From 94223148d697b3beeaa1ec3ca5dc68a73cb9efcd Mon Sep 17 00:00:00 2001
From: Roger Peppe <rogpeppe@gmail.com>
Date: Mon, 6 Feb 2023 10:04:44 +0000
Subject: [PATCH] cue/ast: fix IsValidIdent for _0
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Currently `IsValidIdent` returns false for the identifier `_1`.
This is not correct: the specification defines as valid identifier
as:

	identifier  = [ "#" | "_#" ] letter { letter | unicode_digit } .

where `letter` includes underscore.

This caused an observed panic in `internal/core/export.(*pivotter)` when it calls
`exporter.ident`.

I was wanting to add the test before introducing the fix, but this isn't
possible with the current `IsValidIdentifer` tests because they check
for consistency between `LabelName` and `IsValidIdentifier` and the
current logic is not consistent for this case.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Icb571edbcac97e7f076497d45a03c7b524d92d20
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/549423
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
---
 cue/ast/ident.go      | 17 +++++++++--------
 cue/ast/ident_test.go |  8 ++++++++
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/cue/ast/ident.go b/cue/ast/ident.go
index 3d81806a7cd..20d7b5d6bd0 100644
--- a/cue/ast/ident.go
+++ b/cue/ast/ident.go
@@ -39,25 +39,26 @@ func IsValidIdent(ident string) bool {
 		return false
 	}
 
-	// TODO: use consumed again to allow #0.
-	// consumed := false
+	consumed := false
 	if strings.HasPrefix(ident, "_") {
 		ident = ident[1:]
-		// consumed = true
+		consumed = true
 		if len(ident) == 0 {
 			return true
 		}
 	}
 	if strings.HasPrefix(ident, "#") {
 		ident = ident[1:]
-		// consumed = true
+		// Note: _#0 is not allowed by the spec, although _0 is.
+		// TODO: set consumed to true here to allow #0.
+		consumed = false
 	}
 
-	// if !consumed {
-	if r, _ := utf8.DecodeRuneInString(ident); isDigit(r) {
-		return false
+	if !consumed {
+		if r, _ := utf8.DecodeRuneInString(ident); isDigit(r) {
+			return false
+		}
 	}
-	// }
 
 	for _, r := range ident {
 		if isLetter(r) || isDigit(r) || r == '_' || r == '$' {
diff --git a/cue/ast/ident_test.go b/cue/ast/ident_test.go
index 1093320f519..2437cd42ab1 100644
--- a/cue/ast/ident_test.go
+++ b/cue/ast/ident_test.go
@@ -72,6 +72,14 @@ func TestLabelName(t *testing.T) {
 		in:      &ast.Ident{Name: "_"},
 		out:     "_",
 		isIdent: true,
+	}, {
+		in:      &ast.Ident{Name: "_1"},
+		out:     "_1",
+		isIdent: true,
+	}, {
+		in:  &ast.Ident{Name: "_#1"},
+		out: "",
+		err: true,
 	}, {
 		in:      &ast.Ident{Name: "8ball"},
 		out:     "",