Remove warnings from mutable function arguments (#1606)

### What
Add test coverage for contract functions that use mutable function
arguments. Create new test_mutability contract with a calc function that
accepts a mutable parameter, demonstrating that the SDK macros correctly
handle mut keywords in contract implementations. Remove warnings that
the test coverage surfaced.

  ### Why
The SDK's contract macros already support mutable function parameters,
which are is a Rust pattern. This test ensures the macro expansion
correctly preserves mutability modifiers and validates the generated
code compiles and executes correctly. The test surfaced that warnings
are generated in this case, and the warnings shouldn't be as the user is
writing valid code and the warnings are coming from the generated code.

Close #1605

Author: leighmcculloch

Cargo.lock

@@ -10,7 +10,7 @@ use syn::{
 };
 use syn::{
     spanned::Spanned, token::And, Error, FnArg, Ident, ImplItem, ImplItemFn, ItemImpl, ItemTrait,
-    Lifetime, Pat, PatType, TraitItem, TraitItemFn, Type, TypeReference, Visibility,
+    Lifetime, Pat, PatIdent, PatType, TraitItem, TraitItemFn, Type, TypeReference, Visibility,
 };
 
 /// Gets methods from the implementation that have public visibility. For
@@ -48,6 +48,26 @@ pub fn fn_arg_ident(arg: &FnArg) -> Result<Ident, Error> {
     ))
 }
 
+/// Modifies a Pat removing any 'mut' on an Ident.
+pub fn pat_unwrap_mut(p: Pat) -> Pat {
+    match p {
+        Pat::Ident(PatIdent {
+            attrs,
+            by_ref,
+            mutability: Some(_),
+            ident,
+            subpat,
+        }) => Pat::Ident(PatIdent {
+            attrs,
+            by_ref,
+            mutability: None,
+            ident,
+            subpat,
+        }),
+        _ => p,
+    }
+}
+
 /// Returns a clone of the type from the FnArg.
 pub fn fn_arg_ref_type(arg: &FnArg, lifetime: Option<&Lifetime>) -> Result<Type, Error> {
     if let FnArg::Typed(pat_type) = arg {
@@ -70,13 +90,13 @@ pub fn fn_arg_ref_type(arg: &FnArg, lifetime: Option<&Lifetime>) -> Result<Type,
 }
 
 /// Returns a clone of FnArg with the type as a reference if the arg is a typed
-/// arg and its type is not already a reference.
+/// arg and its type is not already a reference. Mutability from the ident is stripped.
 pub fn fn_arg_make_ref(arg: &FnArg, lifetime: Option<&Lifetime>) -> FnArg {
     if let FnArg::Typed(pat_type) = arg {
         if !matches!(*pat_type.ty, Type::Reference(_)) {
             return FnArg::Typed(PatType {
                 attrs: pat_type.attrs.clone(),
-                pat: pat_type.pat.clone(),
+                pat: Box::new(pat_unwrap_mut(*pat_type.pat.clone())),
                 colon_token: pat_type.colon_token,
                 ty: Box::new(Type::Reference(TypeReference {
                     and_token: And::default(),
@@ -90,12 +110,14 @@ pub fn fn_arg_make_ref(arg: &FnArg, lifetime: Option<&Lifetime>) -> FnArg {
     arg.clone()
 }
 
+/// Returns a clone of FnArg with the type as an Into if the arg is a typed
+/// arg. Mutability from the ident is stripped.
 pub fn fn_arg_make_into(arg: &FnArg) -> FnArg {
     if let FnArg::Typed(pat_type) = arg {
         let ty = &pat_type.ty;
         return FnArg::Typed(PatType {
             attrs: pat_type.attrs.clone(),
-            pat: pat_type.pat.clone(),
+            pat: Box::new(pat_unwrap_mut(*pat_type.pat.clone())),
             colon_token: pat_type.colon_token,
             ty: Box::new(syn::parse_quote! { impl Into<#ty> }),
         });

soroban-sdk-macros/src/syn_ext.rs

@@ -17,6 +17,11 @@ impl Contract {
     pub fn add_with_unused_arg(_e: &Env, a: i32, _b: i32) -> i32 {
         a + 2
     }
+
+    pub fn add_with_mut_arg(_e: &Env, a: i32, mut b: i32) -> i32 {
+        b *= 1;
+        a + b
+    }
 }
 
 #[contract]
@@ -38,6 +43,9 @@ fn test_functional() {
     let b = 12i32;
     let c = ContractClient::new(&e, &contract_id).add(&a, &b);
     assert_eq!(c, 22);
+
+    let c = ContractClient::new(&e, &contract_id).add_with_mut_arg(&a, &b);
+    assert_eq!(c, 22);
 }
 
 #[test]

soroban-sdk/src/tests/contract_fn.rs

@@ -5,6 +5,7 @@
     "mux_id": 0
   },
   "auth": [
+    [],
     [],
     []
   ],