cmd/cue: get go: use constant.Value.ExactString to extract constant

Currently long string constant values break during the import because of
the use of go/constant.Value.String() (which is the short version).

Switch to using go/constant.Value.ExactString() for the complete value.

Add a basic testscript test whilst we are at it.

Fixes #458

Change-Id: I1a2973d008bd517d47474becf5dbbfb3df22ecfa
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6781
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/get_go.go

@@ -695,9 +695,10 @@ func (e *extractor) reportDecl(x *ast.GenDecl) (a []cueast.Decl) {
 				}
 
 				c := e.pkg.TypesInfo.Defs[v.Names[i]].(*types.Const)
-				cv, err := parser.ParseExpr("", c.Val().String())
+				sv := c.Val().ExactString()
+				cv, err := parser.ParseExpr("", sv)
 				if err != nil {
-					panic(err)
+					panic(fmt.Errorf("failed to parse %v: %v", sv, err))
 				}
 
 				// Use orignal Go value if compatible with CUE (octal is okay)

cmd/cue/cmd/get_go_test.go

@@ -22,6 +22,7 @@ import (
 	"testing"
 
 	"cuelang.org/go/internal/copy"
+	"github.com/google/go-cmp/cmp"
 )
 
 func TestGetGo(t *testing.T) {
@@ -81,7 +82,7 @@ func TestGetGo(t *testing.T) {
 			got := loadFile(t, filepath.Join(root, path[len(dst):]))
 
 			if want != got {
-				t.Errorf("contexts for file %s differ", path[len(prefix):])
+				t.Errorf("contexts for file %s differ: \n%s", path[len(prefix):], cmp.Diff(got, want))
 			}
 		})
 		return nil

cmd/cue/cmd/testdata/script/get_go_basic.txt

@@ -0,0 +1,45 @@
+# Test that a basic get go works using golden files to verify output
+
+# Set HOME for go-build cache to be valid
+env HOME=$WORK${/}home
+env USERPROFILE=$HOME
+env LOCALAPPDATA=$WORK${/}appdata
+
+# All the things
+cue get go --local
+cmp blah_go_gen.cue all.cue.golden
+
+-- go.mod --
+module mod.com/blah
+-- blah.go --
+package main
+
+type S struct {
+	Name string
+	T
+}
+
+type T struct {
+	Age int
+}
+
+const (
+	LongStringConst = "This is a really long string. Why are we using a long string? Because that way it ensures we are using go/constant.Value.ExactString() instead of go/constant.Value.String()"
+	IntConst        = "test"
+)
+-- all.cue.golden --
+// Code generated by cue get go. DO NOT EDIT.
+
+//cue:generate cue get go mod.com/blah
+
+package main
+
+#S: {
+	Name: string
+	T:    #T
+}
+
+#T: Age: int
+
+#LongStringConst: "This is a really long string. Why are we using a long string? Because that way it ensures we are using go/constant.Value.ExactString() instead of go/constant.Value.String()"
+#IntConst:        "test"