Add Code Action / Quick Fix to remove unused macro

[robertoaloi]

Context

One of the core features of Erlang LS is to be able to publish a diagnostic message whenever an issue is detected in the user code. Diagnostics are often rendered by the IDE with a squiggle of different colour, depending on the severity of the message (e.g. a warning uses a yellow squiggle, while an error uses a red squiggle).

One of these messages is displayed whenever a macro is defined, but unused. See the picture below.

A Code Action request is sent from the IDE to the language server to compute the possible code actions available for a given document and text range. In the case above, the following request is sent from the IDE to the language server as soon as the user hovers the unused macro:

[Trace - 02:18:26 PM] Sending request 'textDocument/codeAction - (1168)'.
Params: {
  "textDocument": {
    "uri": "file:///Users/robertoaloi/git/github/erlang-ls/erlang_ls/apps/els_lsp/test/els_rename_SUITE.erl"
  },
  "range": {
    "start": {
      "line": 31,
      "character": 13
    },
    "end": {
      "line": 31,
      "character": 13
    }
  },
  "context": {
    "diagnostics": [
      {
        "source": "UnusedMacros",
        "severity": 2,
        "range": {
          "start": {
            "line": 31,
            "character": 8
          },
          "end": {
            "line": 31,
            "character": 16
          }
        },
        "message": "Unused macro: MY_MACRO"
      }
    ]
  }
}


[Trace - 02:18:26 PM] Received response 'textDocument/codeAction - (1168)' in 53ms.
Result: []

As you can see, the request contains the current location (file path, line number, line column) and an additional context (in this case a list containing one diagnostic, which can in turn be used by the server to provide a list of available code actions. Right now, since this feature is not yet implemented, no code actions (i.e. an empty list or []) is returned to the client.

What is Required

In case a textDocument/codeAction request is received by the language server, the server should check for presence of "Unused macro" diagnostics. If one is present, the server should include a code action (say remove-unused-macro) in the response.

Once the possible code action is received by the IDE, the user gets notified. This usually happens via a little bulb icon or similar, which suggests that a code action is available. In this case, the code action would be of type quickfix and suggest to remove the unused macro.

Code Pointers

• Code action requests are dispatched to the els_code_action_provider module, so you probably want to start there

Testing

Don't forget to include a test for the new functionality! You can look at existing examples in the els_code_action_SUITE module and at this code lenses tutorial which includes a testing overview which also applies to code actions.

Problems following the instructions?

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