build: include position information in errors

Updates mmcloughlin#5

build/context.go

@@ -153,7 +153,8 @@ func (c *Context) activeglobal() *avo.Global {
 }
 
 func (c *Context) AddError(err error) {
-	c.errs = append(c.errs, err)
+	e := exterr(err)
+	c.errs = append(c.errs, e)
 }
 
 func (c *Context) AddErrorMessage(msg string) {

build/error.go

@@ -0,0 +1,30 @@
+package build
+
+import (
+	"github.com/mmcloughlin/avo/internal/stack"
+	"github.com/mmcloughlin/avo/src"
+)
+
+// Error represents an error during building, optionally tagged with the position at which it happened.
+type Error struct {
+	Position src.Position
+	Err      error
+}
+
+// exterr constructs an Error with position derived from the first frame in the
+// call stack outside this package.
+func exterr(err error) Error {
+	e := Error{Err: err}
+	if f := stack.ExternalCaller(); f != nil {
+		e.Position = src.FramePosition(*f).Relwd()
+	}
+	return e
+}
+
+func (e Error) Error() string {
+	msg := e.Err.Error()
+	if e.Position.IsValid() {
+		return e.Position.String() + ": " + msg
+	}
+	return msg
+}

src/src.go

@@ -1,13 +1,25 @@
 package src
 
-import "strconv"
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strconv"
+)
 
 // Position represents a position in a source file.
 type Position struct {
 	Filename string
 	Line     int // 1-up
 }
 
+func FramePosition(f runtime.Frame) Position {
+	return Position{
+		Filename: f.File,
+		Line:     f.Line,
+	}
+}
+
 // IsValid reports whether the position is valid: Line must be positive, but
 // Filename may be empty.
 func (p Position) IsValid() bool {
@@ -26,3 +38,23 @@ func (p Position) String() string {
 	s += strconv.Itoa(p.Line)
 	return s
 }
+
+// Rel returns Position relative to basepath. If the given filename cannot be
+// expressed relative to basepath the position will be returned unchanged.
+func (p Position) Rel(basepath string) Position {
+	q := p
+	if rel, err := filepath.Rel(basepath, q.Filename); err == nil {
+		q.Filename = rel
+	}
+	return q
+}
+
+// Relwd returns Position relative to the current working directory. Returns p
+// unchanged if the working directory cannot be determined, or the filename
+// cannot be expressed relative to the working directory.
+func (p Position) Relwd() Position {
+	if wd, err := os.Getwd(); err == nil {
+		return p.Rel(wd)
+	}
+	return p
+}