Wrong "unused record field" when using record field as index

[Alberdi]

Describe the bug
Using a record field as index doesn't count towards marking it as "used".

To Reproduce
Write the following record definition and function:

-record(used_field, {field_c}).

main(R) ->
  element(#used_field.field_c, R).

Expected behavior
No warnings.

Actual behavior
One warning: "Unused record field: #used_field.field_c"

Context

• erlang_ls version (tag/sha): 0.44.1/b151f9d9a4a1dde1118ae2822226500e2fa69125
• Editor used: NVIM v0.6.1
• LSP client used: Nvim LSP client

---

The following change cause the unused_record_fields test from els_diagnostics_SUITE to fail, even though it shouldn't:

diff --git a/apps/els_lsp/priv/code_navigation/src/diagnostics_unused_record_fields.erl b/apps/els_lsp/priv/code_navigation/src/diagnostics_unused_record_fields.erl
index f10efc7..5ddfacd 100644
--- a/apps/els_lsp/priv/code_navigation/src/diagnostics_unused_record_fields.erl
+++ b/apps/els_lsp/priv/code_navigation/src/diagnostics_unused_record_fields.erl
@@ -8,4 +8,4 @@
 main(#used_field{field_a = A, field_b = B}) ->
   {A, B};
 main(R) ->
-  R#unused_field.field_c.
+  element(#unused_field.field_c, R).

Not knowing anything about the internals of erlang_ls (yet?) I've the tried the following without any effect:

diff --git a/apps/els_lsp/src/els_unused_record_fields_diagnostics.erl b/apps/els_lsp/src/els_unused_record_fields_diagnostics.erl
index 4e92e46..7b2e0e3 100644
--- a/apps/els_lsp/src/els_unused_record_fields_diagnostics.erl
+++ b/apps/els_lsp/src/els_unused_record_fields_diagnostics.erl
@@ -55,7 +55,7 @@ source() ->
 -spec find_unused_record_fields(els_dt_document:item()) -> [els_poi:poi()].
 find_unused_record_fields(Document) ->
     Definitions = els_dt_document:pois(Document, [record_def_field]),
-    Usages = els_dt_document:pois(Document, [record_field]),
+    Usages = els_dt_document:pois(Document, [record_field, record_index]),
     UsagesIds = lists:usort([Id || #{id := Id} <- Usages]),
     [POI || #{id := Id} = POI <- Definitions, not lists:member(Id, UsagesIds)].