tui: dump termcap info if -V3 ('verbose' >= 3)

Get terminal debugging info by starting Nvim with 'verbose' level 3:

    nvim -V3log

This is like Vim's `:set termcap`, which was removed in Nvim (and would
be very awkward to restore because of the decoupled UI).

2 files changed, 106 insertions, 2 deletions

diff --git a/src/nvim/tui/terminfo.c b/src/nvim/tui/terminfo.c
index fdc33f0a77..492c1c5e9c 100644
--- a/src/nvim/tui/terminfo.c
+++ b/src/nvim/tui/terminfo.c
@@ -9,7 +9,10 @@
 #include <unibilium.h>
 
 #include "nvim/log.h"
+#include "nvim/globals.h"
 #include "nvim/memory.h"
+#include "nvim/message.h"
+#include "nvim/option.h"
 #include "nvim/tui/terminfo.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
@@ -166,3 +169,87 @@ unibi_term *terminfo_from_builtin(const char *term, char **termname)
   unibi_set_bool(ut, unibi_back_color_erase, false);
   return ut;
 }
+
+/// Dumps termcap info to the messages area.
+/// Serves a similar purpose as Vim `:set termcap` (removed in Nvim).
+///
+/// @note adapted from unibilium unibi-dump.c
+void terminfo_info_msg(const unibi_term *const ut)
+{
+  if (exiting) {
+    return;
+  }
+  msg_puts_title("\n\n--- Terminal info --- {{{\n");
+
+  char *term;
+  get_tty_option("term", &term);
+  msg_printf_attr(0, "&term: %s\n", term);
+  msg_printf_attr(0, "Description: %s\n", unibi_get_name(ut));
+  const char **a = unibi_get_aliases(ut);
+  if (*a) {
+    msg_puts("Aliases: ");
+    do {
+      msg_printf_attr(0, "%s%s\n", *a, a[1] ? " | " : "");
+      a++;
+    } while (*a);
+  }
+
+  msg_puts("Boolean capabilities:\n");
+  for (enum unibi_boolean i = unibi_boolean_begin_ + 1;
+       i < unibi_boolean_end_; i++) {
+    msg_printf_attr(0, "  %-25s %-10s = %s\n", unibi_name_bool(i),
+                    unibi_short_name_bool(i),
+                    unibi_get_bool(ut, i) ? "true" : "false");
+  }
+
+  msg_puts("Numeric capabilities:\n");
+  for (enum unibi_numeric i = unibi_numeric_begin_ + 1;
+       i < unibi_numeric_end_; i++) {
+    int n = unibi_get_num(ut, i);  // -1 means "empty"
+    msg_printf_attr(0, "  %-25s %-10s = %hd\n", unibi_name_num(i),
+                    unibi_short_name_num(i), n);
+  }
+
+  msg_puts("String capabilities:\n");
+  for (enum unibi_string i = unibi_string_begin_ + 1;
+       i < unibi_string_end_; i++) {
+    const char *s = unibi_get_str(ut, i);
+    if (s) {
+      msg_printf_attr(0, "  %-25s %-10s = ", unibi_name_str(i),
+                      unibi_short_name_str(i));
+      // Most of these strings will contain escape sequences.
+      msg_outtrans_special((char_u *)s, false);
+      msg_putchar('\n');
+    }
+  }
+
+  if (unibi_count_ext_bool(ut)) {
+    msg_puts("Extended boolean capabilities:\n");
+    for (size_t i = 0; i < unibi_count_ext_bool(ut); i++) {
+      msg_printf_attr(0, "  %-25s = %s\n",
+                      unibi_get_ext_bool_name(ut, i),
+                      unibi_get_ext_bool(ut, i) ? "true" : "false");
+    }
+  }
+
+  if (unibi_count_ext_num(ut)) {
+    msg_puts("Extended numeric capabilities:\n");
+    for (size_t i = 0; i < unibi_count_ext_num(ut); i++) {
+      msg_printf_attr(0, "  %-25s = %hd\n",
+                      unibi_get_ext_num_name(ut, i),
+                      unibi_get_ext_num(ut, i));
+    }
+  }
+
+  if (unibi_count_ext_str(ut)) {
+    msg_puts("Extended string capabilities:\n");
+    for (size_t i = 0; i < unibi_count_ext_str(ut); i++) {
+      msg_printf_attr(0, "  %-25s = ", unibi_get_ext_str_name(ut, i));
+      msg_outtrans_special((char_u *)unibi_get_ext_str(ut, i), false);
+      msg_putchar('\n');
+    }
+  }
+
+  msg_puts("}}}\n");
+  xfree(term);
+}
diff --git a/src/nvim/tui/tui.c b/src/nvim/tui/tui.c
index 2436295ad4..739f3d18d2 100644
--- a/src/nvim/tui/tui.c
+++ b/src/nvim/tui/tui.c
@@ -354,10 +354,12 @@ static void tui_main(UIBridgeData *bridge, UI *ui)
   tui_terminal_start(ui);
   data->stop = false;
 
-  // allow the main thread to continue, we are ready to start handling UI
-  // callbacks
+  // Allow main thread to continue, we are ready to handle UI callbacks.
   CONTINUE(bridge);
 
+  loop_schedule_deferred(&main_loop,
+                         event_create(show_termcap_event, 1, data->ut));
+
   while (!data->stop) {
     loop_poll_events(&tui_loop, -1);  // tui_loop.events is never processed
   }
@@ -1061,6 +1063,21 @@ static void tui_flush(UI *ui)
   flush_buf(ui, true);
 }
 
+/// Dumps termcap info to the messages area, if 'verbose' >= 3.
+static void show_termcap_event(void **argv)
+{
+  if (p_verbose < 3) {
+    return;
+  }
+  const unibi_term *const ut = argv[0];
+  if (!ut) {
+    abort();
+  }
+  // XXX: (future) if unibi_term is modified (e.g. after a terminal
+  // query-response) this is a race condition.
+  terminfo_info_msg(ut);
+}
+
 #ifdef UNIX
 static void suspend_event(void **argv)
 {