feat: add workspace skeleton and server configuration validation

core/src/error.rs

@@ -0,0 +1,64 @@
+//! The library error type.
+//!
+//! Variants carry the data a caller needs to act or an operator needs to fix
+//! the cause, rather than a pre-formatted sentence.
+
+use std::fmt;
+use std::io;
+use std::path::PathBuf;
+
+#[derive(Debug)]
+pub enum Error {
+    /// Reading or canonicalising a path named by the configuration failed.
+    Io { path: PathBuf, cause: io::Error },
+
+    /// A configuration file is not valid TOML, or does not match the schema.
+    Toml { path: PathBuf, cause: toml::de::Error },
+
+    /// A rule the schema cannot express was violated.
+    Config { message: String },
+
+    /// Two sites claim one hostname, so a request naming it could not be routed.
+    DuplicateHost { host: String, first: String, second: String },
+
+    /// A listener names a format no enabled feature provides. `available` is
+    /// the registry's full id set, which is what tells an operator whether the
+    /// name is a typo or a missing cargo feature.
+    UnknownFormat { listener: String, format: String, available: Vec<String> },
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            // The cause is reachable through `source`, so it is not repeated here.
+            Error::Io { path, .. } => write!(f, "cannot read {}", path.display()),
+            Error::Toml { path, .. } => write!(f, "invalid TOML in {}", path.display()),
+            Error::Config { message } => write!(f, "{message}"),
+            Error::DuplicateHost { host, first, second } => {
+                write!(f, "host '{host}' is claimed by both site '{first}' and site '{second}'")
+            }
+            Error::UnknownFormat { listener, format, available } => write!(
+                f,
+                "listener '{listener}' wants format '{format}', which this build does not \
+                 provide; available: {}",
+                if available.is_empty() { "none".to_string() } else { available.join(", ") }
+            ),
+        }
+    }
+}
+
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Error::Io { cause, .. } => Some(cause),
+            Error::Toml { cause, .. } => Some(cause),
+            _ => None,
+        }
+    }
+}
+
+impl Error {
+    pub(crate) fn config(message: impl Into<String>) -> Self {
+        Error::Config { message: message.into() }
+    }
+}