Revert "Disallow escaped characters in variable expansion"

This reverts commit 555af37616.

CHANGELOG.rst

@@ -9,7 +9,6 @@ Notable improvements and fixes
 Syntax changes and new commands
 -------------------------------
 - Empty variable names are no longer allowed.
-- Escaped characters like ``\x41`` are no longer valid in a variable name (:issue:`7969`).
 
 Deprecations and removed features
 ---------------------------------

src/common.cpp

@@ -1244,9 +1244,6 @@ maybe_t<size_t> read_unquoted_escape(const wchar_t *input, wcstring *result, boo
             }
 
             if (res <= max_val) {
-                // Prepend internal sep so this does not get treated as part of a variable.
-                // See #7969.
-                if (unescape_special) result->push_back(INTERNAL_SEPARATOR);
                 result_char_or_none =
                     static_cast<wchar_t>((byte_literal ? ENCODE_DIRECT_BASE : 0) + res);
             } else {

tests/checks/slices.fish

@@ -78,17 +78,3 @@ echo $list # CHECK: 2 1 3
 set -l list 1 2 3
 set list[2..] $list[-1..2]
 echo $list # CHECK: 1 3 2
-
-# Regression test for #7969.
-set fo shorty
-set foo A B C
-echo $foo[1]
-# CHECK: A
-echo $foo\1331\135
-# CHECK: A[1] B[1] C[1]
-echo $foo\x5b1\x5d
-# CHECK: A[1] B[1] C[1]
-echo $foo\1331\135
-# CHECK: A[1] B[1] C[1]
-echo $fo\157
-# CHECK: shortyo