Add fuzz target for fallback TokenStream parser

fuzz/.gitignore

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+artifacts/
+corpus/
+target/

fuzz/Cargo.toml

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+[package]
+name = "proc-macro2-fuzz"
+version = "0.0.0"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+edition = "2021"
+publish = false
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.4"
+proc-macro2 = { path = ".." }
+
+[[bin]]
+name = "parse_token_stream"
+path = "fuzz_targets/parse_token_stream.rs"
+test = false
+doc = false
+
+[workspace]

fuzz/fuzz_targets/parse_token_stream.rs

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+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use std::str;
+
+fuzz_target!(|bytes: &[u8]| {
+    if bytes.len() < 200 {
+        if let Ok(string) = str::from_utf8(bytes) {
+            _ = string.parse::<proc_macro2::TokenStream>();
+        }
+    }
+});