feat(diagnostic): fromqflist({merge_lines}) #37416

Problem:
`vim.diagnostic.fromqflist` ignores lines that are `item.valid == 0` (see
`getqflist`). Many qflists have messages that span multiple lines, which look
like this:

    collection/src/Modelling/CdOd/Central.hs|496 col 80| error: [GHC-83865]
    ||     • Couldn't match expected type: InstanceWithForm
    ||                                       (FilePath
    ||                                        -> SelectValidCdInstWithForm
    ...

calling `vim.diagnostic.fromqflist(vim.fn.getqflist)` gets a diagnostic message
like this:

    error: [GHC-83865]

only the first line is kept, but often, the remaing lines are useful as well.

Solution:
Introduce `merge_lines` option, which "squashes" lines from invalid qflist items
into the error message of the previous valid item, so that we get this
diagnostic message instead:

    error: [GHC-83865]
         • Couldn't match expected type: InstanceWithForm
                                           (FilePath
                                            -> SelectValidCdInstWithForm

runtime/doc/diagnostic.txt

@@ -802,12 +802,16 @@ enable({enable}, {filter})                           *vim.diagnostic.enable()*
                   • {bufnr}? (`integer`) Buffer number, or 0 for current
                     buffer, or `nil` for all buffers.
 
-fromqflist({list})                               *vim.diagnostic.fromqflist()*
+fromqflist({list}, {opts})                       *vim.diagnostic.fromqflist()*
     Convert a list of quickfix items to a list of diagnostics.
 
     Parameters: ~
-      • {list}  (`table[]`) List of quickfix items from |getqflist()| or
+      • {list}  (`vim.quickfix.entry[]`) List of quickfix items from
-                |getloclist()|.
+                |getqflist()| or |getloclist()|.
+      • {opts}  (`table?`) Configuration table with the following keys:
+                • {merge_lines}? (`boolean`) When true, items with valid=0 are
+                  appended to the previous valid item's message with a
+                  newline. (default: false)
 
     Return: ~
         (`vim.Diagnostic[]`) See |vim.Diagnostic|.

runtime/doc/news.txt

@@ -198,6 +198,8 @@ DIAGNOSTICS
   enabled or disabled diagnostics.
 • |vim.diagnostic.status()| returns a formatted string with current buffer
   diagnostics
+• |vim.diagnostic.fromqflist()| now accepts an `opts` table with
+  `merge_lines` to merge multi-line compiler messages.
 
 EDITOR
 

runtime/lua/vim/diagnostic.lua

@@ -2913,14 +2913,27 @@ function M.toqflist(diagnostics)
   return list
 end
 
+--- Configuration table with the following keys:
+--- @class vim.diagnostic.fromqflist.Opts
+--- @inlinedoc
+---
+--- When true, items with valid=0 are appended to the previous valid item's
+--- message with a newline. (default: false)
+--- @field merge_lines? boolean
+
 --- Convert a list of quickfix items to a list of diagnostics.
 ---
----@param list table[] List of quickfix items from |getqflist()| or |getloclist()|.
+---@param list vim.quickfix.entry[] List of quickfix items from |getqflist()| or |getloclist()|.
+---@param opts? vim.diagnostic.fromqflist.Opts
 ---@return vim.Diagnostic[]
-function M.fromqflist(list)
+function M.fromqflist(list, opts)
   vim.validate('list', list, 'table')
 
+  opts = opts or {}
+  local merge = opts.merge_lines
+
   local diagnostics = {} --- @type vim.Diagnostic[]
+  local last_diag --- @type vim.Diagnostic?
   for _, item in ipairs(list) do
     if item.valid == 1 then
       local lnum = math.max(0, item.lnum - 1)
@@ -2929,7 +2942,7 @@ function M.fromqflist(list)
       local end_col = item.end_col > 0 and (item.end_col - 1) or col
       local code = item.nr > 0 and item.nr or nil
       local severity = item.type ~= '' and M.severity[item.type:upper()] or M.severity.ERROR
-      diagnostics[#diagnostics + 1] = {
+      local diag = {
         bufnr = item.bufnr,
         lnum = lnum,
         col = col,
@@ -2939,6 +2952,10 @@ function M.fromqflist(list)
         message = item.text,
         code = code,
       }
+      diagnostics[#diagnostics + 1] = diag
+      last_diag = diag
+    elseif merge and last_diag then
+      last_diag.message = last_diag.message .. '\n' .. item.text
     end
   end
   return diagnostics

test/functional/lua/diagnostic_spec.lua

@@ -4089,6 +4089,98 @@ describe('vim.diagnostic', function()
       end)
       eq(result[1], result[2])
     end)
+
+    it('merge_lines=true merges continuation lines', function()
+      local result = exec_lua(function()
+        local qflist = {
+          {
+            bufnr = 1,
+            lnum = 10,
+            col = 5,
+            end_lnum = 10,
+            end_col = 10,
+            text = 'error: [GHC-83865]',
+            type = 'E',
+            nr = 0,
+            valid = 1,
+          },
+          {
+            bufnr = 1,
+            lnum = 0,
+            col = 0,
+            end_lnum = 0,
+            end_col = 0,
+            text = "    Couldn't match expected type",
+            type = '',
+            nr = 0,
+            valid = 0,
+          },
+          {
+            bufnr = 1,
+            lnum = 0,
+            col = 0,
+            end_lnum = 0,
+            end_col = 0,
+            text = '    with actual type',
+            type = '',
+            nr = 0,
+            valid = 0,
+          },
+          {
+            bufnr = 1,
+            lnum = 20,
+            col = 1,
+            end_lnum = 20,
+            end_col = 5,
+            text = 'warning: unused',
+            type = 'W',
+            nr = 0,
+            valid = 1,
+          },
+        }
+        return vim.diagnostic.fromqflist(qflist, { merge_lines = true })
+      end)
+
+      eq(2, #result)
+      eq(
+        "error: [GHC-83865]\n    Couldn't match expected type\n    with actual type",
+        result[1].message
+      )
+      eq('warning: unused', result[2].message)
+    end)
+
+    it('merge_lines=false ignores continuation lines', function()
+      local result = exec_lua(function()
+        local qflist = {
+          {
+            bufnr = 1,
+            lnum = 10,
+            col = 5,
+            end_lnum = 10,
+            end_col = 10,
+            text = 'error: main',
+            type = 'E',
+            nr = 0,
+            valid = 1,
+          },
+          {
+            bufnr = 1,
+            lnum = 0,
+            col = 0,
+            end_lnum = 0,
+            end_col = 0,
+            text = 'continuation',
+            type = '',
+            nr = 0,
+            valid = 0,
+          },
+        }
+        return vim.diagnostic.fromqflist(qflist)
+      end)
+
+      eq(1, #result)
+      eq('error: main', result[1].message)
+    end)
   end)
 
   describe('status()', function()