auth-api/tfa: prefer Display over ToString/an inherent function

this fixes two clippy lints that check if either `ToString` or an
inherent `to_string()` function is implement [1, 2]. `Display`
provides `ToString` for free and, thus, is preferable.

[1]:
https://rust-lang.github.io/rust-clippy/master/index.html#/inherent_to_string
[2]:
https://rust-lang.github.io/rust-clippy/master/index.html#to_string_trait_impl

Signed-off-by: Shannon Sterz <s.sterz@proxmox.com>

proxmox-auth-api/src/ticket.rs

@@ -1,6 +1,7 @@
 //! Generate and verify Authentication tickets
 
 use std::borrow::Cow;
+use std::fmt::Display;
 use std::marker::PhantomData;
 
 use anyhow::{bail, format_err, Error};
@@ -14,13 +15,13 @@ use crate::TICKET_LIFETIME;
 /// Stringified ticket data must not contain colons...
 const TICKET_ASCIISET: &AsciiSet = &percent_encoding::CONTROLS.add(b':');
 
-/// An empty type implementing [`ToString`] and [`FromStr`](std::str::FromStr), used for tickets
+/// An empty type implementing [`Display`] and [`FromStr`](std::str::FromStr), used for tickets
 /// with no data.
 pub struct Empty;
 
-impl ToString for Empty {
-    fn to_string(&self) -> String {
-        String::new()
+impl Display for Empty {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "")
     }
 }
 

proxmox-tfa/src/api/webauthn.rs

@@ -1,5 +1,7 @@
 //! Webauthn configuration and challenge data.
 
+use std::fmt::Display;
+
 use anyhow::{format_err, Error};
 use serde::{Deserialize, Serialize};
 use url::Url;
@@ -42,9 +44,9 @@ impl From<OriginUrl> for String {
     }
 }
 
-impl OriginUrl {
-    fn to_string(&self) -> String {
-        self.0.origin().ascii_serialization()
+impl Display for OriginUrl {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0.origin().ascii_serialization())
     }
 }
 
@@ -91,7 +93,7 @@ impl WebauthnConfig {
     pub fn digest(&self) -> [u8; 32] {
         let mut data = format!("rp={:?}\nid={:?}\n", self.rp, self.id,);
         if let Some(origin) = &self.origin {
-            data.push_str(&format!("origin={}\n", origin.to_string()));
+            data.push_str(&format!("origin={}\n", origin));
         }
         openssl::sha::sha256(data.as_bytes())
     }