use -$SHELL rather than $SHELL -l to trigger a login shell

Since the original code was written, Rust grew and stabilized
an interface for specifying argv0 for the child process, so
we can remove the fragile `-l` argument passing.

refs: https://github.com/wez/wezterm/issues/753

docs/changelog.md

@@ -51,6 +51,7 @@ As features stabilize some brief notes about them will accumulate here.
 * Fixed: Numpad Enter reported as CTRL-C on macOS [#739](https://github.com/wez/wezterm/issues/739)
 * Fixed: mouse reporting gbutton state not cleared when focus is lost, eg: from clicking a link [#744](https://github.com/wez/wezterm/issues/744)
 * Improved: better looking curly underline. Thanks to [@ModProg](https://github.com/ModProg)! [#733](https://github.com/wez/wezterm/pull/733)
+* Fixed: wezterm now sets argv0 to `-$SHELL` to invoke a login shell, rather than running `$SHELL -l`. [#753](https://github.com/wez/wezterm/issues/753)
 
 ### 20210405-110924-a5bb5be8
 

docs/config/launch.md

@@ -14,6 +14,9 @@ Your shell is determined by the following rules:
 a login shell.  A login shell generally loads additional startup files
 and sets up more environment than a non-login shell.
 
+*Since: nightly builds*: instead of passing `-l` to the shell, wezterm
+will spawn the shell as `-$SHELL` to invoke it as a login shell.
+
 Note: if you have recently changed your shell using `chsh` and you
 have `$SHELL` set in the environment, you will need to sign out and
 sign back in again for the environment to pick up your new `$SHELL`

pty/src/cmdbuilder.rs

@@ -131,14 +131,18 @@ impl CommandBuilder {
 
     /// Convert the CommandBuilder to a `std::process::Command` instance.
     pub(crate) fn as_command(&self) -> anyhow::Result<std::process::Command> {
+        use std::os::unix::process::CommandExt;
+
         let mut cmd = if self.is_default_prog() {
-            let mut cmd = std::process::Command::new(&Self::get_shell()?);
-            // Run the shell as a login shell.  This is a little shaky; it just
-            // happens to be the case that bash, zsh, fish and tcsh use -l
-            // to indicate that they are login shells.  Ideally we'd just
-            // tell the command builder to prefix argv[0] with a `-`, but
-            // Rust doesn't support that.
-            cmd.arg("-l");
+            let shell = Self::get_shell()?;
+
+            let mut cmd = std::process::Command::new(&shell);
+
+            // Run the shell as a login shell by prefixing the shell's
+            // basename with `-` and setting that as argv0
+            let basename = shell.rsplit('/').next().unwrap_or(&shell);
+            cmd.arg0(&format!("-{}", basename));
+
             let home = Self::get_home_dir()?;
             let dir: &OsStr = self
                 .cwd