feat(cli): Output runtime errors pretty-printed for readability

src/bin/sile.rs

@@ -1,18 +1,42 @@
+use sile::cli::Cli;
+
+use snafu::prelude::*;
+
 use clap::{CommandFactory, FromArgMatches};
 
-use sile::cli::Cli;
-use sile::Result;
+#[derive(Snafu)]
+enum Error {
+    #[snafu(display("{}", source))]
+    Args { source: clap::error::Error },
+
+    #[snafu(display("{}", source))]
+    Runtime { source: anyhow::Error },
+
+    #[snafu(display("{}", source))]
+    Version { source: anyhow::Error },
+}
+
+// Deeper error types are reported using the Debug trait, but we handle them via Snafu and the Display trait.
+// So we delegate. c.f. https://github.com/shepmaster/snafu/issues/110
+impl std::fmt::Debug for Error {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {
+        std::fmt::Display::fmt(self, fmt)
+    }
+}
+
+type Result<T, E = Error> = std::result::Result<T, E>;
 
 fn main() -> Result<()> {
     let version = option_env!("VERGEN_GIT_DESCRIBE").unwrap_or_else(|| env!("CARGO_PKG_VERSION"));
     let version = version.replacen('-', ".r", 1);
-    let long_version = sile::version()?
+    let long_version = sile::version()
+        .context(VersionSnafu)?
         .strip_prefix("SILE ")
         .unwrap_or("")
         .to_string();
     let app = Cli::command().version(version).long_version(long_version);
     let matches = app.get_matches();
-    let args = Cli::from_arg_matches(&matches).expect("Unable to parse arguments");
+    let args = Cli::from_arg_matches(&matches).context(ArgsSnafu)?;
     sile::run(
         args.input,
         args.backend,
@@ -29,6 +53,7 @@ fn main() -> Result<()> {
         args.r#use,
         args.quiet,
         args.traceback,
-    )?;
+    )
+    .context(RuntimeSnafu)?;
     Ok(())
 }