feat(diagnostic): is_enabled, enable(…, enable:boolean)

Problem:
`vim.diagnostic.is_disabled` and `vim.diagnostic.disable` are unnecessary
and inconsistent with the "toggle" pattern (established starting with
`vim.lsp.inlay_hint`, see https://github.com/neovim/neovim/pull/25512#pullrequestreview-1676750276

As a reminder, the rationale is:
- we always need `enable()`
- we always end up needing `is_enabled()`
- "toggle" can be achieved via `enable(not is_enabled())`
- therefore,
    - `toggle()` and `disable()` are redundant
    - `is_disabled()` is a needless inconsistency

Solution:
- Introduce `vim.diagnostic.is_enabled`, and `vim.diagnostic.enable(…, enable:boolean)`
    - Note: Future improvement would be to add an `enable()` overload `enable(enable:boolean, opts: table)`.
- Deprecate `vim.diagnostic.is_disabled`, `vim.diagnostic.disable`

7 files changed, 153 insertions, 95 deletions

diff --git a/runtime/doc/deprecated.txt b/runtime/doc/deprecated.txt
index 5ac4ad4ce2..1b57f34896 100644
--- a/runtime/doc/deprecated.txt
+++ b/runtime/doc/deprecated.txt
@@ -80,12 +80,15 @@ HIGHLIGHTS
 - *hl-VertSplit*	Use |hl-WinSeparator| instead.
 
 LSP DIAGNOSTICS
+- *vim.diagnostic.disable()*		Use |vim.diagnostic.enable()|
+- *vim.diagnostic.is_disabled()*	Use |vim.diagnostic.is_enabled()|
+
 For each of the functions below, use the corresponding function in
 |vim.diagnostic| instead (unless otherwise noted). For example, use
 |vim.diagnostic.get()| instead of |vim.lsp.diagnostic.get()|.
 
 - *vim.lsp.diagnostic.clear()*		Use |vim.diagnostic.hide()| instead.
-- *vim.lsp.diagnostic.disable()*
+- *vim.lsp.diagnostic.disable()*          Use |vim.diagnostic.enable()| instead.
 - *vim.lsp.diagnostic.display()*	Use |vim.diagnostic.show()| instead.
 - *vim.lsp.diagnostic.enable()*
 - *vim.lsp.diagnostic.get()*
diff --git a/runtime/doc/develop.txt b/runtime/doc/develop.txt
index 767f46ad1e..a7373c84b3 100644
--- a/runtime/doc/develop.txt
+++ b/runtime/doc/develop.txt
@@ -383,9 +383,12 @@ Use existing common {verb} names (actions) if possible:
     - try_{verb}:   Best-effort operation, failure returns null or error obj
 
 Do NOT use these deprecated verbs:
+    - disable:      Prefer `enable(enable: boolean)`.
+    - is_disabled:  Prefer `is_enabled()`.
     - list:         Redundant with "get"
-    - show:         Redundant with "print", "echo"
     - notify:       Redundant with "print", "echo"
+    - show:         Redundant with "print", "echo"
+    - toggle:       Prefer `enable(not is_enabled())`.
 
 Use consistent names for {noun} (nouns) in API functions: buffer is called
 "buf" everywhere, not "buffer" in some places and "buf" in others.
diff --git a/runtime/doc/diagnostic.txt b/runtime/doc/diagnostic.txt
index a9172119d0..1f3f1c80fe 100644
--- a/runtime/doc/diagnostic.txt
+++ b/runtime/doc/diagnostic.txt
@@ -616,23 +616,19 @@ count({bufnr}, {opts})                                *vim.diagnostic.count()*
         (`table`) Table with actually present severity values as keys (see
         |diagnostic-severity|) and integer counts as values.
 
-disable({bufnr}, {namespace})                       *vim.diagnostic.disable()*
-    Disable diagnostics in the given buffer.
+enable({bufnr}, {namespace}, {enable})               *vim.diagnostic.enable()*
+    Enables or disables diagnostics.
 
-    Parameters: ~
-      • {bufnr}      (`integer?`) Buffer number, or 0 for current buffer. When
-                     omitted, disable diagnostics in all buffers.
-      • {namespace}  (`integer?`) Only disable diagnostics for the given
-                     namespace.
-
-enable({bufnr}, {namespace})                         *vim.diagnostic.enable()*
-    Enable diagnostics in the given buffer.
+    To "toggle", pass the inverse of `is_enabled()`: >lua
+        vim.diagnostic.enable(0, not vim.diagnostic.is_enabled())
+<
 
     Parameters: ~
-      • {bufnr}      (`integer?`) Buffer number, or 0 for current buffer. When
-                     omitted, enable diagnostics in all buffers.
-      • {namespace}  (`integer?`) Only enable diagnostics for the given
-                     namespace.
+      • {bufnr}      (`integer?`) Buffer number, or 0 for current buffer, or
+                     `nil` for all buffers.
+      • {namespace}  (`integer?`) Only for the given namespace, or `nil` for
+                     all.
+      • {enable}     (`boolean?`) true/nil to enable, false to disable
 
 fromqflist({list})                               *vim.diagnostic.fromqflist()*
     Convert a list of quickfix items to a list of diagnostics.
@@ -733,7 +729,7 @@ hide({namespace}, {bufnr})                             *vim.diagnostic.hide()*
     diagnostics, use |vim.diagnostic.reset()|.
 
     To hide diagnostics and prevent them from re-displaying, use
-    |vim.diagnostic.disable()|.
+    |vim.diagnostic.enable()|.
 
     Parameters: ~
       • {namespace}  (`integer?`) Diagnostic namespace. When omitted, hide
@@ -741,14 +737,16 @@ hide({namespace}, {bufnr})                             *vim.diagnostic.hide()*
       • {bufnr}      (`integer?`) Buffer number, or 0 for current buffer. When
                      omitted, hide diagnostics in all buffers.
 
-is_disabled({bufnr}, {namespace})               *vim.diagnostic.is_disabled()*
-    Check whether diagnostics are disabled in a given buffer.
+is_enabled({bufnr}, {namespace})                 *vim.diagnostic.is_enabled()*
+    Check whether diagnostics are enabled.
+
+    Note: ~
+      • This API is pre-release (unstable).
 
     Parameters: ~
       • {bufnr}      (`integer?`) Buffer number, or 0 for current buffer.
-      • {namespace}  (`integer?`) Diagnostic namespace. When omitted, checks
-                     if all diagnostics are disabled in {bufnr}. Otherwise,
-                     only checks if diagnostics from {namespace} are disabled.
+      • {namespace}  (`integer?`) Diagnostic namespace, or `nil` for all
+                     diagnostics in {bufnr}.
 
     Return: ~
         (`boolean`)
diff --git a/runtime/doc/news-0.9.txt b/runtime/doc/news-0.9.txt
index 789bc9e0bc..f1f3ac6954 100644
--- a/runtime/doc/news-0.9.txt
+++ b/runtime/doc/news-0.9.txt
@@ -162,7 +162,7 @@ The following new APIs or features were added.
 • |vim.diagnostic| now supports LSP DiagnosticsTag.
   See: https://microsoft.github.io/language-server-protocol/specification/#diagnosticTag
 
-• |vim.diagnostic.is_disabled()| checks if diagnostics are disabled in a given
+• vim.diagnostic.is_disabled() checks if diagnostics are disabled in a given
   buffer or namespace.
 
 • Treesitter captures can now be transformed by directives. This will allow
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index ae5f808a9c..8609d84506 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -328,6 +328,8 @@ The following new APIs and features were added.
   |vim.diagnostic.get()| when only the number of diagnostics is needed, but
   not the diagnostics themselves.
 
+• Introduced |vim.diagnostic.is_enabled()|
+
 • |vim.deepcopy()| has a `noref` argument to avoid hashing table values.
 
 • Terminal buffers emit a |TermRequest| autocommand event when the child
@@ -419,6 +421,8 @@ The following changes to existing APIs or features add new behavior.
 • |vim.diagnostic.get()| and |vim.diagnostic.count()| accept multiple
   namespaces rather than just a single namespace.
 
+• |vim.diagnostic.enable()| accepts an `enabled` boolean parameter.
+
 • Extmarks now fully support multi-line ranges, and a single extmark can be
   used to highlight a range of arbitrary length. The |nvim_buf_set_extmark()|
   API function already allowed you to define such ranges, but highlight regions
@@ -520,6 +524,10 @@ release.
   - |nvim_win_get_option()|	Use |nvim_get_option_value()| instead.
   - |nvim_win_set_option()|	Use |nvim_set_option_value()| instead.
 
