examples: Add error output demonstration

Add an `errors` example that creates and prints various error types from
the crate. This is useful for two main purposes:

- Helps us catch mistakes by exercising error paths.
- Helps us check that error output is useful for debugging.
- Helps us check that output for "alloc" builds is good/useful (no-std)

Run with appropriate features to verify the output, e.g.,

  `cargo run --example errors --no-default-features --features=alloc`

Author: tcharding

Diff for: Cargo.toml

@@ -18,3 +18,7 @@ alloc = []
 
 [target.'cfg(mutate)'.dev-dependencies]
 mutagen = { git = "https://github.com/llogiq/mutagen" }
+
+[[example]]
+name = "errors"
+required-features = []

Diff for: examples/errors.rs

@@ -0,0 +1,133 @@
+//! Demonstrate output from the various crate errors.
+
+use core::fmt;
+use std::error::Error;
+
+use bech32::{Fe32, Hrp};
+
+fn main() {
+    crate_decode();
+    // crate::encode only returns `fmt::Error` errors.
+
+    crate_segwit_decode();
+    crate_segwit_encode();
+
+    // TODO: Do the other primitives modules.
+    primitives_hrp();
+}
+
+/// Demonstrates `bech32::decode` errors.
+fn crate_decode() {
+    use bech32::decode;
+    let function = "bech32::decode";
+
+    // The arguments to pass to `function`.
+    let strings = vec!["1qqq", "hrp1abc"];
+
+    for s in strings {
+        let err = decode(s).unwrap_err();
+        println!("\n\n* Call `{}(\"{}\")` -> {:?}", function, s, err);
+        println!("\n------------");
+        print_source(&err);
+        println!("------------");
+    }
+}
+
+/// Demonstrates the `Hrp::Error` variants.
+fn primitives_hrp() {
+    use bech32::primitives::hrp::Error::*;
+
+    println!("\n\n* All errors when parsing an invalid HRP");
+    let errs = vec![
+        TooLong(99),
+        Empty,
+        NonAsciiChar(char::from('\u{e9}')),
+        InvalidAsciiByte(200),
+        MixedCase,
+    ];
+    println!("\n------------");
+
+    let last = errs.len() - 1;
+    for (i, e) in errs.iter().enumerate() {
+        println!("Debug: {:?}\nError: {}", e.clone(), e);
+        if i != last {
+            println!("");
+        }
+    }
+    println!("------------");
+}
+
+// TODO: Generate address strings to trigger:
+// - Padding(PaddingError)
+// - WitnessLength(WitnessLengthError)
+/// Demonstrates `bech32::segwit::decode` errors.
+fn crate_segwit_decode() {
+    use bech32::segwit::decode;
+    let function = "bech32::segwit::decode";
+
+    // The arguments to pass to `function`.
+    let strings = vec!["1qpppppp", "bc1qabc", "bc1", "bc1mpppppp", "bc1qppppp"];
+
+    for s in strings {
+        let err = decode(s).unwrap_err();
+        println!("\n\n* Call `{}(\"{}\")` -> {:?}", function, s, err);
+        println!("\n------------");
+        print_source(&err);
+        println!("------------");
+    }
+}
+
+/// Demonstrates `bech32::segwit::encode` errors.
+fn crate_segwit_encode() {
+    use bech32::segwit::encode;
+    let function = "bech32::segwit::encode";
+
+    // The arguments to pass to `function`.
+    let hrp = Hrp::parse("bc").expect("a valid HRP string");
+
+    let invalid_witness_version = Fe32::M;
+
+    let valid_witness_program_segwit_v1 = [0x00, 0x01];
+    let invalid_witness_program_too_short = [0x00];
+    let invalid_witness_program_too_long = [0x00; 50];
+
+    let print = |err| {
+        println!("\n\n* Call `{}({}, [])` -> {:?}", function, hrp, err);
+        println!("\n------------");
+        print_source(&err);
+        println!("------------");
+    };
+
+    let err = encode(hrp, invalid_witness_version, &valid_witness_program_segwit_v1).unwrap_err();
+    print(err);
+
+    let err = encode(hrp, Fe32::P, &invalid_witness_program_too_short).unwrap_err();
+    print(err);
+
+    let err = encode(hrp, Fe32::P, &invalid_witness_program_too_long).unwrap_err();
+    print(err);
+
+    let err = encode(hrp, Fe32::Q, &valid_witness_program_segwit_v1).unwrap_err();
+    print(err);
+}
+
+/// Prints `e` in a similar fashion to the output created by `anyhow`.
+#[cfg(feature = "std")]
+fn print_source(mut e: &dyn Error) {
+    println!("Error: {}", e);
+
+    if e.source().is_some() {
+        let mut counter = 0;
+        println!("\nCaused by: ");
+
+        while e.source().is_some() {
+            let inner = e.source().unwrap();
+            println!("\t{}: {}", counter, inner);
+            e = e.source().unwrap();
+            counter += 1;
+        }
+    }
+}
+
+#[cfg(not(feature = "std"))]
+fn print_source(e: &dyn fmt::Display) { println!("{}", e) }

Diff for: src/primitives/decode.rs

@@ -955,12 +955,13 @@ mod tests {
                 fn $test_name() {
                     let res = SegwitHrpstring::new($address);
                     if res.is_ok() {
-                        panic!("{} sting should not be valid: {}", $address, $reason);
+                        panic!("{} string should not be valid: {}", $address, $reason);
                     }
                 }
             )*
         }
     }
+    // TODO: We are not asserting on the error value, at least one of these is not failing for the reason stated.
     check_invalid_segwit_addresses! {
         invalid_segwit_address_0, "missing hrp", "1qar0srrr7xfkvy5l643lydnw9re59gtzzwf5mdq";
         invalid_segwit_address_1, "missing data-checksum", "91111";