retain initial spaces in string expressions (#243)

Initially, any whitespaces after the opening quote in a string expression
are trimmed. This was meant only for indentations but it also removes
whitespaces that are just spaces. This commit changes that behavior
to retain initial spaces after the opening quote.

syntax/compile_xstr.go

@@ -17,6 +17,7 @@ var (
 	indentRE      = regexp.MustCompile(`(\n[\t ]*)\z`)
 	firstIndentRE = regexp.MustCompile(`\A((\n[\t ]+)(?:\n)|(\n))`)
 	lastSpacesRE  = regexp.MustCompile(`\n([ \t]*)\z`)
+	spaces        = regexp.MustCompile(`\A[\t ]+`)
 )
 
 func (pc ParseContext) compileExpandableString(ctx context.Context, b ast.Branch, c ast.Children) (rel.Expr, error) {
@@ -25,6 +26,13 @@ func (pc ParseContext) compileExpandableString(ctx context.Context, b ast.Branch
 	parts := []interface{}{}
 
 	ws := quote[2:]
+
+	// retain initial spaces
+	if spaces.MatchString(ws) {
+		parts = append(parts, ws)
+		ws = ""
+	}
+
 	trim := ""
 	trimIndent := func(s string) {
 		s = ws + s

syntax/parse_string_test.go

@@ -16,6 +16,18 @@ func TestXStringSimple(t *testing.T) {
 	AssertCodesEvalToSameValue(t, `"a42k3.142z"     `, `$"a${6*7}k${//math.pi:.3f}z"`)
 }
 
+func TestXStringRetainInitialSpaces(t *testing.T) {
+	t.Parallel()
+
+	AssertCodesEvalToSameValue(t, `"    "     `, `$"    "         `)
+	AssertCodesEvalToSameValue(t, `"    x"    `, `$"    x"        `)
+	AssertCodesEvalToSameValue(t, `"     abc" `, `$"     ${'abc'}"`)
+	AssertCodesEvalToSameValue(t, `"      abc"`, `$"     ${$' abc'}"`)
+
+	// suppress empty line
+	AssertCodesEvalToSameValue(t, `""`, `$"     ${''}"`)
+}
+
 func TestXStringBackquote(t *testing.T) {
 	t.Parallel()
 	AssertCodesEvalToSameValue(t, `""      `, "$``")