[Feature] Enhanced Unused Detection

[le-blue]

Feature Request: Enhanced Unused Detection

Enhance the existing unused detection in order to find symbols that are

• assigned but never read (dead code)
• read but never assigned or initialized in declaration (X)

Besides, it would be useful to distinguish between different types of "unused" scenarios and set a severity level for each scenario in the toml, since the consequences of these scenarios differ:

• Never read port inputs or generics - Interface compatibility?
• Never referenced or never read locals - Dead code, impacts readability but optimized out in synthesis
• Never assigned port outputs or read but never assigned locals - Most likely bug

Current Behavior

Currently, VHDL-LS detects unused declarations only when they are never referenced at all.

Also, unused (never read) port inputs and generics might be acceptable for interface compatibility. The current workaround is to embrace them with -- vhdl_ls off ... -- vhdl_ls on.

library ieee;
   use ieee.std_logic_1164.all;

entity test is
   generic(
      g_UNUSED   : boolean := true -- ❌ Flagged (but maybe acceptable)
   );
   port(
      i_unused   : in std_logic;   -- ❌ Flagged (but maybe acceptable)
      o_unused   : out std_logic;  -- ❌ NOT flagged (but is never assigned)
      o_used     : out std_logic   -- ✅ Correctly used
   );
end entity test;

architecture example of test is

signal never_used      : std_logic; -- ✅ Correctly flagged as unused
signal only_read       : std_logic; -- ❌ NOT flagged (but is read-only)
signal only_written    : std_logic; -- ❌ NOT flagged (but is write-only)
signal properly_used   : std_logic; -- ✅ Correctly used

begin

only_written    <= '1';
properly_used   <= '0';

process (properly_used, only_read) is
begin

   o_used   <= properly_used and only_read;

end process;

end architecture example;