Disallow creation of circular variable alias chains

Make `defvaralias` signal an error upon attempts to create variable
alias cycles.  This detects errors earlier and makes the alias
traversal during execution simpler and faster since no cycle detection
is needed elsewhere.
Now variable and function aliases are handled identically in these
respects.

* src/lisp.h (indirect_variable): Remove declaration.
* src/data.c (indirect_variable): Remove.
(Findirect_variable): Update doc string.  Simplify alias resolution.
(Fboundp, find_symbol_value, set_internal, default_value)
(set_default_internal, Fmake_variable_buffer_local)
(Fmake_local_variable, Fkill_local_variable, Flocal_variable_p)
(Flocal_variable_if_set_p, Fvariable_binding_locus):
* src/buffer.c (buffer_local_value):
* src/eval.c (specbind): Simplify variable alias resolution.
(Fdefvaralias): Update doc string.  Check for cycles.
* doc/lispref/variables.texi (Variable Aliases):
Mention that `defvaralias` can signal `cyclic-variable-indirection`
but `indirect-variable` cannot.
* etc/NEWS: Announce the change.
* test/src/eval-tests.el (eval-tests-defvaralias): New test.

1 files changed, 16 insertions, 0 deletions

diff --git a/test/src/eval-tests.el b/test/src/eval-tests.el
index e0a27439ba2..4589763b2f5 100644
--- a/test/src/eval-tests.el
+++ b/test/src/eval-tests.el
@@ -266,4 +266,20 @@ expressions works for identifiers starting with period."
     )
   (should (eq eval-test--local-var 'global)))
 
+(ert-deftest eval-tests-defvaralias ()
+  (defvar eval-tests--my-var 'coo)
+  (defvaralias 'eval-tests--my-var1 'eval-tests--my-var)
+  (defvar eval-tests--my-var1)
+  (should (equal eval-tests--my-var 'coo))
+  (should (equal eval-tests--my-var1 'coo))
+
+  (defvaralias 'eval-tests--my-a 'eval-tests--my-b)
+  (defvaralias 'eval-tests--my-b 'eval-tests--my-c)
+
+  (should-error (defvaralias 'eval-tests--my-c 'eval-tests--my-c)
+                :type 'cyclic-variable-indirection)
+  (defvaralias 'eval-tests--my-d 'eval-tests--my-a)
+  (should-error (defvaralias 'eval-tests--my-c 'eval-tests--my-d)
+                :type 'cyclic-variable-indirection))
+
 ;;; eval-tests.el ends here