add support for RESTIC_PASSWORD env variable

The restic tool doesn't have a CLI parameter for this, but it allows
setting an env variable. We can check for that too and keep this as an
env variable only.

This way we keep the safety of knowing that all necessary parameters
(i.e. the repo and password options) are set.

src/args.rs

@@ -30,6 +30,8 @@ impl Args {
                 Password::Command(command)
             } else if let Some(file) = cli.password_file {
                 Password::File(file)
+            } else if let Some(str) = cli.restic_password {
+                Password::Plain(str)
             } else {
                 unreachable!("Error in Config: neither password_command nor password_file found. Please open an issue if you see this.")
             },
@@ -84,7 +86,7 @@ impl Args {
 #[command(group(
     ArgGroup::new("password")
         .required(true)
-        .args(["password_command", "password_file"]),
+        .args(["password_command", "password_file", "restic_password"]),
 ))]
 struct Cli {
     #[arg(short = 'r', long, env = "RESTIC_REPOSITORY")]
@@ -99,6 +101,9 @@ struct Cli {
     #[arg(long, value_name = "FILE", env = "RESTIC_PASSWORD_FILE")]
     password_file: Option<String>,
 
+    #[arg(value_name = "RESTIC_PASSWORD", env = "RESTIC_PASSWORD")]
+    restic_password: Option<String>,
+
     ///  How many restic subprocesses to spawn concurrently.
     ///
     /// If you get ssh-related errors or too much memory use try lowering this.

src/restic.rs

@@ -110,6 +110,8 @@ pub enum Repository {
 
 #[derive(Debug)]
 pub enum Password {
+    /// A plain string (restic: RESTIC_PASSWORD env variable)
+    Plain(String),
     /// A password command (restic: --password-command)
     Command(String),
     /// A password file (restic: --password-file)
@@ -216,6 +218,8 @@ impl Restic {
             Password::Command(command) =>
                 cmd.arg("--password-command").arg(command),
             Password::File(file) => cmd.arg("--password-file").arg(file),
+            // Nothing to do, the password is given as an env variable already.
+            Password::Plain(_str) => &cmd,
         };
         if self.no_cache {
             cmd.arg("--no-cache");