feat: warn when payable fallback function found but no receive function

crates/sema/src/typeck/mod.rs

@@ -5,19 +5,38 @@ use crate::{
 };
 use rayon::prelude::*;
 use solar_data_structures::{map::FxHashSet, parallel};
+use solar_interface::error_code;
 
 pub(crate) fn check(gcx: Gcx<'_>) {
     parallel!(
         gcx.sess,
         gcx.hir.par_contract_ids().for_each(|id| {
             check_duplicate_definitions(gcx, &gcx.symbol_resolver.contract_scopes[id]);
+            check_payable_fallback_without_receive(gcx, id);
         }),
         gcx.hir.par_source_ids().for_each(|id| {
             check_duplicate_definitions(gcx, &gcx.symbol_resolver.source_scopes[id]);
         }),
     );
 }
 
+fn check_payable_fallback_without_receive(gcx: Gcx<'_>, contract_id: hir::ContractId) {
+    let contract = gcx.hir.contract(contract_id);
+
+    let payable_fallback_found =
+        contract.fallback.is_some_and(|f| gcx.hir.function(f).state_mutability.is_payable());
+
+    let receive_found = contract.receive.is_some();
+
+    if payable_fallback_found && !receive_found {
+        gcx.dcx()
+            .warn("contract has a payable fallback function, but no receive ether function")
+            .code(error_code!(3628))
+            .span_help(contract.name.span, "consider changing fallback to receive")
+            .emit();
+    }
+}
+
 /// Checks for definitions that have the same name and parameter types in the given scope.
 fn check_duplicate_definitions(gcx: Gcx<'_>, scope: &Declarations) {
     let is_duplicate = |a: Declaration, b: Declaration| -> bool {

tests/ui/parser/payable_fallback_without_receive.sol

@@ -0,0 +1,15 @@
+contract P1 {
+    fallback() external payable {}
+}
+
+contract P2 is P1 {
+    receive() external payable {}
+}
+
+contract P3 {
+    fallback() external payable {}
+
+    receive() external payable {}
+}
+
+contract P4 is P1 {}

tests/ui/parser/payable_fallback_without_receive.stderr

@@ -0,0 +1,16 @@
+warning[3628]: contract has a payable fallback function, but no receive ether function
+  --> ROOT/tests/ui/parser/payable_fallback_without_receive.sol:LL:CC
+   |
+LL | contract P1 {
+   |          -- help: consider changing fallback to receive
+   |
+
+warning[3628]: contract has a payable fallback function, but no receive ether function
+  --> ROOT/tests/ui/parser/payable_fallback_without_receive.sol:LL:CC
+   |
+LL | contract P4 is P1 {}
+   |          -- help: consider changing fallback to receive
+   |
+
+warning: 2 warnings emitted
+