label-mode: replace conceal with virtual text in nvim ≥ 0.5

Problem: Using conceal for label interferes with usual syntax highlights
that use conceal.

Solution: In nvim, set decoration provider that renders labels as
virtual text only in the current window.

autoload/sneak/label.vim

@@ -5,9 +5,15 @@
 let g:sneak#target_labels = get(g:, 'sneak#target_labels', ";sftunq/SFGHLTUNRMQZ?0")
 
 let s:matchmap = {}
-let s:match_ids = []
 let s:orig_conceal_matches = []
 
+let s:use_virt_text = has('nvim-0.5')
+if s:use_virt_text
+  call luaeval('require("sneak").init()')
+else
+  let s:match_ids = []
+endif
+
 if exists('*strcharpart')
   func! s:strchar(s, i) abort
     return strcharpart(a:s, a:i, 1)
@@ -20,9 +26,13 @@ endif
 
 func! s:placematch(c, pos) abort
   let s:matchmap[a:c] = a:pos
-  let pat = '\%'.a:pos[0].'l\%'.a:pos[1].'c.'
-  let id = matchadd('Conceal', pat, 999, -1, { 'conceal': a:c })
-  call add(s:match_ids, id)
+  if s:use_virt_text
+    call luaeval('require("sneak").placematch(_A[1], _A[2], _A[3])', [a:c, a:pos[0] - 1, a:pos[1] - 1])
+  else
+    let pat = '\%'.a:pos[0].'l\%'.a:pos[1].'c.'
+    let id = matchadd('Conceal', pat, 999, -1, { 'conceal': a:c })
+    call add(s:match_ids, id)
+  endif
 endf
 
 func! s:save_conceal_matches() abort
@@ -119,14 +129,17 @@ endf "}}}
 func! s:after() abort
   autocmd! sneak_label_cleanup
   try | call matchdelete(s:sneak_cursor_hl) | catch | endtry
-  call map(s:match_ids, 'matchdelete(v:val)')
-  let s:match_ids = []
-  " Remove temporary highlight links.
-  exec 'hi! link Conceal '.s:orig_hl_conceal
-  call s:restore_conceal_matches()
+  if s:use_virt_text
+    call luaeval('require("sneak").after()')
+  else
+    call map(s:match_ids, 'matchdelete(v:val)')
+    let s:match_ids = []
+    " Remove temporary highlight links.
+    exec 'hi! link Conceal '.s:orig_hl_conceal
+    call s:restore_conceal_matches()
+    let [&l:concealcursor,&l:conceallevel]=[s:o_cocu,s:o_cole]
+  endif
   exec 'hi! link Sneak '.s:orig_hl_sneak
-
-  let [&l:concealcursor,&l:conceallevel]=[s:o_cocu,s:o_cole]
 endf
 
 func! s:disable_conceal_in_other_windows() abort
@@ -149,20 +162,25 @@ endf
 
 func! s:before() abort
   let s:matchmap = {}
-  for o in ['cocu', 'cole']
-    exe 'let s:o_'.o.'=&l:'.o
-  endfor
-
-  setlocal concealcursor=ncv conceallevel=2
 
   " Highlight the cursor location (because cursor is hidden during getchar()).
   let s:sneak_cursor_hl = matchadd("SneakScope", '\%#', 11, -1)
 
-  let s:orig_hl_conceal = sneak#util#links_to('Conceal')
-  call s:save_conceal_matches()
+  if s:use_virt_text
+    call luaeval('require("sneak").before()')
+  else
+    for o in ['cocu', 'cole']
+      exe 'let s:o_'.o.'=&l:'.o
+    endfor
+    setlocal concealcursor=ncv conceallevel=2
+
+    let s:orig_hl_conceal = sneak#util#links_to('Conceal')
+    call s:save_conceal_matches()
+    " Set temporary link to our custom 'conceal' highlight.
+    hi! link Conceal SneakLabel
+  endif
+
   let s:orig_hl_sneak   = sneak#util#links_to('Sneak')
-  " Set temporary link to our custom 'conceal' highlight.
-  hi! link Conceal SneakLabel
   " Set temporary link to hide the sneak search targets.
   hi! link Sneak SneakLabelMask
 

lua/sneak.lua

@@ -0,0 +1,43 @@
+local M = {}
+
+local ns = vim.api.nvim_create_namespace('sneak')
+
+local matches
+
+function M.before()
+  matches = {}
+end
+
+function M.placematch(c, row, col)
+  matches[#matches+1] = { c, row, col }
+end
+
+function M.after()
+  matches = nil
+end
+
+function M.init()
+  vim.api.nvim_set_decoration_provider(ns, {
+    on_start = function(_, _)
+      if not matches then
+        return false
+      end
+    end,
+    on_win = function(_, win, _, _, _)
+      if win ~= vim.api.nvim_get_current_win() then
+        return false
+      end
+      for _, m in ipairs(matches) do
+        local c, row, col = unpack(m)
+        vim.api.nvim_buf_set_extmark(0, ns, row, col, {
+          priority = 1000,
+          virt_text = { {c, 'SneakLabel'} },
+          virt_text_pos = 'overlay',
+          ephemeral = true,
+        })
+      end
+    end
+  })
+end
+
+return M