lexpr-macros: Provide a Display implementation for ParseError

This improvement was prompted by `clippy`, which now warns about
unused values contained in enum variants of `ParseError`, even if they
are used by a derived `Debug` implementation.

As a bonus to fixing the clippy warning, the error messages are now a
bit nicer.

lexpr-macros/src/lib.rs

@@ -14,7 +14,7 @@ use quote::quote;
 pub fn sexp(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
     let output = match expand(TokenStream::from(input)) {
         Err(e) => {
-            let msg = format!("could not parse s-expression: {:?}", e);
+            let msg = format!("could not parse s-expression: {}", e);
             quote! { compile_error!(#msg) }
         }
         Ok(output) => output,

lexpr-macros/src/parser.rs

@@ -1,3 +1,5 @@
+use std::fmt;
+
 use crate::value::Value;
 
 use proc_macro2::{Delimiter, Literal, Spacing, TokenStream, TokenTree};
@@ -17,6 +19,28 @@ pub enum ParseError {
     UnexpectedEnd,
 }
 
+impl fmt::Display for ParseError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::ExpectedStringLiteral(literal) => {
+                write!(f, "expected string literal `{}`", literal)
+            }
+            Self::UnexpectedToken(tt) => write!(f, "unexpected token `{}`", tt),
+            Self::UnexpectedChar(c) => write!(f, "unexpected character `{}`", c),
+            Self::UnexpectedDelimiter(delim) => {
+                let delim = match delim {
+                    Delimiter::Brace => "brace",
+                    Delimiter::Bracket => "bracket",
+                    Delimiter::Parenthesis => "parenthesis",
+                    Delimiter::None => "non",
+                };
+                write!(f, "unexpected {}-delimited expression", delim)
+            }
+            Self::UnexpectedEnd => write!(f, "unexpected end of input"),
+        }
+    }
+}
+
 impl Parser {
     fn new(tokens: Vec<TokenTree>) -> Self {
         Parser { tokens, index: 0 }

lexpr/NEWS.md

@@ -14,6 +14,9 @@ Changes:
   octothorpe, as in Emacs Lisp and Common Lisp, allowing for a
   less-noisy spelling; e.g.: `:foo` instead of `#:foo`. Feature
   request (#99) and initial implementation (#96) by @samuel-jimenez.
+- The errors produced by the `sexp!` macro now are a bit more
+  human-oriented, as the `lexpr-macros` `ParseError` type gained a
+  `Display` implementation.
 
 Maintenance-related changes: