fuzz: Add fuzz harness for zkas compilation

Create a fuzz harness to test the ./zkas binary compilation process. The
whole pipeline is tested: Lexer, Parser, Compiler, Analyzer. This is
performed by copying the relevant code from bin/zkas/src/main.rs.

Testing the entire pipeline like this is not very efficient in terms of
fuzzing cycles but on the other hand it is a quick-and-dirty way to find
results. It also benefits from testing the actual inputs to the binary
in the way it's expecting, rather than checking each of the components
in a piecemeal way using interfaces that aren't expected to be exposed
anyway.

fuzz/honggfuzz/Cargo.toml

@@ -46,3 +46,9 @@ name = "varint-differential"
 path = "src/varint_differential.rs"
 test = false
 doc = false
+
+[[bin]]
+name = "zkas-compile"
+path = "src/zkas_compile.rs"
+test = false
+doc = false

fuzz/honggfuzz/src/zkas_compile.rs

@@ -0,0 +1,84 @@
+/* This file is part of DarkFi (https://dark.fi)
+ *
+ * Copyright (C) 2020-2023 Dyne.org foundation
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+// extern crate darkfi_serial;
+
+use honggfuzz::fuzz;
+use darkfi::zkas::{Lexer, Parser, Compiler, Analyzer};
+use std::str;
+
+fn main() {
+    loop {
+        fuzz!(|data: &[u8]| {
+            // The lex, parse, compile code below is taken from bin/zkas/src/main.rs
+            
+            // Use only inputs that can be encoded as .chars(), as this is what the
+            // zkas binary uses
+            let chars = match str::from_utf8(data) {
+                Ok(v) => v.chars(),
+                Err(_e) => return,
+            };
+
+            let filename = "/dev/null";
+            let lexer = Lexer::new(filename, chars.clone());
+            let tokens = match lexer.lex() {
+                Ok(v) => v,
+                Err(_) => return,
+            };
+
+            // The parser goes over the tokens provided by the lexer and builds
+            // the initial AST, not caring much about the semantics, just enforcing
+            // syntax and general structure.
+            let parser = Parser::new(filename, chars.clone(), tokens);
+            let (namespace, k, constants, witnesses, statements) = match parser.parse() {
+                Ok(v) => v,
+                Err(_) => return,
+            };
+
+            // The analyzer goes through the initial AST provided by the parser and
+            // converts return and variable types to their correct forms, and also
+            // checks that the semantics of the ZK script are correct.
+            let mut analyzer = Analyzer::new(filename, chars.clone(), constants, witnesses, statements);
+            if analyzer.analyze_types().is_err() {
+                return
+            }
+
+            if analyzer.analyze_semantic().is_err() {
+                return
+            }
+
+            let compiler = Compiler::new(
+                filename,
+                chars.clone(),
+                namespace,
+                k,
+                analyzer.constants,
+                analyzer.witnesses,
+                analyzer.statements,
+                analyzer.literals,
+                false, // a guess
+            );
+
+            match compiler.compile() {
+                Ok(v) => v,
+                Err(_) => return,
+            };
+                
+        });
+    }
+}