mini.base16: undercurl for LSP diagnostics

[wroyca]

Modern terminals now handle undercurl appropriately, and we already use it with spell-checking diagnostics. Following that, undercurl for LSP diagnostics feels like a natural transition, and the possibility to opt in would be, I believe, quite welcome :)

Here's a draft implementation that can serve as the basis:

From 0d2b08461e0f6ac5ef74c3a22caab7409454bce3 Mon Sep 17 00:00:00 2001
From: William Roy <wroy@proton.me>
Date: Thu, 5 Oct 2023 17:58:32 -0400
Subject: [PATCH] (mini.base16) Undercurl diagnostic configuration

---
 lua/mini/base16.lua | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/lua/mini/base16.lua b/lua/mini/base16.lua
index a4c45a5..1e0cd96 100644
--- a/lua/mini/base16.lua
+++ b/lua/mini/base16.lua
@@ -200,6 +200,11 @@ MiniBase16.config = {
   -- more information.
   use_cterm = nil,
 
+  -- Whether to support modern terminal undercurl diagnostic. Can be boolean,
+  -- or `nil` (same as `false`). See `setup()` documentation for
+  -- more information.
+  use_undercurl_diagnostic = nil,
+
   -- Plugin integrations. Use `default = false` to disable all integrations.
   -- Also can be set per plugin (see |MiniBase16.config|).
   plugins = { default = true },
@@ -361,7 +366,8 @@ end
 H.apply_config = function(config)
   MiniBase16.config = config
 
-  H.apply_palette(config.palette, config.use_cterm)
+  H.apply_palette(
+    config.palette, config.use_cterm, config.use_undercurl_diagnostic)
 end
 
 -- Validators -----------------------------------------------------------------
@@ -434,7 +440,7 @@ H.validate_hex = function(x, x_name)
 end
 
 -- Highlighting ---------------------------------------------------------------
-H.apply_palette = function(palette, use_cterm)
+H.apply_palette = function(palette, use_cterm, use_undercurl_diagnostic)
   -- Prepare highlighting application. Notes:
   -- - Clear current highlight only if other theme was loaded previously.
   -- - No need to `syntax reset` because *all* syntax groups are defined later.
@@ -450,6 +456,13 @@ H.apply_palette = function(palette, use_cterm)
     p, hi = palette, H.highlight_gui
   end
 
+  local dattr
+  if use_undercurl_diagnostic then
+    dattr = 'undercurl'
+  else
+    dattr = 'underline'
+  end
+
   -- NOTE: recommendations for adding new highlight groups:
   -- - Put all related groups (like for new plugin) in single paragraph.
   -- - Sort within group alphabetically (by hl-group name) ignoring case.
@@ -612,11 +625,11 @@ H.apply_palette = function(palette, use_cterm)
   hi('DiagnosticSignOk',    {link='DiagnosticFloatingOk'})
   hi('DiagnosticSignWarn',  {link='DiagnosticFloatingWarn'})
 
-  hi('DiagnosticUnderlineError', {fg=nil, bg=nil, attr='underline', sp=p.base08})
-  hi('DiagnosticUnderlineHint',  {fg=nil, bg=nil, attr='underline', sp=p.base0D})
-  hi('DiagnosticUnderlineInfo',  {fg=nil, bg=nil, attr='underline', sp=p.base0C})
-  hi('DiagnosticUnderlineOk',    {fg=nil, bg=nil, attr='underline', sp=p.base0B})
-  hi('DiagnosticUnderlineWarn',  {fg=nil, bg=nil, attr='underline', sp=p.base0E})
+  hi('DiagnosticUnderlineError', {fg=nil, bg=nil, attr=dattr, sp=p.base08})
+  hi('DiagnosticUnderlineHint',  {fg=nil, bg=nil, attr=dattr, sp=p.base0D})
+  hi('DiagnosticUnderlineInfo',  {fg=nil, bg=nil, attr=dattr, sp=p.base0C})
+  hi('DiagnosticUnderlineOk',    {fg=nil, bg=nil, attr=dattr, sp=p.base0B})
+  hi('DiagnosticUnderlineWarn',  {fg=nil, bg=nil, attr=dattr, sp=p.base0E})
 
   -- Built-in LSP
   hi('LspReferenceText',  {fg=nil, bg=p.base02, attr=nil, sp=nil})
-- 
2.41.0

[echasnovski]

Thanks for suggestion!

Having this type of granular configuration in color scheme is something I perceive as "unnecessary complication". Users can override those without much troubles on their end.