feat(git): syntax-highlight deleted lines in the diff overlay

lua/git/hunks.lua

@@ -22,6 +22,7 @@ local NS_OVERLAY = vim.api.nvim_create_namespace("ow.git.hunks.overlay")
 ---@field rel string
 ---@field index string[]?
 ---@field index_sha string?
+---@field index_hl { src: string[], lines: table[][]? }?
 ---@field hunks ow.Git.Hunks.Hunk[]
 ---@field overlay boolean
 ---@field autocmds integer[]
@@ -153,20 +154,117 @@ local function render_signs(buf)
     end
 end
 
+local SKIP_CAPTURES = { spell = true, nospell = true, conceal = true }
+
+---@param buf integer
+---@param lines string[]
+---@return table[][]?
+local function highlight_index(buf, lines)
+    if not vim.treesitter.highlighter.active[buf] then
+        return nil
+    end
+    local got, parser = pcall(vim.treesitter.get_parser, buf)
+    if not got or not parser then
+        return nil
+    end
+    local lang = parser:lang()
+    local query = vim.treesitter.query.get(lang, "highlights")
+    if not query then
+        return nil
+    end
+    local source = table.concat(lines, "\n")
+    local got_root, root = pcall(function()
+        local trees = vim.treesitter.get_string_parser(source, lang):parse()
+        local tree = trees and trees[1]
+        return tree and tree:root()
+    end)
+    if not got_root or not root then
+        return nil
+    end
+    ---@type table<integer, table<integer, string>>
+    local groups = {}
+    for id, node in query:iter_captures(root, source) do
+        local name = query.captures[id]
+        if name and name:sub(1, 1) ~= "_" and not SKIP_CAPTURES[name] then
+            local sr, sc, er, ec = node:range()
+            for row = sr, math.min(er, #lines - 1) do
+                local row_groups = groups[row] or {}
+                groups[row] = row_groups
+                local from = row == sr and sc or 0
+                local to = row == er and ec or #(lines[row + 1] or "")
+                for col = from, to - 1 do
+                    row_groups[col] = name
+                end
+            end
+        end
+    end
+    local out = {}
+    for row = 0, #lines - 1 do
+        local line = lines[row + 1] or ""
+        local row_groups = groups[row] or {}
+        local chunks = {}
+        local col = 0
+        while col < #line do
+            local name = row_groups[col]
+            local stop = col + 1
+            while stop < #line and row_groups[stop] == name do
+                stop = stop + 1
+            end
+            local hl ---@type string|string[]
+            if name then
+                hl = { "GitHunkDeleteLine", "@" .. name }
+            else
+                hl = "GitHunkDeleteLine"
+            end
+            table.insert(chunks, { line:sub(col + 1, stop), hl })
+            col = stop
+        end
+        out[row + 1] = chunks
+    end
+    return out
+end
+
 ---@param h ow.Git.Hunks.Hunk
+---@param hl_lines table[][]?  per-index-line syntax chunks, or nil
 ---@return table[]
-local function delete_virt_lines(h)
+local function delete_virt_lines(h, hl_lines)
     local width = vim.o.columns
     local virt = {}
-    for _, line in ipairs(h.old_lines) do
+    for i, line in ipairs(h.old_lines) do
         local pad = math.max(width - vim.api.nvim_strwidth(line), 0)
-        table.insert(virt, {
-            { line .. string.rep(" ", pad), "GitHunkDeleteLine" },
-        })
+        local cached = hl_lines and hl_lines[h.old_start + i - 1]
+        if cached then
+            local chunks = vim.list_extend({}, cached)
+            table.insert(chunks, {
+                string.rep(" ", pad),
+                "GitHunkDeleteLine",
+            })
+            table.insert(virt, chunks)
+        else
+            table.insert(virt, {
+                { line .. string.rep(" ", pad), "GitHunkDeleteLine" },
+            })
+        end
     end
     return virt
 end
 
+---@param state ow.Git.Hunks.BufState
+---@param buf integer
+---@return table[][]?
+local function index_spans(state, buf)
+    if not state.index then
+        return nil
+    end
+    local cache = state.index_hl
+    if cache and cache.src == state.index then
+        return cache.lines
+    end
+    local lines = highlight_index(buf, state.index)
+    state.index_hl = { src = state.index, lines = lines }
+    return lines
+end
+
 ---@param buf integer
 local function render_overlay(buf)
     if not vim.api.nvim_buf_is_valid(buf) then
@@ -178,6 +276,7 @@ local function render_overlay(buf)
         return
     end
     local line_count = vim.api.nvim_buf_line_count(buf)
+    local hl_lines = index_spans(state, buf)
     for _, h in ipairs(state.hunks) do
         if h.type ~= "delete" then
             for r = h.new_start, h.new_start + h.new_count - 1 do
@@ -211,7 +310,7 @@ local function render_overlay(buf)
                 row, above = math.max(h.new_start - 1, 0), true
             end
             pcall(vim.api.nvim_buf_set_extmark, buf, NS_OVERLAY, row, 0, {
-                virt_lines = delete_virt_lines(h),
+                virt_lines = delete_virt_lines(h, hl_lines),
                 virt_lines_above = above,
                 right_gravity = false,
                 invalidate = true,