Give warning on unused require

[thiamsantos]

This a draft PR for the for the proposal: https://groups.google.com/g/elixir-lang-core/c/d9ZjViQaOY8/m/tZN0jn6LAAAJ. The implementation is based on the unused import warning.

I plan to compile a project like plug to check the impact that such warning would have in codebases.

[josevalim]

Maybe instead of from_import we could do [{warn, false} | Opts] instead?

[thiamsantos]

Done :)

[josevalim]

We already test these different conditions in the unit tests in lexical tracker. In here, we need only a single test that verifies the format of the warning itself.

[thiamsantos]

Done :)

[thiamsantos]

The results of the compilation of some popular libraries using this branch are promising.
The new warnings appear to be legit.

plug

# plug > mix compile Compiling 1 file (.ex) warning: unused require Logger │ 59 │ require Logger │ ~ │ └─ lib/plug/request_id.ex:59:3 warning: unused require Bitwise │ 105 │ require Bitwise │ ~ │ └─ lib/plug/csrf_protection.ex:105:3 warning: the variable "prefix_size" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator │ 153 │ <<prefix::binary-size(prefix_size), char, suffix::binary-size(suffix_size)>> = segment │ ~ │ └─ lib/plug/router/utils.ex:153:31: Plug.Router.Utils.build_path_clause/7 warning: the variable "suffix_size" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator │ 153 │ <<prefix::binary-size(prefix_size), char, suffix::binary-size(suffix_size)>> = segment │ ~ │ └─ lib/plug/router/utils.ex:153:71: Plug.Router.Utils.build_path_clause/7 Generated plug app 

ecto_sql

# ecto_sql ~/dev/thiamsantos/ecto_sql (master) > mix compile warning: setting :preferred_cli_env in your mix.exs "def project" is deprecated, set it inside "def cli" instead: def cli do [preferred_envs: ["test.all": :test, "test.adapters": :test]] end (mix 1.20.0-dev) lib/mix/cli.ex:189: Mix.CLI.preferred_cli_env/3 (mix 1.20.0-dev) lib/mix/cli.ex:172: Mix.CLI.maybe_change_env_and_target/2 (mix 1.20.0-dev) lib/mix/cli.ex:59: Mix.CLI.proceed/1 /Users/thiago.santos/dev/thiamsantos/elixir/bin/mix:7: (file) (elixir 1.20.0-dev) src/elixir_compiler.erl:81: :elixir_compiler.dispatch/4 (elixir 1.20.0-dev) src/elixir_compiler.erl:56: :elixir_compiler.compile/4 Compiling 25 files (.ex) warning: the variable "size" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator │ 1977 │ <<val::size(size)>> = value │ ~ │ └─ lib/ecto/adapters/postgres/connection.ex:1977:19: Ecto.Adapters.Postgres.Connection.bitstring_literal/1 warning: unused require Ecto.Query │ 107 │ require Ecto.Query │ ~ │ └─ lib/ecto/migrator.ex:107:3 warning: unused require Ecto.Query │ 140 │ require Ecto.Query │ ~ │ └─ lib/ecto/adapters/tds.ex:140:3 warning: unused require Logger │ 139 │ require Logger │ ~ │ └─ lib/ecto/adapters/tds.ex:139:3 warning: unused require Ecto.Schema │ 8 │ require Ecto.Schema │ ~ │ └─ lib/ecto/adapters/tds/connection.ex:8:5 warning: unused require Logger │ 4 │ require Logger │ ~ │ └─ lib/ecto/adapters/tds/connection.ex:4:5 Generated ecto_sql app 

phoenix

# phoenix ~/dev/thiamsantos/phoenix (master) > mix compile warning: unused require Phoenix.Endpoint │ 6 │ require Phoenix.Endpoint │ ~ │ └─ lib/phoenix/channel/server.ex:6:3 warning: unused require Phoenix.Endpoint │ 6 │ require Phoenix.Endpoint │ ~ │ └─ lib/phoenix/controller.ex:6:3 warning: unused require Logger │ 101 │ require Logger │ ~ │ └─ lib/phoenix/token.ex:101:3 warning: unused require Logger │ 69 │ require Logger │ ~ │ └─ lib/phoenix/socket/pool_supervisor.ex:69:3 warning: the variable "prefix_size" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator │ 46 │ <<prefix::binary-size(prefix_size), ^suffix::binary>> -> prefix │ ~ │ └─ lib/phoenix/naming.ex:46:29: Phoenix.Naming.unsuffix/2 warning: unused require Phoenix.Endpoint │ 17 │ require Phoenix.Endpoint │ ~ │ └─ lib/phoenix/endpoint/render_errors.ex:17:3 warning: unused require Logger │ 283 │ require Logger │ ~ │ └─ lib/phoenix/endpoint.ex:283:3 warning: unused require Phoenix.Endpoint │ 199 │ require Phoenix.Endpoint │ ~ │ └─ lib/phoenix/socket.ex:199:3 warning: unused require Logger │ 9 │ require Logger │ ~ │ └─ lib/phoenix/config.ex:9:3 Generated phoenix app 

[josevalim]

💚 💙 💜 💛 ❤️

[lukaszsamson]

@josevalim@thiamsantos I suspect this may not work correctly with require Foo, as: Bar when Foo is not used in macro calls but Bar alias is used. There are not tests covering that case.

[josevalim]

In this case the require is not used, so it should warn (and the user should use an alias instead)! A test would be nice indeed!

[lukaszsamson]

Yes, the warning should say that, e.g. unused require Foo. Consider using alias Foo, as: Bar instead

[thiamsantos]

In this case the require is not used, so it should warn (and the user should use an alias instead)! A test would be nice indeed!

I am not sure if I followed the as: in the require statement is something discouraged?

--
Another point:
I think we are missing some documentation about the warnings.
Probably a note on the documentation similar to alias docs would be nice as well.

[sabiwara]

I am not sure if I followed the as: in the require statement is something discouraged?

Not discouraged if you need both require + alias, but in the case no macro is used, it should be a regular alias 🙂

[thiamsantos]

@sabiwara yeah, makes sense. Now I was able to connect the dots 😂 thank you ♥️

[michallepicki]

Shouldn't this warning be suppressed for requires injected by macros? For a regular Phoenix project I'm seeing a flood of warnings for things like Phoenix.Endpoint, Phoenix.LiveView.Engine, Phoenix.Template, Phoenix.LiveView.TagEngine etc even though our project codebase doesn't require them explicitly anywhere in the controller or component modules that now have that warning

[josevalim]

Yes, it should. I will take a look at it.

[josevalim]

Fixed in main, thank you!