cmd/cue: support more escape codes from Go string constants

We were using go/constant's ExactString to obtain a CUE expression,
which worked for the most part given how Go basic literals such as
strings are quite similar to CUE.

However, one crucial difference is that Go allows \x hexadecimal
escape sequences in strings, which ExactString prefers to use at times,
whereas CUE only allows them only in byte sequences:

    Hexadecimal and octal escapes are only allowed within byte sequences (single quotes).

There is room for further improvement, as we should not be using
go/constant's ExactString for other kinds of basic literals either.
That's left for a separate change to make bisecting easier
if we introduce any unintentional regressions.

Fixes #2915.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Iea040b34a321e7b95004f35431d42ab63604edd7
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1211847
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: mvdan

Diff for: cmd/cue/cmd/get_go.go

@@ -18,6 +18,7 @@ import (
 	"bytes"
 	"fmt"
 	"go/ast"
+	"go/constant"
 	"go/token"
 	"go/types"
 	"os"
@@ -787,11 +788,23 @@ func (e *extractor) reportDecl(x *ast.GenDecl) (a []cueast.Decl) {
 				}
 
 				typ := e.pkg.TypesInfo.TypeOf(name)
-				c := e.pkg.TypesInfo.Defs[v.Names[i]].(*types.Const)
-				sv := c.Val().ExactString()
-				cv, err := parser.ParseExpr("", sv)
-				if err != nil {
-					panic(fmt.Errorf("failed to parse %v: %v", sv, err))
+				v := e.pkg.TypesInfo.Defs[v.Names[i]].(*types.Const).Val()
+				var cv cueast.Expr
+				switch v.Kind() {
+				case constant.String:
+					cv = &cueast.BasicLit{
+						Kind:  cuetoken.STRING,
+						Value: literal.String.Quote(constant.StringVal(v)),
+					}
+
+				default:
+					// TODO(mvdan): replace this with switch cases for Bool/Int/Float
+					sv := v.ExactString()
+					var err error
+					cv, err = parser.ParseExpr("", sv)
+					if err != nil {
+						panic(fmt.Errorf("failed to parse %v: %v", sv, err))
+					}
 				}
 
 				// Use the original Go value if compatible with CUE (octal is okay)

Diff for: cmd/cue/cmd/testdata/script/get_go_json_compat.txtar

@@ -8,13 +8,13 @@ cmp decls/p_go_gen.cue decls/p_go_gen.cue.golden
 exec go run ./marshal
 cp stdout go_marshal.json
 cmp go_marshal.json go_marshal.json.golden
-! exec cue vet go_marshal.json schema.cue
+exec cue vet go_marshal.json schema.cue
 
 # Export from CUE and Unmarshal via Go's encoding/json.
-! exec cue export ./decls
-# cmp stdout cue_export.json.golden
-# stdin stdout
-# exec go run ./unmarshal
+exec cue export ./decls
+cmp stdout cue_export.json.golden
+stdin stdout
+exec go run ./unmarshal
 
 -- schema.cue --
 package schema
@@ -80,9 +80,9 @@ package decls
 
 #ValueSecond: int & 3000000000
 
-#ErrorPrefix: "\x1b[31mError:\x1b[0m" // "\033[31mError:\033[0m"
+#ErrorPrefix: "\u001b[31mError:\u001b[0m" // "\033[31mError:\033[0m"
 
-#ResetColor: "\x1b[0m" // "\u001b[0m"
+#ResetColor: "\u001b[0m"
 -- marshal/main.go --
 package main