fix(vim.deprecate): show deprecation warning in devel versions as well

Problem:

On devel(nightly) versions, deprecation warnings for hard-deprecated
features are not being displayed. E.g.,
  - to be removed in: 0.11
  - hard-deprecation since 0.10
  - soft-deprecation since 0.9

then 0.10-nightly (0.10.0-dev) versions as well as 0.10.0 (stable)
should display the deprecation warning message.

Solution:

Improve the code and logic on `vim.deprecate()`, and improve
test cases with mocked `vim.version()`.

2 files changed, 118 insertions, 35 deletions

diff --git a/runtime/lua/vim/_editor.lua b/runtime/lua/vim/_editor.lua
index 52a21587f4..fd49f63558 100644
--- a/runtime/lua/vim/_editor.lua
+++ b/runtime/lua/vim/_editor.lua
@@ -1033,21 +1033,44 @@ end
 ---
 ---@return string|nil # Deprecated message, or nil if no message was shown.
 function vim.deprecate(name, alternative, version, plugin, backtrace)
+  vim.validate {
+    name = { name, 'string' },
+    alternative = { alternative, 'string', true },
+    version = { version, 'string', true },
+    plugin = { plugin, 'string', true },
+  }
+  plugin = plugin or 'Nvim'
+
   -- Only issue warning if feature is hard-deprecated as specified by MAINTAIN.md.
-  if plugin == nil then
-    local current_version = vim.version()
-    local deprecated_version = assert(vim.version.parse(version))
-    local soft_deprecated_version =
-      { deprecated_version.major, deprecated_version.minor - 1, deprecated_version.patch }
-    local deprecate = vim.version.lt(current_version, soft_deprecated_version)
-    if deprecate then
+  -- e.g., when planned to be removed in version = '0.12' (soft-deprecated since 0.10-dev),
+  -- show warnings since 0.11, including 0.11-dev (hard_deprecated_since = 0.11-dev).
+  if plugin == 'Nvim' then
+    local current_version = vim.version() ---@type Version
+    local removal_version = assert(vim.version.parse(version))
+    local is_hard_deprecated ---@type boolean
+
+    if removal_version.minor > 0 then
+      local hard_deprecated_since = assert(vim.version._version({
+        major = removal_version.major,
+        minor = removal_version.minor - 1,
+        patch = 0,
+        prerelease = 'dev', -- Show deprecation warnings in devel (nightly) version as well
+      }))
+      is_hard_deprecated = (current_version >= hard_deprecated_since)
+    else
+      -- Assume there will be no next minor version before bumping up the major version;
+      -- therefore we can always show a warning.
+      assert(removal_version.minor == 0, vim.inspect(removal_version))
+      is_hard_deprecated = true
+    end
+
+    if not is_hard_deprecated then
       return
     end
   end
 
   local msg = ('%s is deprecated'):format(name)
-  plugin = plugin or 'Nvim'
-  msg = alternative and ('%s, use %s instead.'):format(msg, alternative) or msg
+  msg = alternative and ('%s, use %s instead.'):format(msg, alternative) or (msg .. '.')
   msg = ('%s%s\nThis feature will be removed in %s version %s'):format(
     msg,
     (plugin == 'Nvim' and ' :help deprecated' or ''),
diff --git a/test/functional/lua/vim_spec.lua b/test/functional/lua/vim_spec.lua
index 98968f3695..6e05728b0c 100644
--- a/test/functional/lua/vim_spec.lua
+++ b/test/functional/lua/vim_spec.lua
@@ -128,33 +128,93 @@ describe('lua stdlib', function()
     eq(1, fn.luaeval('vim.stricmp("\\0C\\0", "\\0B\\0")'))
   end)
 
