p is before q if the filename
+// of p sorts lexicographically before the filename of q.
+func (p Pos) Cmp(q Pos) int {
+	pname := p.RelFilename()
+	qname := q.RelFilename()
+	switch {
+	case pname < qname:
+		return -1
+	case pname > qname:
+		return +1
+	}
+
+	pline := p.Line()
+	qline := q.Line()
+	switch {
+	case pline < qline:
+		return -1
+	case pline > qline:
+		return +1
+	}
+
+	pcol := p.Col()
+	qcol := q.Col()
+	switch {
+	case pcol < qcol:
+		return -1
+	case pcol > qcol:
+		return +1
+	}
+
+	return 0
+}
+
 func (pos Pos) String() string {
 	rel := position_{pos.RelFilename(), pos.RelLine(), pos.RelCol()}
 	abs := position_{pos.Base().Pos().RelFilename(), pos.Line(), pos.Col()}
diff --git a/src/cmd/compile/internal/syntax/testing.go b/src/cmd/compile/internal/syntax/testing.go
index 3e02dc1c5d..6a97dc0c2a 100644
--- a/src/cmd/compile/internal/syntax/testing.go
+++ b/src/cmd/compile/internal/syntax/testing.go
@@ -33,10 +33,10 @@ var errRx = regexp.MustCompile(`^ *ERROR *"?([^"]*)"?`)
 // for each Error is the position of the token immediately preceding
 // the comment, the Error message is the message msg extracted from
 // the comment, with all errors that are on the same line collected
-// in a slice. If there is no preceding token (the `ERROR` comment
-// appears in the beginning of the file), then the recorded position
-// is unknown (line, col = 0, 0). If there are no ERROR comments, the
-// result is nil.
+// in a slice, in source order. If there is no preceding token (the
+// `ERROR` comment appears in the beginning of the file), then the
+// recorded position is unknown (line, col = 0, 0). If there are no
+// ERROR comments, the result is nil.
 func ErrorMap(src io.Reader) (errmap map[uint][]Error) {
 	// position of previous token
 	var base *PosBase