fix(treesitter): don't add final newline if not present #35970

**Problem(?):** Buffers that (for whatever reason) aren't meant to have
a final newline are still parsed with a final newline in `treesitter.c`.

**Solution:** Don't add the newline to the last buffer line if it
shouldn't be there. (This more closely matches the approach of
`read_buffer_into()`.)

This allows us to, say, use a scratch buffer with `noeol` and `nofixeol`
behind the scenes in `get_string_parser()`.

...which would allow us to track injection trees with extmarks in that
case.

...which would allow us to not drop previous trees after reparsing a
different range with `get_parser():parse()`.

...which would prevent flickering when editing a buffer that has 2+
windows to it in view at a time.

...which would allow us to keep our sanity!!!

(one step at a time...)

src/nvim/lua/treesitter.c

@@ -443,8 +443,9 @@ static const char *input_cb(void *payload, uint32_t byte_index, TSPoint position
     *bytes_read = 0;
     return "";
   }
-  char *line = ml_get_buf(bp, (linenr_T)position.row + 1);
-  size_t len = (size_t)ml_get_buf_len(bp, (linenr_T)position.row + 1);
+  linenr_T lnum = (linenr_T)position.row + 1;
+  char *line = ml_get_buf(bp, lnum);
+  size_t len = (size_t)ml_get_buf_len(bp, lnum);
   if (position.column > len) {
     *bytes_read = 0;
     return "";
@@ -456,10 +457,13 @@ static const char *input_cb(void *payload, uint32_t byte_index, TSPoint position
   memchrsub(buf, '\n', NUL, tocopy);
   *bytes_read = (uint32_t)tocopy;
   if (tocopy < BUFSIZE) {
-    // now add the final \n. If it didn't fit, input_cb will be called again
-    // on the same line with advanced column.
-    buf[tocopy] = '\n';
-    (*bytes_read)++;
+    // now add the final \n, if it is meant to be present for this buffer. If it didn't fit,
+    // input_cb will be called again on the same line with advanced column.
+    if (lnum != bp->b_ml.ml_line_count || (!bp->b_p_bin && bp->b_p_fixeol)
+        || (lnum != bp->b_no_eol_lnum && bp->b_p_eol)) {
+      buf[tocopy] = '\n';
+      (*bytes_read)++;
+    }
   }
   return buf;
 #undef BUFSIZE

test/functional/treesitter/parser_spec.lua

@@ -91,6 +91,60 @@ describe('treesitter parser API', function()
     eq(true, exec_lua('return parser:parse()[1] == tree2'))
   end)
 
+  it('respects eol settings when parsing buffer', function()
+    insert([[
+      int main() {
+        int x = 3;
+      } // :D]])
+
+    exec_lua(function()
+      vim.bo.eol = false
+      vim.bo.fixeol = false
+      _G.parser = vim.treesitter.get_parser(0, 'c')
+      _G.tree = _G.parser:parse()[1]
+      _G.root = _G.tree:root()
+      _G.lang = vim.treesitter.language.inspect('c')
+    end)
+
+    eq(
+      '<node translation_unit>',
+      exec_lua(function()
+        return tostring(_G.root)
+      end)
+    )
+    eq(
+      { 0, 0, 0, 2, 7, 33 },
+      exec_lua(function()
+        return { _G.root:range(true) }
+      end)
+    )
+
+    -- NOTE: Changing these settings marks the buffer as `modified` but does not fire `on_bytes`,
+    -- meaning this test case does not pass... is this intended?
+    -- exec_lua(function()
+    --   vim.bo.eol = true
+    --   vim.bo.fixeol = true
+    --   vim.cmd.update()
+    --   _G.parser = vim.treesitter.get_parser(0, 'c')
+    --   _G.tree = _G.parser:parse()[1]
+    --   _G.root = _G.tree:root()
+    --   _G.lang = vim.treesitter.language.inspect('c')
+    -- end)
+    --
+    -- eq(
+    --   '<node translation_unit>',
+    --   exec_lua(function()
+    --     return tostring(root)
+    --   end)
+    -- )
+    -- eq(
+    --   { 0, 0, 0, 3, 0, 34 },
+    --   exec_lua(function()
+    --     return { root:range(true) }
+    --   end)
+    -- )
+  end)
+
   it('parses buffer asynchronously', function()
     insert([[
       int main() {