internal/core/export: fix a recent nil pointer regression

Before the fix, the added test fails with a panic due a nil pointer.
The fix prevents accessing the offending pointer.
After the fix, the new test passes.

Note that we might find a better fix down the line for the root cause;
a TODO is added as a reminder.

Fixes #2584.

Closes #2690 as merged as of commit dec11d9.

Signed-off-by: Rudi Farkas <[email protected]>
Change-Id: Ic24562fee250e9efcfaaf1a0bf95a514817675a5
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1172874
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rudifa

internal/core/export/adt.go

@@ -329,7 +329,8 @@ func (e *exporter) resolve(env *adt.Environment, r adt.Resolver) ast.Expr {
 		// comprehensions originate from a single source and do not need to be
 		// handled.
 		if env != nil { // for generated stuff
-			if v := env.Vertex; !v.IsDynamic {
+			// TODO: note that env.Vertex should never be nil; investigate and replace the nil check below.
+			if v := env.Vertex; v != nil && !v.IsDynamic {
 				if v = v.Lookup(x.Label); v != nil {
 					e.linkIdentifier(v, ident)
 				}

internal/core/export/testdata/main/issue2584.txtar

@@ -0,0 +1,100 @@
+# issue #2584
+
+-- in.cue --
+F1: sub1: sub2: L
+F2: {
+	(string): _
+	L
+}
+D: {}
+let L = D
+-- out/default --
+-- out/definition --
+
+let L = D
+F1: {
+	sub1: {
+		sub2: L
+	}
+}
+F2: {
+	(string): _
+	L
+}
+D: {}
+-- out/doc --
+[]
+[F1]
+[F1 sub1]
+[F1 sub1 sub2]
+[F2]
+[F2 _]
+[D]
+[L]
+-- out/value --
+== Simplified
+{
+	let L = D
+	F1: {
+		sub1: {
+			sub2: {}
+		}
+	}
+	F2: {
+		(string): _
+		L
+	}
+	D: {}
+}
+== Raw
+{
+	let L = D
+	F1: {
+		sub1: {
+			sub2: {}
+		}
+	}
+	F2: {
+		(string): _
+		L
+	}
+	D: {}
+}
+== Final
+{
+	F1: {
+		sub1: {
+			sub2: {}
+		}
+	}
+	F2: _|_ // F2: invalid non-ground value string (must be concrete string)
+	D: {}
+}
+== All
+{
+	let L = D
+	F1: {
+		sub1: {
+			sub2: {}
+		}
+	}
+	F2: {
+		(string): _
+		L
+	}
+	D: {}
+}
+== Eval
+{
+	let L = D
+	F1: {
+		sub1: {
+			sub2: {}
+		}
+	}
+	F2: {
+		(string): _
+		L
+	}
+	D: {}
+}