issue: cannot use databases by reference when using handlers

crates/revm/Cargo.toml

@@ -114,6 +114,11 @@ name = "generate_block_traces"
 path = "../../examples/generate_block_traces.rs"
 required-features = ["std", "serde-json", "ethersdb"]
 
+[[example]]
+name = "db_by_ref"
+path = "../../examples/db_by_ref.rs"
+required-features = ["std", "serde-json"]
+
 [[bench]]
 name = "bench"
 path = "benches/bench.rs"

crates/revm/src/handler/register.rs

@@ -8,7 +8,7 @@ pub type EvmHandler<'a, EXT, DB> = Handler<'a, Evm<'a, EXT, DB>, EXT, DB>;
 pub type EvmInstructionTables<'a, EXT, DB> = InstructionTables<'a, Evm<'a, EXT, DB>>;
 
 // Handle register
-pub type HandleRegister<'a, EXT, DB> = fn(&mut EvmHandler<'a, EXT, DB>);
+pub type HandleRegister<'a, EXT, DB> = for<'evm> fn(&'evm mut EvmHandler<'a, EXT, DB>);
 
 // Boxed handle register
 pub type HandleRegisterBox<'a, EXT, DB> = Box<dyn Fn(&mut EvmHandler<'a, EXT, DB>)>;

examples/db_by_ref.rs

@@ -0,0 +1,74 @@
+use revm::{
+    db::{CacheDB, EmptyDB, WrapDatabaseRef},
+    handler::register::HandleRegister,
+    inspector_handle_register,
+    inspectors::TracerEip3155,
+    primitives::ResultAndState,
+    Database, DatabaseCommit, Evm,
+};
+
+struct DebugContext<'evm, EXT, DB: Database> {
+    ext: EXT,
+    register_handles_fn: HandleRegister<'evm, EXT, DB>,
+}
+
+fn run_transaction<'db, 'evm, EXT>(
+    db: &'db CacheDB<EmptyDB>,
+    ext: EXT,
+    register_handles_fn: HandleRegister<'evm, EXT, WrapDatabaseRef<&'evm CacheDB<EmptyDB>>>,
+) -> anyhow::Result<ResultAndState>
+where
+    'db: 'evm,
+{
+    let mut evm = Evm::builder()
+        .with_ref_db(db)
+        .with_external_context(ext)
+        .append_handler_register(register_handles_fn)
+        .build();
+
+    let result = evm.transact()?;
+
+    Ok(result)
+}
+
+fn run_transaction_and_commit_with_ext<'db, 'evm, EXT>(
+    db: &'db mut CacheDB<EmptyDB>,
+    ext: EXT,
+    register_handles_fn: HandleRegister<'evm, EXT, WrapDatabaseRef<&'evm CacheDB<EmptyDB>>>,
+) -> anyhow::Result<()>
+where
+    'db: 'evm,
+{
+    let ResultAndState { state: changes, .. } = {
+        let db: &'evm _ = &*db;
+        run_transaction(db, ext, register_handles_fn)?
+    };
+
+    // Compile error: error[E0502]: cannot borrow `*db` as mutable because it is also borrowed as immutable
+    // The lifetime of `'evm` is extended beyond this function's scope because it is used in the `HandleRegister` function
+    db.commit(changes);
+
+    Ok(())
+}
+
+fn run_transaction_and_commit<'db>(db: &mut CacheDB<EmptyDB>) -> anyhow::Result<()> {
+    let mut evm = Evm::builder().with_ref_db(db).build();
+
+    let ResultAndState { state: changes, .. } = evm.transact()?;
+
+    // No compiler error because there is no lifetime parameter for the `HandleRegister` function
+    db.commit(changes);
+
+    Ok(())
+}
+
+fn main() -> anyhow::Result<()> {
+    let mut cache_db = CacheDB::new(EmptyDB::default());
+
+    let mut tracer = TracerEip3155::new(Box::new(std::io::stdout()), true, true);
+
+    run_transaction_and_commit_with_ext(&mut cache_db, &mut tracer, inspector_handle_register)?;
+    run_transaction_and_commit(&mut cache_db)?;
+
+    Ok(())
+}