chore(avm-transpiler): minor rust fixes (#4889)

* Validate that the catch-all for foreign functions looks like a getter. When I was implementing NOTEHASHEXISTS I was getting a weird assertion failing but what was happening was that I was running the old transpiler on a contract using NOTEHASHEXISTS. The transpiler would go into the catch-all for NOTEHASHEXISTS and then I'd get the assertion that the inputs where not empty. This should make it better. Even better is to have the getter to opcode transformation in the match, but I didn't like the formatting. Using a global Map is not possible in rust.
* `&String` is rare. `&str` is better and `&String` autoconverts.

avm-transpiler/src/transpile.rs

@@ -233,14 +233,14 @@ pub fn brillig_to_avm(brillig: &Brillig) -> Vec<u8> {
 /// - TODO: support for avm external calls through this function
 fn handle_foreign_call(
     avm_instrs: &mut Vec<AvmInstruction>,
-    function: &String,
+    function: &str,
     destinations: &Vec<ValueOrArray>,
     inputs: &Vec<ValueOrArray>,
 ) {
-    match function.as_str() {
+    match function {
         "avmOpcodeNoteHashExists" => handle_note_hash_exists(avm_instrs, destinations, inputs),
         "emitNoteHash" | "emitNullifier" => handle_emit_note_hash_or_nullifier(
-            function.as_str() == "emitNullifier",
+            function == "emitNullifier",
             avm_instrs,
             destinations,
             inputs,
@@ -252,7 +252,15 @@ fn handle_foreign_call(
         "poseidon" => {
             handle_single_field_hash_instruction(avm_instrs, function, destinations, inputs)
         }
-        _ => handle_getter_instruction(avm_instrs, function, destinations, inputs),
+        // Getters.
+        _ if inputs.len() == 0 && destinations.len() == 1 => {
+            handle_getter_instruction(avm_instrs, function, destinations, inputs)
+        }
+        // Anything else.
+        _ => panic!(
+            "Transpiler doesn't know how to process ForeignCall function {}",
+            function
+        ),
     }
 }
 
@@ -384,7 +392,7 @@ fn handle_nullifier_exists(
 /// to reason about. In order to decrease user friction we will use two field outputs.
 fn handle_2_field_hash_instruction(
     avm_instrs: &mut Vec<AvmInstruction>,
-    function: &String,
+    function: &str,
     destinations: &[ValueOrArray],
     inputs: &[ValueOrArray],
 ) {
@@ -405,7 +413,7 @@ fn handle_2_field_hash_instruction(
         _ => panic!("Keccak | Poseidon address destination should be a single value"),
     };
 
-    let opcode = match function.as_str() {
+    let opcode = match function {
         "keccak256" => AvmOpcode::KECCAK,
         "sha256" => AvmOpcode::SHA256,
         _ => panic!(
@@ -443,7 +451,7 @@ fn handle_2_field_hash_instruction(
 /// representation.
 fn handle_single_field_hash_instruction(
     avm_instrs: &mut Vec<AvmInstruction>,
-    function: &String,
+    function: &str,
     destinations: &[ValueOrArray],
     inputs: &[ValueOrArray],
 ) {
@@ -461,7 +469,7 @@ fn handle_single_field_hash_instruction(
         _ => panic!("Poseidon address destination should be a single value"),
     };
 
-    let opcode = match function.as_str() {
+    let opcode = match function {
         "poseidon" => AvmOpcode::POSEIDON,
         _ => panic!(
             "Transpiler doesn't know how to process ForeignCall function {:?}",
@@ -497,20 +505,21 @@ fn handle_single_field_hash_instruction(
 /// - ...
 fn handle_getter_instruction(
     avm_instrs: &mut Vec<AvmInstruction>,
-    function: &String,
+    function: &str,
     destinations: &Vec<ValueOrArray>,
     inputs: &Vec<ValueOrArray>,
 ) {
     // For the foreign calls we want to handle, we do not want inputs, as they are getters
     assert!(inputs.is_empty());
     assert!(destinations.len() == 1);
+
     let dest_offset_maybe = destinations[0];
     let dest_offset = match dest_offset_maybe {
         ValueOrArray::MemoryAddress(dest_offset) => dest_offset.0,
         _ => panic!("ForeignCall address destination should be a single value"),
     };
 
-    let opcode = match function.as_str() {
+    let opcode = match function {
         "address" => AvmOpcode::ADDRESS,
         "storageAddress" => AvmOpcode::STORAGEADDRESS,
         "origin" => AvmOpcode::ORIGIN,
@@ -529,6 +538,7 @@ fn handle_getter_instruction(
             function
         ),
     };
+
     avm_instrs.push(AvmInstruction {
         opcode,
         indirect: Some(ALL_DIRECT),