From bcbe967fdd1d26bc138092d9d6a36e28c55940d7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Mart=C3=AD?= <mvdan@mvdan.cc>
Date: Thu, 27 Jul 2023 15:02:37 +0100
Subject: [PATCH] internal/core/compile: reject "_" as an alias name
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"_" stands for "top", and it was already being rejected as a label,
but it was still being allowed for alias or let declarations,
causing confusing errors such as:

    unreferenced alias or let clause _

Reject all three the same way.

Fixes #2381.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Iad909813756c5df4421e96e0bdb2a071dc7c66f1
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/556985
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>
---
 cue/testdata/compile/err_top.txtar | 48 ++++++++++++++++++++++++++++++
 cue/testdata/compile/labels.txtar  | 20 -------------
 internal/core/compile/compile.go   |  3 ++
 3 files changed, 51 insertions(+), 20 deletions(-)
 create mode 100644 cue/testdata/compile/err_top.txtar

diff --git a/cue/testdata/compile/err_top.txtar b/cue/testdata/compile/err_top.txtar
new file mode 100644
index 00000000000..cb869d8a2c2
--- /dev/null
+++ b/cue/testdata/compile/err_top.txtar
@@ -0,0 +1,48 @@
+-- in.cue --
+disallowTopAsAlias: {
+	_=X: 1
+}
+
+disallowTopAsLet: {
+	let _ = 1
+}
+
+disallowTopAsLabel: {
+	_: 1
+	a: _ // Should not compile to a reference.
+}
+
+// TODO: disallow dollar as label? This is according to the spec, but it
+// will be a breaking change and $ was reserved for referring to the root of
+// a file, which we very likely will never implement.
+// disallowDollarAsLabel: {
+//   $: 1
+// }
+
+-- out/compile --
+disallowTopAsAlias: cannot use _ as alias or let clause:
+    ./in.cue:2:2
+disallowTopAsLet: cannot use _ as alias or let clause:
+    ./in.cue:6:6
+disallowTopAsLabel: cannot use _ as label:
+    ./in.cue:10:2
+--- in.cue
+{
+  disallowTopAsAlias: {
+    X: 1
+  }
+  disallowTopAsLet: {
+    let _ = 1
+  }
+  disallowTopAsLabel: {
+    _|_(cannot use _ as label)
+    a: _
+  }
+}
+-- out/eval --
+disallowTopAsAlias: cannot use _ as alias or let clause:
+    ./in.cue:2:2
+disallowTopAsLet: cannot use _ as alias or let clause:
+    ./in.cue:6:6
+disallowTopAsLabel: cannot use _ as label:
+    ./in.cue:10:2
diff --git a/cue/testdata/compile/labels.txtar b/cue/testdata/compile/labels.txtar
index 563c3a40f54..0c9c60f37c6 100644
--- a/cue/testdata/compile/labels.txtar
+++ b/cue/testdata/compile/labels.txtar
@@ -24,18 +24,6 @@ ok11: [list.FlattenN([string], 1)]: string   // disallowed in evaluator
 
 bad1: [ for x in [1, 2, 3] {x}]: string
 
-disallowTopAsLabel: {
-	_: 1
-	a: _ // Should not compile to a reference.
-}
-
-// TODO: disallow dollar as label? This is according to the spec, but it
-// will be a breaking change and $ was reserved for referring to the root of
-// a file, which we very likely will never implement.
-// disallowDollarAsLabel: {
-//   $: 1
-// }
-
 saneReferencesInComprehensions: {
 	for _ in [1] {
 		a: _ // Should not compile to a reference.
@@ -44,8 +32,6 @@ saneReferencesInComprehensions: {
 -- out/compile --
 bad1: comprehension values not allowed in this position:
     ./in.cue:24:9
-disallowTopAsLabel: cannot use _ as label:
-    ./in.cue:27:2
 --- in.cue
 {
   dis1: ("dev"|"prd")
@@ -95,10 +81,6 @@ disallowTopAsLabel: cannot use _ as label:
   bad1: {
     [_|_(comprehension values not allowed in this position)]: string
   }
-  disallowTopAsLabel: {
-    _|_(cannot use _ as label)
-    a: _
-  }
   saneReferencesInComprehensions: {
     for _, _ in [
       1,
@@ -110,5 +92,3 @@ disallowTopAsLabel: cannot use _ as label:
 -- out/eval --
 bad1: comprehension values not allowed in this position:
     ./in.cue:24:9
-disallowTopAsLabel: cannot use _ as label:
-    ./in.cue:27:2
diff --git a/internal/core/compile/compile.go b/internal/core/compile/compile.go
index 99f7b67ad27..f870b3ec05b 100644
--- a/internal/core/compile/compile.go
+++ b/internal/core/compile/compile.go
@@ -189,6 +189,9 @@ func (c *compiler) insertAlias(id *ast.Ident, a aliasEntry) *adt.Bottom {
 			"alias %q already declared; previous declaration at %s",
 			id.Name, e.source.Pos())
 	}
+	if id.Name == "_" {
+		return c.errf(id, "cannot use _ as alias or let clause")
+	}
 
 	m[id.Name] = a
 	return nil