1 files changed, 101 insertions, 0 deletions
diff --git a/src/cmd/go/internal/mvs/errors.go b/src/cmd/go/internal/mvs/errors.go
new file mode 100644
index 0000000000..5564965fb5
--- /dev/null
+++ b/src/cmd/go/internal/mvs/errors.go
@@ -0,0 +1,101 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mvs
+
+import (
+	"fmt"
+	"strings"
+
+	"golang.org/x/mod/module"
+)
+
+// BuildListError decorates an error that occurred gathering requirements
+// while constructing a build list. BuildListError prints the chain
+// of requirements to the module where the error occurred.
+type BuildListError struct {
+	Err   error
+	stack []buildListErrorElem
+}
+
+type buildListErrorElem struct {
+	m module.Version
+
+	// nextReason is the reason this module depends on the next module in the
+	// stack. Typically either "requires", or "updating to".
+	nextReason string
+}
+
+// NewBuildListError returns a new BuildListError wrapping an error that
+// occurred at a module found along the given path of requirements and/or
+// upgrades, which must be non-empty.
+//
+// The isUpgrade function reports whether a path step is due to an upgrade.
+// A nil isUpgrade function indicates that none of the path steps are due to upgrades.
+func NewBuildListError(err error, path []module.Version, isUpgrade func(from, to module.Version) bool) *BuildListError {
+	stack := make([]buildListErrorElem, 0, len(path))
+	for len(path) > 1 {
+		reason := "requires"
+		if isUpgrade != nil && isUpgrade(path[0], path[1]) {
+			reason = "updating to"
+		}
+		stack = append(stack, buildListErrorElem{
+			m:          path[0],
+			nextReason: reason,
+		})
+		path = path[1:]
+	}
+	stack = append(stack, buildListErrorElem{m: path[0]})
+
+	return &BuildListError{
+		Err:   err,
+		stack: stack,
+	}
+}
+
+// Module returns the module where the error occurred. If the module stack
+// is empty, this returns a zero value.
+func (e *BuildListError) Module() module.Version {
+	if len(e.stack) == 0 {
+		return module.Version{}
+	}
+	return e.stack[len(e.stack)-1].m
+}
+
+func (e *BuildListError) Error() string {
+	b := &strings.Builder{}
+	stack := e.stack
+
+	// Don't print modules at the beginning of the chain without a
+	// version. These always seem to be the main module or a
+	// synthetic module ("target@").
+	for len(stack) > 0 && stack[0].m.Version == "" {
+		stack = stack[1:]
+	}
+
+	if len(stack) == 0 {
+		b.WriteString(e.Err.Error())
+	} else {
+		for _, elem := range stack[:len(stack)-1] {
+			fmt.Fprintf(b, "%s %s\n\t", elem.m, elem.nextReason)
+		}
+		// Ensure that the final module path and version are included as part of the
+		// error message.
+		m := stack[len(stack)-1].m
+		if mErr, ok := e.Err.(*module.ModuleError); ok {
+			actual := module.Version{Path: mErr.Path, Version: mErr.Version}
+			if v, ok := mErr.Err.(*module.InvalidVersionError); ok {
+				actual.Version = v.Version
+			}
+			if actual == m {
+				fmt.Fprintf(b, "%v", e.Err)
+			} else {
+				fmt.Fprintf(b, "%s (replaced by %s): %v", m, actual, mErr.Err)
+			}
+		} else {
+			fmt.Fprintf(b, "%v", module.VersionError(m, e.Err))
+		}
+	}
+	return b.String()
+}