vim-patch:8.2.0997: cannot execute a register containing line continuation

Problem:    Cannot execute a register containing line continuation.
Solution:   Concatenate lines where needed. (Yegappan Lakshmanan,
            closes vim/vim#6272)
856c1110c1

According to #2542 the "Future:" part was removed intentionally.
Use size_t in more places to reduce type casts.

runtime/doc/repeat.txt

@@ -161,6 +161,11 @@ Q			Repeat the last recorded register [count] times.
 			result of evaluating the expression is executed as an
 			Ex command.
 			Mappings are not recognized in these commands.
+			When the |line-continuation| character (\) is present
+			at the beginning of a line in a linewise register,
+			then it is combined with the previous line. This is
+			useful for yanking and executing parts of a Vim
+			script.
 
 							*:@:*
 :[addr]@:		Repeat last command-line.  First set cursor at line

src/nvim/ops.c

@@ -1023,6 +1023,60 @@ static int stuff_yank(int regname, char_u *p)
 
 static int execreg_lastc = NUL;
 
+/// When executing a register as a series of ex-commands, if the
+/// line-continuation character is used for a line, then join it with one or
+/// more previous lines. Note that lines are processed backwards starting from
+/// the last line in the register.
+///
+/// @param lines list of lines in the register
+/// @param idx   index of the line starting with \ or "\. Join this line with all the immediate
+///              predecessor lines that start with a \ and the first line that doesn't start
+///              with a \. Lines that start with a comment "\ character are ignored.
+/// @returns the concatenated line. The index of the line that should be
+///          processed next is returned in idx.
+static char_u *execreg_line_continuation(char_u **lines, size_t *idx)
+{
+  size_t i = *idx;
+  assert(i > 0);
+  const size_t cmd_end = i;
+
+  garray_T ga;
+  ga_init(&ga, (int)sizeof(char_u), 400);
+
+  char_u *p;
+
+  // search backwards to find the first line of this command.
+  // Any line not starting with \ or "\ is the start of the
+  // command.
+  while (--i > 0) {
+    p = skipwhite(lines[i]);
+    if (*p != '\\' && (p[0] != '"' || p[1] != '\\' || p[2] != ' ')) {
+      break;
+    }
+  }
+  const size_t cmd_start = i;
+
+  // join all the lines
+  ga_concat(&ga, (char *)lines[cmd_start]);
+  for (size_t j = cmd_start + 1; j <= cmd_end; j++) {
+    p = skipwhite(lines[j]);
+    if (*p == '\\') {
+      // Adjust the growsize to the current length to
+      // speed up concatenating many lines.
+      if (ga.ga_len > 400) {
+        ga_set_growsize(&ga, MIN(ga.ga_len, 8000));
+      }
+      ga_concat(&ga, (char *)(p + 1));
+    }
+  }
+  ga_append(&ga, NUL);
+  char_u *str = vim_strsave(ga.ga_data);
+  ga_clear(&ga);
+
+  *idx = i;
+  return str;
+}
+
 /// Execute a yank register: copy it into the stuff buffer
 ///
 /// @param colon   insert ':' before each line
@@ -1111,7 +1165,21 @@ int do_execreg(int regname, int colon, int addcr, int silent)
           return FAIL;
         }
       }
-      escaped = vim_strsave_escape_ks(reg->y_array[i]);
+
+      // Handle line-continuation for :@<register>
+      char_u *str = reg->y_array[i];
+      bool free_str = false;
+      if (colon && i > 0) {
+        p = skipwhite(str);
+        if (*p == '\\' || (p[0] == '"' && p[1] == '\\' && p[2] == ' ')) {
+          str = execreg_line_continuation(reg->y_array, &i);
+          free_str = true;
+        }
+      }
+      escaped = vim_strsave_escape_ks(str);
+      if (free_str) {
+        xfree(str);
+      }
       retval = ins_typebuf(escaped, remap, 0, true, silent);
       xfree(escaped);
       if (retval == FAIL) {

src/nvim/testdir/test_registers.vim

@@ -482,6 +482,82 @@ func Test_v_register()
   bwipe!
 endfunc
 
+" Test for executing the contents of a register as an Ex command with line
+" continuation.
+func Test_execute_reg_as_ex_cmd()
+  " Line continuation with just two lines
+  let code =<< trim END
+    let l = [
+      \ 1]
+  END
+  let @r = code->join("\n")
+  let l = []
+  @r
+  call assert_equal([1], l)
+
+  " Line continuation with more than two lines
+  let code =<< trim END
+    let l = [
+      \ 1,
+      \ 2,
+      \ 3]
+  END
+  let @r = code->join("\n")
+  let l = []
+  @r
+  call assert_equal([1, 2, 3], l)
+
+  " use comments interspersed with code
+  let code =<< trim END
+    let l = [
+      "\ one
+      \ 1,
+      "\ two
+      \ 2,
+      "\ three
+      \ 3]
+  END
+  let @r = code->join("\n")
+  let l = []
+  @r
+  call assert_equal([1, 2, 3], l)
+
+  " use line continuation in the middle
+  let code =<< trim END
+    let a = "one"
+    let l = [
+      \ 1,
+      \ 2]
+    let b = "two"
+  END
+  let @r = code->join("\n")
+  let l = []
+  @r
+  call assert_equal([1, 2], l)
+  call assert_equal("one", a)
+  call assert_equal("two", b)
+
+  " only one line with a \
+  let @r = "\\let l = 1"
+  call assert_fails('@r', 'E10:')
+
+  " only one line with a "\
+  let @r = '   "\ let i = 1'
+  @r
+  call assert_false(exists('i'))
+
+  " first line also begins with a \
+  let @r = "\\let l = [\n\\ 1]"
+  call assert_fails('@r', 'E10:')
+
+  " Test with a large number of lines
+  let @r = "let str = \n"
+  let @r ..= repeat("  \\ 'abcdefghijklmnopqrstuvwxyz' ..\n", 312)
+  let @r ..= '  \ ""'
+  @r
+  call assert_equal(repeat('abcdefghijklmnopqrstuvwxyz', 312), str)
+endfunc
+
 func Test_ve_blockpaste()
   new
   set ve=all