Improve the parser error type

keyvalues-parser/fuzz/Cargo.toml

@@ -24,3 +24,10 @@ name = "parse"
 path = "fuzz_targets/parse.rs"
 test = false
 doc = false
+
+[[bin]]
+name = "error_invariants"
+path = "fuzz_targets/error_invariants.rs"
+test = false
+doc = false
+bench = false

keyvalues-parser/fuzz/fuzz_targets/error_invariants.rs

@@ -0,0 +1,16 @@
+#![no_main]
+
+use keyvalues_parser::Vdf;
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|text: &str| {
+    if let Err(err) = Vdf::parse(text) {
+        // Lots of fiddly logic in displaying that can panic
+        err.to_string();
+
+        // The error snippet should match the original text sliced using the error span
+        let from_orig = err.index_span().slice(text);
+        let from_snippet = err.error_snippet();
+        assert_eq!(from_orig, from_snippet);
+    }
+});