Change error returned from start functions to custom enum (#571)

This helps future-proofing the API. Thanks @FSMaxB for
suggesting this in #568

gotham/Cargo.toml

@@ -49,13 +49,13 @@ rand = "0.8"
 rand_chacha = "0.3"
 regex = "1.0"
 serde = { version = "1.0", features = ["derive"] }
+thiserror = "1.0"
 tokio = { version = "1.11.0", features = ["net", "rt-multi-thread", "time", "fs", "io-util"] }
 tokio-rustls = { version = "0.23", optional = true }
 uuid = { version = "0.8", features = ["v4"] }
 
 [dev-dependencies]
 futures-executor = "0.3.14"
-thiserror = "1.0"
 tokio = { version = "1.11.0", features = ["macros", "test-util"] }
 
 [package.metadata.docs.rs]

gotham/src/lib.rs

@@ -63,6 +63,7 @@ use std::future::Future;
 use std::io;
 use std::net::ToSocketAddrs;
 use std::sync::Arc;
+use thiserror::Error;
 use tokio::io::{AsyncRead, AsyncWrite};
 use tokio::net::{TcpListener, TcpStream};
 use tokio::runtime::{self, Runtime};
@@ -74,6 +75,15 @@ pub use plain::*;
 #[cfg(feature = "rustls")]
 pub use tls::start as start_with_tls;
 
+/// The error that can occur when starting the gotham server.
+#[derive(Debug, Error)]
+#[non_exhaustive]
+pub enum StartError {
+    /// I/O error.
+    #[error("I/O Error: {0}")]
+    IoError(#[from] io::Error),
+}
+
 fn new_runtime(threads: usize) -> Runtime {
     runtime::Builder::new_multi_thread()
         .worker_threads(threads)

gotham/src/plain/mod.rs

@@ -1,16 +1,15 @@
 use futures_util::future;
 use log::info;
-use std::io;
 use std::net::ToSocketAddrs;
 
 use super::handler::NewHandler;
-use super::{bind_server, new_runtime, tcp_listener};
+use super::{bind_server, new_runtime, tcp_listener, StartError};
 
 #[cfg(feature = "testing")]
 pub mod test;
 
 /// Starts a Gotham application on plain, unsecured HTTP.
-pub fn start<NH, A>(addr: A, new_handler: NH) -> io::Result<()>
+pub fn start<NH, A>(addr: A, new_handler: NH) -> Result<(), StartError>
 where
     NH: NewHandler + 'static,
     A: ToSocketAddrs + 'static + Send,
@@ -19,7 +18,11 @@ where
 }
 
 /// Starts a Gotham application with a designated number of threads.
-pub fn start_with_num_threads<NH, A>(addr: A, new_handler: NH, threads: usize) -> io::Result<()>
+pub fn start_with_num_threads<NH, A>(
+    addr: A,
+    new_handler: NH,
+    threads: usize,
+) -> Result<(), StartError>
 where
     NH: NewHandler + 'static,
     A: ToSocketAddrs + 'static + Send,
@@ -33,7 +36,7 @@ where
 /// This is used internally, but exposed in case the developer intends on doing any
 /// manual wiring that isn't supported by the Gotham API. It's unlikely that this will
 /// be required in most use cases; it's mainly exposed for shutdown handling.
-pub async fn init_server<NH, A>(addr: A, new_handler: NH) -> io::Result<()>
+pub async fn init_server<NH, A>(addr: A, new_handler: NH) -> Result<(), StartError>
 where
     NH: NewHandler + 'static,
     A: ToSocketAddrs + 'static + Send,

gotham/src/tls/mod.rs

@@ -1,19 +1,22 @@
 use futures_util::future::{MapErr, TryFutureExt};
 use log::{error, info};
-use std::io;
 use std::net::ToSocketAddrs;
 use std::sync::Arc;
 use tokio::net::TcpStream;
 use tokio_rustls::{rustls, Accept, TlsAcceptor};
 
 use super::handler::NewHandler;
-use super::{bind_server, new_runtime, tcp_listener};
+use super::{bind_server, new_runtime, tcp_listener, StartError};
 
 #[cfg(feature = "testing")]
 pub mod test;
 
 /// Starts a Gotham application with the default number of threads.
-pub fn start<NH, A>(addr: A, new_handler: NH, tls_config: rustls::ServerConfig) -> io::Result<()>
+pub fn start<NH, A>(
+    addr: A,
+    new_handler: NH,
+    tls_config: rustls::ServerConfig,
+) -> Result<(), StartError>
 where
     NH: NewHandler + 'static,
     A: ToSocketAddrs + 'static + Send,
@@ -27,7 +30,7 @@ pub fn start_with_num_threads<NH, A>(
     new_handler: NH,
     tls_config: rustls::ServerConfig,
     threads: usize,
-) -> io::Result<()>
+) -> Result<(), StartError>
 where
     NH: NewHandler + 'static,
     A: ToSocketAddrs + 'static + Send,
@@ -45,7 +48,7 @@ pub async fn init_server<NH, A>(
     addr: A,
     new_handler: NH,
     tls_config: rustls::ServerConfig,
-) -> io::Result<()>
+) -> Result<(), StartError>
 where
     NH: NewHandler + 'static,
     A: ToSocketAddrs + 'static + Send,