+• vim.diagnostic functions:
+  - |vim.diagnostic.disable()|
+  - |vim.diagnostic.is_disabled()|
+
 • vim.lsp functions:
   - |vim.lsp.util.get_progress_messages()|	Use |vim.lsp.status()| instead.
   - |vim.lsp.get_active_clients()|		Use |vim.lsp.get_clients()| instead.
diff --git a/runtime/lua/vim/diagnostic.lua b/runtime/lua/vim/diagnostic.lua
index 57491d155d..26374aa055 100644
--- a/runtime/lua/vim/diagnostic.lua
+++ b/runtime/lua/vim/diagnostic.lua
@@ -1492,7 +1492,7 @@ end
 --- diagnostics, use |vim.diagnostic.reset()|.
 ---
 --- To hide diagnostics and prevent them from re-displaying, use
---- |vim.diagnostic.disable()|.
+--- |vim.diagnostic.enable()|.
 ---
 ---@param namespace integer? Diagnostic namespace. When omitted, hide
 ---                          diagnostics from all namespaces.
@@ -1517,25 +1517,35 @@ function M.hide(namespace, bufnr)
   end
 end
 
---- Check whether diagnostics are disabled in a given buffer.
+--- Check whether diagnostics are enabled.
 ---
----@param bufnr integer? Buffer number, or 0 for current buffer.
----@param namespace integer? Diagnostic namespace. When omitted, checks if
----                          all diagnostics are disabled in {bufnr}.
----                          Otherwise, only checks if diagnostics from
----                          {namespace} are disabled.
----@return boolean
-function M.is_disabled(bufnr, namespace)
+--- @param bufnr integer? Buffer number, or 0 for current buffer.
+--- @param namespace integer? Diagnostic namespace, or `nil` for all diagnostics in {bufnr}.
+--- @return boolean
+--- @since 12
+function M.is_enabled(bufnr, namespace)
   bufnr = get_bufnr(bufnr)
   if namespace and M.get_namespace(namespace).disabled then
-    return true
+    return false
   end
 
   if type(diagnostic_disabled[bufnr]) == 'table' then
-    return diagnostic_disabled[bufnr][namespace]
+    return not diagnostic_disabled[bufnr][namespace]
   end
 
-  return diagnostic_disabled[bufnr] ~= nil
+  return diagnostic_disabled[bufnr] == nil
+end
+
+--- @deprecated use `vim.diagnostic.is_enabled()`
+function M.is_disabled(bufnr, namespace)
+  vim.deprecate(
+    'Defining diagnostic signs with :sign-define or sign_define()',
+    'vim.diagnostic.is_enabled()',
+    '0.12',
+    nil,
+    false
+  )
+  return not M.is_enabled(bufnr, namespace)
 end
 
 --- Display diagnostics for the given namespace and buffer.
@@ -1923,71 +1933,75 @@ function M.setloclist(opts)
   set_list(true, opts)
 end
 
---- Disable diagnostics in the given buffer.
----
----@param bufnr integer? Buffer number, or 0 for current buffer. When
----                      omitted, disable diagnostics in all buffers.
----@param namespace integer? Only disable diagnostics for the given namespace.
+--- @deprecated use `vim.diagnostic.enabled(…, false)`
 function M.disable(bufnr, namespace)
-  vim.validate({ bufnr = { bufnr, 'n', true }, namespace = { namespace, 'n', true } })
-  if bufnr == nil then
-    if namespace == nil then
-      -- Disable everything (including as yet non-existing buffers and
-      -- namespaces) by setting diagnostic_disabled to an empty table and set
-      -- its metatable to always return true. This metatable is removed
-      -- in enable()
-      diagnostic_disabled = setmetatable({}, {
-        __index = function()
-          return true
-        end,
-      })
-    else
-      local ns = M.get_namespace(namespace)
-      ns.disabled = true
-    end
-  else
-    bufnr = get_bufnr(bufnr)
-    if namespace == nil then
-      diagnostic_disabled[bufnr] = true
-    else
-      if type(diagnostic_disabled[bufnr]) ~= 'table' then
-        diagnostic_disabled[bufnr] = {}
-      end
-      diagnostic_disabled[bufnr][namespace] = true
-    end
-  end
-
-  M.hide(namespace, bufnr)
+  vim.deprecate(
+    'vim.diagnostic.disable()',
+    'vim.diagnostic.enabled(…, false)',
+    '0.12',
+    nil,
+    false
+  )
+  M.enable(bufnr, namespace, false)
 end
 
