cmdline: CTRL-R: <Space> instead of CR between lines.

^M isn't any more "correct" than space: the "technically correct"
interpretation is to execute the first line that is seen (and this is
what happens on middle-click paste in Vim). ^M is only intended to
defuse the newline, so that the user can review the command. We can do
that with a space instead, and then the command can be executed without
having to fix it up first.

runtime/doc/vim_diff.txt

@@ -238,6 +238,9 @@ newly allocated memory all over the place) and fail on types which cannot be
 coerced to strings. See |id()| for more details, currently it uses 
 `printf("%p", {expr})` internally.
 
+|c_CTRL-R| pasting a non-special register into the |cmdline| separates lines
+by <Space> instead of <CR>.
+
 ==============================================================================
 5. Missing legacy features				 *nvim-features-missing*
 				     *if_lua* *if_perl* *if_mzscheme* *if_tcl*

src/nvim/ex_getln.c

@@ -2471,10 +2471,10 @@ void restore_cmdline_alloc(char_u *p)
 ///
 /// @param regname   Register name.
 /// @param literally Insert text literally instead of "as typed".
-/// @param remcr     When true, remove trailing CR.
+/// @param remspc    When true, remove trailing <Space>.
 ///
 /// @returns FAIL for failure, OK otherwise
-static bool cmdline_paste(int regname, bool literally, bool remcr)
+static bool cmdline_paste(int regname, bool literally, bool remspc)
 {
   long i;
   char_u              *arg;
@@ -2539,7 +2539,7 @@ static bool cmdline_paste(int regname, bool literally, bool remcr)
     return OK;
   }
 
-  return cmdline_paste_reg(regname, literally, remcr);
+  return cmdline_paste_reg(regname, literally, remspc);
 }
 
 /*

src/nvim/ops.c

@@ -1260,16 +1260,16 @@ int get_spec_reg(
 
 /// Paste a yank register into the command line.
 /// Only for non-special registers.
-/// Used by CTRL-R command in command-line mode
+/// Used by CTRL-R in command-line mode.
 /// insert_reg() can't be used here, because special characters from the
 /// register contents will be interpreted as commands.
 ///
 /// @param regname   Register name.
 /// @param literally Insert text literally instead of "as typed".
-/// @param remcr     When true, don't add CR characters.
+/// @param remspc    When true, don't add <Space> characters.
 ///
 /// @returns FAIL for failure, OK otherwise
-bool cmdline_paste_reg(int regname, bool literally, bool remcr)
+bool cmdline_paste_reg(int regname, bool literally, bool remspc)
 {
   yankreg_T *reg = get_yank_register(regname, YREG_PASTE);
   if (reg->y_array == NULL)
@@ -1278,10 +1278,9 @@ bool cmdline_paste_reg(int regname, bool literally, bool remcr)
   for (size_t i = 0; i < reg->y_size; i++) {
     cmdline_paste_str(reg->y_array[i], literally);
 
-    // Insert ^M between lines and after last line if type is kMTLineWise.
+    // Insert space between lines, unless `remspc` is true.
-    // Don't do this when "remcr" is true.
+    if (i < reg->y_size - 1 && !remspc) {
-    if ((reg->y_type == kMTLineWise || i < reg->y_size - 1) && !remcr) {
+      cmdline_paste_str((char_u *)" ", literally);
-      cmdline_paste_str((char_u *)"\r", literally);
     }
 
     /* Check for CTRL-C, in case someone tries to paste a few thousand

test/functional/cmdline/ctrl_r_spec.lua

@@ -0,0 +1,34 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, insert, funcs, eq, feed =
+  helpers.clear, helpers.insert, helpers.funcs, helpers.eq, helpers.feed
+
+describe('cmdline CTRL-R', function()
+  before_each(clear)
+
+  it('pasting non-special register inserts <Space> between lines', function()
+    insert([[
+    line1abc
+    line2somemoretext
+    ]])
+    -- Yank 2 lines linewise, then paste to cmdline.
+    feed([[<C-\><C-N>gg0yj:<C-R>0]])
+    -- <Space> inserted *between* lines, not after the final line.
+    eq('line1abc line2somemoretext', funcs.getcmdline())
+
+    -- Yank 2 lines characterwise, then paste to cmdline.
+    feed([[<C-\><C-N>gg05lyvj:<C-R>0]])
+    -- <Space> inserted *between* lines, not after the final line.
+    eq('abc line2', funcs.getcmdline())
+
+    -- Yank 1 line linewise, then paste to cmdline.
+    feed([[<C-\><C-N>ggyy:<C-R>0]])
+    -- No spaces inserted.
+    eq('line1abc', funcs.getcmdline())
+  end)
+
+  it('pasting special register inserts <CR>, <NL>', function()
+    feed([[:<C-R>="foo\nbar\rbaz"<CR>]])
+    eq('foo\nbar\rbaz', funcs.getcmdline())
+  end)
+end)
+