ast: restore location on unsafe-var errors from rewritten head vars

v1/ast/compile.go

@@ -1596,11 +1596,16 @@ func (c *Compiler) checkSafetyRuleHeads() {
 				if vars.DiffCount(vis.vars) > 0 {
 					unsafe := vars.Diff(vis.vars)
 					for v := range unsafe {
+						// Capture the location before v is rewritten back to its
+						// original name below: vars is keyed by the (possibly
+						// rewritten) head var, so looking up vars[v] after the
+						// reassignment would miss the location. See #8719.
+						loc := vars[v].Location
 						if w, ok := c.RewrittenVars[v]; ok {
 							v = w
 						}
 						if !v.IsGenerated() {
-							if !c.err(NewError(UnsafeVarErr, vars[v].Location, "var %v is unsafe", v)) {
+							if !c.err(NewError(UnsafeVarErr, loc, "var %v is unsafe", v)) {
 								return true
 							}
 						}

v1/ast/compile_test.go

@@ -1899,6 +1899,26 @@ obj := {
 				},
 			},
 		},
+		{
+			// Regression test for #8719: an unsafe variable that appears in a
+			// rule head and is rewritten (e.g. declared with `some`) must still
+			// report its location.
+			module: `package test
+
+r[x] := y if {
+	some x
+	y := [_]
+}`,
+			expectedErrs: []struct {
+				message  string
+				location int
+			}{
+				{
+					"var x is unsafe",
+					3,
+				},
+			},
+		},
 	}
 
 	for _, tc := range tests {