Fix compound attributes in let bindings

Fix an issue where compound attributes would discard previous bindings
in a let statement.

`let a=1; b.b=2; c=3; in a` would fail, where
`let a=1; b.b=2; c=3; in c` would work as expected.

src/Nix/Eval.hs

@@ -207,8 +207,8 @@ attrSetAlter (k : ks) pos m p val = case M.lookup k m of
     ( M.insert
       k
       (toValue @(AttrSet v, AttrSet SourcePos) =<< (, mempty) <$> sequence st')
-      st
-    , M.insert k pos sp
+      m
+    , M.insert k pos p
     )
 
 desugarBinds :: forall r . ([Binding r] -> r) -> [Binding r] -> [Binding r]

tests/EvalTests.hs

@@ -139,6 +139,12 @@ case_find_file_failure_invalid_arg_no_path =
 case_infinite_recursion =
     assertNixEvalThrows "let foo = a: bar a; bar = a: foo a; in foo 3"
 
+case_nested_let =
+    constantEqualText "3" "let a = 3; x.x = 2; in a"
+
+case_nested_nested_let =
+    constantEqualText "3" "let a = 3; x.x = let b = a; in b; c = x.x; in c"
+
 case_inherit_in_rec_set =
     constantEqualText "1" "let x = 1; in (rec { inherit x; }).x"