---- Enable diagnostics in the given buffer.
+--- Enables or disables diagnostics.
 ---
----@param bufnr integer? Buffer number, or 0 for current buffer. When
----                      omitted, enable diagnostics in all buffers.
----@param namespace integer? Only enable diagnostics for the given namespace.
-function M.enable(bufnr, namespace)
-  vim.validate({ bufnr = { bufnr, 'n', true }, namespace = { namespace, 'n', true } })
+--- To "toggle", pass the inverse of `is_enabled()`:
+---
+--- ```lua
+--- vim.diagnostic.enable(0, not vim.diagnostic.is_enabled())
+--- ```
+---
+--- @param bufnr integer? Buffer number, or 0 for current buffer, or `nil` for all buffers.
+--- @param namespace integer? Only for the given namespace, or `nil` for all.
+--- @param enable (boolean|nil) true/nil to enable, false to disable
+function M.enable(bufnr, namespace, enable)
+  vim.validate({
+    bufnr = { bufnr, 'n', true },
+    namespace = { namespace, 'n', true },
+    enable = { enable, 'b', true },
+  })
+  enable = enable == nil and true or enable
   if bufnr == nil then
     if namespace == nil then
-      -- Enable everything by setting diagnostic_disabled to an empty table
-      diagnostic_disabled = {}
+      diagnostic_disabled = (
+        enable
+          -- Enable everything by setting diagnostic_disabled to an empty table.
+          and {}
+        -- Disable everything (including as yet non-existing buffers and namespaces) by setting
+        -- diagnostic_disabled to an empty table and set its metatable to always return true.
+        or setmetatable({}, {
+          __index = function()
+            return true
+          end,
+        })
+      )
     else
       local ns = M.get_namespace(namespace)
-      ns.disabled = false
+      ns.disabled = not enable
     end
   else
     bufnr = get_bufnr(bufnr)
     if namespace == nil then
-      diagnostic_disabled[bufnr] = nil
+      diagnostic_disabled[bufnr] = (not enable) and true or nil
     else
       if type(diagnostic_disabled[bufnr]) ~= 'table' then
-        return
+        if enable then
+          return
+        else
+          diagnostic_disabled[bufnr] = {}
+        end
       end
-      diagnostic_disabled[bufnr][namespace] = nil
+      diagnostic_disabled[bufnr][namespace] = (not enable) and true or nil
     end
   end
 
-  M.show(namespace, bufnr)
+  if enable then
+    M.show(namespace, bufnr)
+  else
+    M.hide(namespace, bufnr)
+  end
 end
 
 --- Parse a diagnostic from a string.
diff --git a/test/functional/lua/diagnostic_spec.lua b/test/functional/lua/diagnostic_spec.lua
index afdf619526..972e2baa71 100644
--- a/test/functional/lua/diagnostic_spec.lua
+++ b/test/functional/lua/diagnostic_spec.lua
@@ -329,7 +329,7 @@ describe('vim.diagnostic', function()
     eq(
       { 1, 1, 2, 0, 2 },
       exec_lua [[
-      vim.diagnostic.disable(diagnostic_bufnr, diagnostic_ns)
+      vim.diagnostic.enable(diagnostic_bufnr, diagnostic_ns, false)
       return {
         count_diagnostics(diagnostic_bufnr, vim.diagnostic.severity.ERROR, diagnostic_ns),
         count_diagnostics(diagnostic_bufnr, vim.diagnostic.severity.WARN, other_ns),
@@ -371,7 +371,7 @@ describe('vim.diagnostic', function()
       vim.diagnostic.set(diagnostic_ns, diagnostic_bufnr, ns_1_diags)
       vim.diagnostic.set(other_ns, diagnostic_bufnr, ns_2_diags)
 
-      vim.diagnostic.disable(diagnostic_bufnr, diagnostic_ns)
+      vim.diagnostic.enable(diagnostic_bufnr, diagnostic_ns, false)
 
       return {
         count_extmarks(diagnostic_bufnr, diagnostic_ns),
@@ -384,7 +384,7 @@ describe('vim.diagnostic', function()
       { 4, 0 },
       exec_lua [[
       vim.diagnostic.enable(diagnostic_bufnr, diagnostic_ns)
-      vim.diagnostic.disable(diagnostic_bufnr, other_ns)
+      vim.diagnostic.enable(diagnostic_bufnr, other_ns, false)
 
       return {
         count_extmarks(diagnostic_bufnr, diagnostic_ns),
@@ -500,7 +500,7 @@ describe('vim.diagnostic', function()
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns))
 
