From de4c8a7f448d2af8be86717c3e5bd5b850515e9f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bastian=20M=C3=BCller?= <bastian@axiomzen.co>
Date: Fri, 31 Jul 2020 17:19:04 -0700
Subject: [PATCH] include available declarations in "not declared" error

---
 runtime/sema/check_import_declaration.go | 52 ++++++++++++++++++++----
 runtime/sema/errors.go                   | 13 ++++++
 2 files changed, 57 insertions(+), 8 deletions(-)

diff --git a/runtime/sema/check_import_declaration.go b/runtime/sema/check_import_declaration.go
index 02d17c47e4..511279d143 100644
--- a/runtime/sema/check_import_declaration.go
+++ b/runtime/sema/check_import_declaration.go
@@ -100,19 +100,21 @@ func (checker *Checker) declareImportDeclaration(declaration *ast.ImportDeclarat
 
 	// Attempt to import the requested value declarations
 
+	allValueElements := imp.AllValueElements()
 	foundValues, invalidAccessedValues := checker.importElements(
 		checker.valueActivations,
 		declaration.Identifiers,
-		imp.AllValueElements(),
+		allValueElements,
 		imp.IsImportableValue,
 	)
 
 	// Attempt to import the requested type declarations
 
+	allTypeElements := imp.AllTypeElements()
 	foundTypes, invalidAccessedTypes := checker.importElements(
 		checker.typeActivations,
 		declaration.Identifiers,
-		imp.AllTypeElements(),
+		allTypeElements,
 		imp.IsImportableType,
 	)
 
@@ -141,22 +143,55 @@ func (checker *Checker) declareImportDeclaration(declaration *ast.ImportDeclarat
 		)
 	}
 
+	identifierCount := len(declaration.Identifiers)
+
 	// Determine which requested declarations could neither be found
 	// in the value nor in the type declarations of the imported program.
 	// For each missing import, report an error and declare both a value
 	// with an invalid type and an invalid type to avoid spurious errors
-	// due to uses of the inaccessible value or type
+	// due to uses of the inaccessible value or type.
+	//
+	// Also show which declarations are available, to help with debugging.
+
+	missing := make([]ast.Identifier, 0, identifierCount)
 
-	missing := make(map[ast.Identifier]bool, len(declaration.Identifiers))
 	for _, identifier := range declaration.Identifiers {
 		if foundValues[identifier] || foundTypes[identifier] {
 			continue
 		}
 
-		missing[identifier] = true
+		missing = append(missing, identifier)
 	}
 
-	checker.handleMissingImports(missing, location)
+	if len(missing) > 0 {
+		capacity := len(allValueElements) + len(allTypeElements)
+		available := make([]string, 0, capacity)
+		availableSet := make(map[string]struct{}, capacity)
+
+		for identifier := range allValueElements {
+			if _, ok := availableSet[identifier]; ok {
+				continue
+			}
+			if !imp.IsImportableValue(identifier) {
+				continue
+			}
+			availableSet[identifier] = struct{}{}
+			available = append(available, identifier)
+		}
+
+		for identifier := range allTypeElements {
+			if _, ok := availableSet[identifier]; ok {
+				continue
+			}
+			if !imp.IsImportableType(identifier) {
+				continue
+			}
+			availableSet[identifier] = struct{}{}
+			available = append(available, identifier)
+		}
+
+		checker.handleMissingImports(missing, available, location)
+	}
 
 	return nil
 }
@@ -202,12 +237,13 @@ func (checker *Checker) EnsureLoaded(location ast.Location, loadProgram func() *
 	return subChecker, checkerErr
 }
 
-func (checker *Checker) handleMissingImports(missing map[ast.Identifier]bool, importLocation ast.Location) {
-	for identifier := range missing {
+func (checker *Checker) handleMissingImports(missing []ast.Identifier, available []string, importLocation ast.Location) {
+	for _, identifier := range missing {
 		checker.report(
 			&NotExportedError{
 				Name:           identifier.Identifier,
 				ImportLocation: importLocation,
+				Available:      available,
 				Pos:            identifier.Pos,
 			},
 		)
diff --git a/runtime/sema/errors.go b/runtime/sema/errors.go
index db2c2113c9..474a31e0e8 100644
--- a/runtime/sema/errors.go
+++ b/runtime/sema/errors.go
@@ -21,6 +21,7 @@ package sema
 import (
 	"fmt"
 	"math/big"
+	"strings"
 
 	"github.com/onflow/cadence/runtime/ast"
 	"github.com/onflow/cadence/runtime/common"
@@ -1024,6 +1025,7 @@ func (*RepeatedImportError) isSemanticError() {}
 type NotExportedError struct {
 	Name           string
 	ImportLocation ast.Location
+	Available      []string
 	Pos            ast.Position
 }
 
@@ -1035,6 +1037,17 @@ func (e *NotExportedError) Error() string {
 	)
 }
 
+func (e *NotExportedError) SecondaryError() string {
+	var builder strings.Builder
+	builder.WriteString("available exported declarations are:\n")
+
+	for _, available := range e.Available {
+		builder.WriteString(fmt.Sprintf(" - `%s`\n", available))
+	}
+
+	return builder.String()
+}
+
 func (*NotExportedError) isSemanticError() {}
 
 func (e *NotExportedError) StartPosition() ast.Position {