refactor(sources): warn and prevent registration if unsupported

lua/null-ls/sources.lua

@@ -1,4 +1,5 @@
 local methods = require("null-ls.methods")
+local u = require("null-ls.utils")
 
 local validate = vim.validate
 
@@ -153,6 +154,11 @@ M.validate_and_transform = function(source)
         method_map[method] = true
     end
 
+    if method_map[methods.internal.DIAGNOSTICS_ON_SAVE] and not u.has_version("0.6.0") then
+        u.echo("WarningMsg", string.format("source %s is not supported on nvim versions < 0.6.0", name))
+        return
+    end
+
     return {
         name = name,
         generator = generator,

test/spec/sources_spec.lua

@@ -1,6 +1,8 @@
 local stub = require("luassert.stub")
+local mock = require("luassert.mock")
 
 local methods = require("null-ls.methods")
+local u = mock(require("null-ls.utils"), true)
 
 describe("sources", function()
     local sources = require("null-ls.sources")
@@ -270,6 +272,7 @@ describe("sources", function()
     describe("validate_and_transform", function()
         local mock_source
         before_each(function()
+            u.has_version.returns(true)
             mock_source = {
                 generator = { fn = function() end, opts = {}, async = false },
                 name = "mock generator",
@@ -278,6 +281,10 @@ describe("sources", function()
             }
         end)
 
+        after_each(function()
+            u.has_version:clear()
+        end)
+
         it("should validate and return transformed source", function()
             local validated = sources.validate_and_transform(mock_source)
 
@@ -344,6 +351,15 @@ describe("sources", function()
             assert.falsy(validated)
         end)
 
+        it("should return nil if nvim version does not support method", function()
+            mock_source.method = methods.internal.DIAGNOSTICS_ON_SAVE
+            u.has_version.returns(false)
+
+            local validated = sources.validate_and_transform(mock_source)
+
+            assert.falsy(validated)
+        end)
+
         it("should throw if generator is invalid", function()
             mock_source.generator = nil