-  it('vim.deprecate', function()
+  local function test_vim_deprecate(current_version)
     -- vim.deprecate(name, alternative, version, plugin, backtrace)
-    eq(
-      dedent [[
-      foo.bar() is deprecated, use zub.wooo{ok=yay} instead. :help deprecated
-      This feature will be removed in Nvim version 0.10]],
-      exec_lua('return vim.deprecate(...)', 'foo.bar()', 'zub.wooo{ok=yay}', '0.10')
-    )
-    -- Same message, skipped.
-    eq(vim.NIL, exec_lua('return vim.deprecate(...)', 'foo.bar()', 'zub.wooo{ok=yay}', '0.10'))
-    -- Don't show error if not hard deprecated
-    eq(vim.NIL, exec_lua('return vim.deprecate(...)', 'foo.bar()', 'nil', '5000.0.0'))
-    -- When `plugin` is specified, don't show ":help deprecated". #22235
-    eq(
-      dedent [[
-      foo.bar() is deprecated, use zub.wooo{ok=yay} instead.
-      This feature will be removed in my-plugin.nvim version 0.3.0]],
-      exec_lua(
-        'return vim.deprecate(...)',
-        'foo.bar()',
-        'zub.wooo{ok=yay}',
-        '0.3.0',
-        'my-plugin.nvim',
-        false
-      )
-    )
-  end)
+    -- See MAINTAIN.md for the soft/hard deprecation policy
+
+    describe(('vim.deprecate [current_version = %s]'):format(current_version), function()
+      before_each(function()
+        -- mock vim.version() behavior, should be pinned for consistent testing
+        exec_lua(
+          [[
+            local current_version_mock = vim.version.parse(...)
+            getmetatable(vim.version).__call = function()
+              return current_version_mock
+            end
+          ]],
+          current_version
+        )
+      end)
+
+      it('when plugin = nil', function()
+        eq(
+          dedent [[
+            foo.bar() is deprecated, use zub.wooo{ok=yay} instead. :help deprecated
+            This feature will be removed in Nvim version 0.10]],
+          exec_lua('return vim.deprecate(...)', 'foo.bar()', 'zub.wooo{ok=yay}', '0.10')
+        )
+        -- Same message, skipped.
+        eq(vim.NIL, exec_lua('return vim.deprecate(...)', 'foo.bar()', 'zub.wooo{ok=yay}', '0.10'))
+
+        -- Don't show error if not hard-deprecated (only soft-deprecated)
+        eq(
+          vim.NIL,
+          exec_lua('return vim.deprecate(...)', 'foo.baz()', 'foo.better_baz()', '0.12.0')
+        )
+
+        -- Show error if hard-deprecated
+        eq(
+          dedent [[
+            foo.hard_dep() is deprecated, use vim.new_api() instead. :help deprecated
+            This feature will be removed in Nvim version 0.11]],
+          exec_lua('return vim.deprecate(...)', 'foo.hard_dep()', 'vim.new_api()', '0.11')
+        )
+
+        -- To be deleted in the next major version (1.0)
+        eq(
+          dedent [[
+            foo.baz() is deprecated. :help deprecated
+            This feature will be removed in Nvim version 1.0]],
+          exec_lua [[ return vim.deprecate('foo.baz()', nil, '1.0') ]]
+        )
+      end)
+
+      it('when plugin is specified', function()
+        -- When `plugin` is specified, don't show ":help deprecated". #22235
+        eq(
+          dedent [[
+            foo.bar() is deprecated, use zub.wooo{ok=yay} instead.
+            This feature will be removed in my-plugin.nvim version 0.3.0]],
+          exec_lua(
+            'return vim.deprecate(...)',
+            'foo.bar()',
+            'zub.wooo{ok=yay}',
+            '0.3.0',
+            'my-plugin.nvim',
+            false
+          )
+        )
+
+        -- plugins: no soft deprecation period
+        eq(
+          dedent [[
+            foo.bar() is deprecated, use zub.wooo{ok=yay} instead.
+            This feature will be removed in my-plugin.nvim version 0.11.0]],
+          exec_lua(
+            'return vim.deprecate(...)',
+            'foo.bar()',
+            'zub.wooo{ok=yay}',
+            '0.11.0',
+            'my-plugin.nvim',
+            false
+          )
+        )
+      end)
+    end)
+  end
+
+  test_vim_deprecate('0.10')
+  test_vim_deprecate('0.10-dev+g0000000')
 
   it('vim.startswith', function()
     eq(true, fn.luaeval('vim.startswith("123", "1")'))