Suggest keyword argument based on spelling in UnpackArgs (google#393)

Users of tools such as Bazel and ours arguably edit mostly
the kwargs of functions the tool provides. Typos often slip
through and spelling suggestion can save some headache.

This change may affect the message string of some errors.

Signed-off-by: Pierre Fenoll <[email protected]>

starlark/testdata/function.star

@@ -321,3 +321,9 @@ b(1, y=2, )
 #c(1, *[], )
 #d(1, *[], z=None, )
 #e(1, *[], z=None, *{}, )
+
+---
+# Unpack provides spell check for argument names.
+load("assert.star", "assert")
+
+assert.fails(lambda: min([], keg=1), ".+did you mean key\\?")

starlark/unpack.go

@@ -8,6 +8,8 @@ import (
 	"log"
 	"reflect"
 	"strings"
+
+	"go.starlark.net/internal/spell"
 )
 
 // An Unpacker defines custom argument unpacking behavior.
@@ -149,7 +151,16 @@ kwloop:
 				continue kwloop
 			}
 		}
-		return fmt.Errorf("%s: unexpected keyword argument %s", fnname, name)
+		err := fmt.Errorf("%s: unexpected keyword argument %s", fnname, name)
+		names := make([]string, 0, nparams)
+		for i := 0; i < nparams; i += 2 {
+			param, _ := paramName(pairs[i])
+			names = append(names, param)
+		}
+		if n := spell.Nearest(string(name), names); n != "" {
+			err = fmt.Errorf("%s (did you mean %s?)", err.Error(), n)
+		}
+		return err
 	}
 
 	// Check that all non-optional parameters are defined.