feat(lua): implement Iter:join() (#26416)

runtime/doc/lua.txt

@@ -3410,6 +3410,19 @@ Iter:fold({init}, {f})                                           *Iter:fold()*
     Return: ~
         any
 
+Iter:join({delim})                                               *Iter:join()*
+    Collect the iterator into a delimited string.
+
+    Each element in the iterator is joined into a string separated by {delim}.
+
+    Consumes the iterator.
+
+    Parameters: ~
+      • {delim}  (string) Delimiter
+
+    Return: ~
+        (string)
+
 Iter:last()                                                      *Iter:last()*
     Drains the iterator and returns the last item.
 

runtime/lua/vim/iter.lua

@@ -356,6 +356,18 @@ function ListIter.totable(self)
   return self._table
 end
 
+--- Collect the iterator into a delimited string.
+---
+--- Each element in the iterator is joined into a string separated by {delim}.
+---
+--- Consumes the iterator.
+---
+--- @param delim string Delimiter
+--- @return string
+function Iter.join(self, delim)
+  return table.concat(self:totable(), delim)
+end
+
 --- Folds ("reduces") an iterator into a single value.
 ---
 --- Examples:

test/functional/lua/iter_spec.lua

@@ -91,6 +91,11 @@ describe('vim.iter', function()
     end
   end)
 
+  it('join()', function()
+    eq('1, 2, 3', vim.iter({1, 2, 3}):join(', '))
+    eq('a|b|c|d', vim.iter(vim.gsplit('a|b|c|d', '|')):join('|'))
+  end)
+
   it('next()', function()
     local it = vim.iter({1, 2, 3}):map(function(v) return 2 * v end)
     eq(2, it:next())