feat(main): expand file ~\ or ~/ prefix on Windows

In command_line_scan() for MSWIN, expand "~\" or "~/" prefixed paths to
the USERPROFILE environment variable for the user's profile directory.

Rename the static os_homedir() to os_uv_homedir() to emphasize that it
is a wrapper around a libuv function.

Add the function os_get_homedir() to os/env.c to return the cached
homedir value as a const. Must be called after homedir is initialized or
it fails.

The difference between this function and the static os_uv_homedir() is
that the latter gets the homedir from libuv and is used to initialize
homedir in init_homedir(), while os_get_homedir() just returns homedir
as a const if it's initialized and is public.

Use the os_get_homedir() accessor for ~/ expansion on Windows to make
the code more concise.

Add a Windows section to main_spec.lua with tests for expanding ~/ and
~\ prefixes for files passed in on the command-line.

Fix #23901

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

src/nvim/main.c

@@ -1442,6 +1442,17 @@ scripterror:
       ga_grow(&global_alist.al_ga, 1);
       char *p = xstrdup(argv[0]);
 
+      // On Windows expand "~\" or "~/" prefix in file names to profile directory.
+#ifdef MSWIN
+      if (*p == '~' && (p[1] == '\\' || p[1] == '/')) {
+        size_t size = strlen(os_get_homedir()) + strlen(p);
+        char *tilde_expanded = xmalloc(size);
+        snprintf(tilde_expanded, size, "%s%s", os_get_homedir(), p + 1);
+        xfree(p);
+        p = tilde_expanded;
+      }
+#endif
+
       if (parmp->diff_mode && os_isdir(p) && GARGCOUNT > 0
           && !os_isdir(alist_name(&GARGLIST[0]))) {
         char *r = concat_fnames(p, path_tail(alist_name(&GARGLIST[0])), true);

src/nvim/os/env.c

@@ -400,17 +400,28 @@ void os_get_hostname(char *hostname, size_t size)
 ///   2. if $HOME is not set, try the following
 /// For Windows:
 ///   1. assemble homedir using HOMEDRIVE and HOMEPATH
-///   2. try os_homedir()
+///   2. try os_uv_homedir()
 ///   3. resolve a direct reference to another system variable
 ///   4. guess C drive
 /// For Unix:
-///   1. try os_homedir()
+///   1. try os_uv_homedir()
 ///   2. go to that directory
 ///     This also works with mounts and links.
 ///     Don't do this for Windows, it will change the "current dir" for a drive.
 ///   3. fall back to current working directory as a last resort
 static char *homedir = NULL;
-static char *os_homedir(void);
+static char *os_uv_homedir(void);
+
+/// Public accessor for the cached "real", resolved user home directory. See
+/// comment on `homedir`.
+const char *os_get_homedir(void)
+{
+  if (!homedir) {
+    emsg("os_get_homedir failed: homedir not initialized");
+    return NULL;
+  }
+  return homedir;
+}
 
 void init_homedir(void)
 {
@@ -440,7 +451,7 @@ void init_homedir(void)
     }
   }
   if (var == NULL) {
-    var = os_homedir();
+    var = os_uv_homedir();
   }
 
   // Weird but true: $HOME may contain an indirect reference to another
@@ -471,7 +482,7 @@ void init_homedir(void)
 
 #ifdef UNIX
   if (var == NULL) {
-    var = os_homedir();
+    var = os_uv_homedir();
   }
 
   // Get the actual path.  This resolves links.
@@ -492,7 +503,7 @@ void init_homedir(void)
 
 static char homedir_buf[MAXPATHL];
 
-static char *os_homedir(void)
+static char *os_uv_homedir(void)
 {
   homedir_buf[0] = NUL;
   size_t homedir_size = MAXPATHL;

test/functional/core/main_spec.lua

@@ -193,4 +193,26 @@ describe('command-line option', function()
     matches('Run "nvim %-V1 %-v"', fn.system({ nvim_prog_abs(), '-v' }))
     matches('Compilation: .*Run :checkhealth', fn.system({ nvim_prog_abs(), '-V1', '-v' }))
   end)
+
+  if is_os('win') then
+    for _, prefix in ipairs({ '~/', '~\\' }) do
+      it('expands ' .. prefix .. ' on Windows', function()
+        local fname = os.getenv('USERPROFILE') .. '\\nvim_test.txt'
+        finally(function()
+          os.remove(fname)
+        end)
+        write_file(fname, 'some text')
+        eq(
+          'some text',
+          fn.system({
+            nvim_prog_abs(),
+            '-es',
+            '+%print',
+            '+q',
+            prefix .. 'nvim_test.txt',
+          }):gsub('\n', '')
+        )
+      end)
+    end
+  end
 end)