Print relative filenames in errors

Error messages could get quite lengthy as the code was written previously,
because we always used the complete absolute path for the file in question.
This change "prettifies" this to be relative to whatever contextual path
the user has chosen during compilation.  This shortens messages considerably.

pkg/compiler/opts.go

@@ -12,8 +12,8 @@ type Options struct {
 }
 
 // DefaultOpts returns the default set of compiler options.
-func DefaultOpts() Options {
+func DefaultOpts(pwd string) Options {
 	return Options{
-		Diag: diag.DefaultSink(),
+		Diag: diag.DefaultSink(pwd),
 	}
 }

pkg/diag/pos.go

@@ -4,13 +4,18 @@ package diag
 
 // Pos represents a position in a file.
 type Pos struct {
-	Row int
+	Ln  int
 	Col int
 }
 
 // EmptyPos may be used when no position is needed.
 var EmptyPos = Pos{0, 0}
 
+// IsEmpty returns true if the Pos information is missing.
+func (pos Pos) IsEmpty() bool {
+	return pos.Ln == 0 && pos.Col == 0
+}
+
 // Location represents a region spanning two positions in a file.
 type Location struct {
 	Start Pos
@@ -19,3 +24,8 @@ type Location struct {
 
 // EmptyLocation may be used when no position information is available.
 var EmptyLocation = Location{EmptyPos, EmptyPos}
+
+// IsEmpty returns true if the Location information is missing.
+func (loc Location) IsEmpty() bool {
+	return loc.Start.IsEmpty() && loc.End.IsEmpty()
+}

pkg/diag/sink.go

@@ -6,6 +6,7 @@ import (
 	"bytes"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strconv"
 
 	"github.com/golang/glog"
@@ -27,14 +28,15 @@ type Sink interface {
 }
 
 // DefaultDiags returns a default sink that simply logs output to stderr/stdout.
-func DefaultSink() Sink {
-	return &defaultSink{}
+func DefaultSink(pwd string) Sink {
+	return &defaultSink{pwd: pwd}
 }
 
 // defaultSink is the default sink which logs output to stderr/stdout.
 type defaultSink struct {
-	errors   int
-	warnings int
+	pwd      string // an optional present working directory to which output paths will be relative to.
+	errors   int    // the number of errors that have been issued.
+	warnings int    // the number of warnings that have been issued.
 }
 
 func (d *defaultSink) Count() int {
@@ -79,10 +81,16 @@ func (d *defaultSink) stringify(diag *Diag, prefix string, args ...interface{})
 	}
 
 	if diag.Doc != nil {
-		buffer.WriteString(diag.Doc.File)
-		if diag.Loc != nil {
+		rel := diag.Doc.File
+		if d.pwd != "" {
+			// If a PWD is available, convert the file to be relative to it.
+			rel, _ = filepath.Rel(d.pwd, rel)
+		}
+		buffer.WriteString(rel)
+
+		if diag.Loc != nil && !diag.Loc.IsEmpty() {
 			buffer.WriteRune(':')
-			buffer.WriteString(strconv.Itoa(diag.Loc.Start.Row))
+			buffer.WriteString(strconv.Itoa(diag.Loc.Start.Ln))
 			buffer.WriteRune(':')
 			buffer.WriteString(strconv.Itoa(diag.Loc.Start.Col))
 		}