-        vim.diagnostic.disable()
+        vim.diagnostic.enable(nil, nil, false)
 
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns))
@@ -561,7 +561,7 @@ describe('vim.diagnostic', function()
                              count_extmarks(diagnostic_bufnr, other_ns) +
                              count_extmarks(other_bufnr, diagnostic_ns))
 
-        vim.diagnostic.disable(diagnostic_bufnr)
+        vim.diagnostic.enable(diagnostic_bufnr, nil, false)
 
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns) +
@@ -573,7 +573,7 @@ describe('vim.diagnostic', function()
                              count_extmarks(diagnostic_bufnr, other_ns) +
                              count_extmarks(other_bufnr, diagnostic_ns))
 
-        vim.diagnostic.disable(other_bufnr)
+        vim.diagnostic.enable(other_bufnr, nil, false)
 
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns) +
@@ -610,7 +610,7 @@ describe('vim.diagnostic', function()
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns))
 
-        vim.diagnostic.disable(nil, diagnostic_ns)
+        vim.diagnostic.enable(nil, diagnostic_ns, false)
 
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns))
@@ -620,7 +620,7 @@ describe('vim.diagnostic', function()
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns))
 
-        vim.diagnostic.disable(nil, other_ns)
+        vim.diagnostic.enable(nil, other_ns, false)
 
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns))
@@ -663,13 +663,13 @@ describe('vim.diagnostic', function()
                              count_extmarks(diagnostic_bufnr, other_ns) +
                              count_extmarks(other_bufnr, diagnostic_ns))
 
-        vim.diagnostic.disable(diagnostic_bufnr, diagnostic_ns)
+        vim.diagnostic.enable(diagnostic_bufnr, diagnostic_ns, false)
 
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns) +
                              count_extmarks(other_bufnr, diagnostic_ns))
 
-        vim.diagnostic.disable(diagnostic_bufnr, other_ns)
+        vim.diagnostic.enable(diagnostic_bufnr, other_ns, false)
 
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns) +
@@ -682,7 +682,7 @@ describe('vim.diagnostic', function()
                              count_extmarks(other_bufnr, diagnostic_ns))
 
         -- Should have no effect
-        vim.diagnostic.disable(other_bufnr, other_ns)
+        vim.diagnostic.enable(other_bufnr, other_ns, false)
 
         table.insert(result, count_extmarks(diagnostic_bufnr, diagnostic_ns) +
                              count_extmarks(diagnostic_bufnr, other_ns) +
@@ -1742,7 +1742,7 @@ describe('vim.diagnostic', function()
       eq(
         0,
         exec_lua [[
-        vim.diagnostic.disable(diagnostic_bufnr, diagnostic_ns)
+        vim.diagnostic.enable(diagnostic_bufnr, diagnostic_ns, false)
         vim.diagnostic.set(diagnostic_ns, diagnostic_bufnr, {
           make_error('Diagnostic From Server 1:1', 1, 1, 1, 1),
         })
@@ -2711,7 +2711,39 @@ describe('vim.diagnostic', function()
       )
     end)
 
-    it('checks if diagnostics are disabled in a buffer', function()
+    it('is_enabled', function()
+      eq(
+        { false, false, false, false },
+        exec_lua [[
+        vim.diagnostic.set(diagnostic_ns, diagnostic_bufnr, {
+          make_error('Diagnostic #1', 1, 1, 1, 1),
+        })
+        vim.api.nvim_set_current_buf(diagnostic_bufnr)
+        vim.diagnostic.enable(nil, nil, false)
+        return {
+          vim.diagnostic.is_enabled(),
+          vim.diagnostic.is_enabled(diagnostic_bufnr),
+          vim.diagnostic.is_enabled(diagnostic_bufnr, diagnostic_ns),
+          vim.diagnostic.is_enabled(_, diagnostic_ns),
+        }
+      ]]
+      )
+
+      eq(
+        { true, true, true, true },
+        exec_lua [[
+        vim.diagnostic.enable()
+        return {
+          vim.diagnostic.is_enabled(),
+          vim.diagnostic.is_enabled(diagnostic_bufnr),
+          vim.diagnostic.is_enabled(diagnostic_bufnr, diagnostic_ns),
+          vim.diagnostic.is_enabled(_, diagnostic_ns),
+        }
+      ]]
+      )
+    end)
+
+    it('is_disabled (deprecated)', function()
       eq(
         { true, true, true, true },
         exec_lua [[