fix(diagnostic): always return copies of diagnostic items (#25010)

runtime/doc/diagnostic.txt

@@ -480,6 +480,9 @@ fromqflist({list})                               *vim.diagnostic.fromqflist()*
 get({bufnr}, {opts})                                    *vim.diagnostic.get()*
     Get current diagnostics.
 
+    Modifying diagnostics in the returned table has no effect. To set
+    diagnostics in a buffer, use |vim.diagnostic.set()|.
+
     Parameters: ~
       • {bufnr}  (integer|nil) Buffer number to get diagnostics from. Use 0
                  for current buffer or nil for all buffers.

runtime/lua/vim/diagnostic.lua

@@ -386,6 +386,7 @@ local function get_diagnostics(bufnr, opts, clamp)
 
   local function add(b, d)
     if not opts.lnum or d.lnum == opts.lnum then
+      d = vim.deepcopy(d)
       if clamp and api.nvim_buf_is_loaded(b) then
         local line_count = buf_line_count[b] - 1
         if
@@ -396,7 +397,6 @@ local function get_diagnostics(bufnr, opts, clamp)
           or d.col < 0
           or d.end_col < 0
         then
-          d = vim.deepcopy(d)
           d.lnum = math.max(math.min(d.lnum, line_count), 0)
           d.end_lnum = math.max(math.min(d.end_lnum, line_count), 0)
           d.col = math.max(d.col, 0)
@@ -750,6 +750,8 @@ end
 
 --- Get current diagnostics.
 ---
+--- Modifying diagnostics in the returned table has no effect. To set diagnostics in a buffer, use |vim.diagnostic.set()|.
+---
 ---@param bufnr integer|nil Buffer number to get diagnostics from. Use 0 for
 ---                        current buffer or nil for all buffers.
 ---@param opts table|nil A table with the following keys:

test/functional/lua/diagnostic_spec.lua

@@ -181,6 +181,18 @@ describe('vim.diagnostic', function()
     eq(0, #diagnostics)
   end)
 
+  it('always returns a copy of diagnostic tables', function()
+    local result = exec_lua [[
+      vim.diagnostic.set(diagnostic_ns, diagnostic_bufnr, {
+        make_error('Diagnostic #1', 1, 1, 1, 1),
+      })
+      local diag = vim.diagnostic.get()
+      diag[1].col = 10000
+      return vim.diagnostic.get()[1].col == 10000
+    ]]
+    eq(result, false)
+  end)
+
   it('resolves buffer number 0 to the current buffer', function()
     eq(2, exec_lua [[
       vim.api.nvim_set_current_buf(diagnostic_bufnr)