Do not restore $hp when returning from a context

CHANGELOG.md

@@ -8,6 +8,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 
+- [#525](https://github.com/FuelLabs/fuel-vm/pull/525): The `$hp` register is no longer restored to it's previous value when returning from a call, making it possible to return heap-allocated types from `CALL`.
+
+
 #### Breaking
 
 - [#514](https://github.com/FuelLabs/fuel-vm/pull/514/): Add `ChainId` and `GasCosts` to `ConsensusParameters`. 
@@ -29,7 +32,6 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - [#499](https://github.com/FuelLabs/fuel-vm/pull/499/): The `wasm_bindgen` support of `fuel-asm` and `fuel-types`.
     Each new release also publish a typescript analog of the `fuel-asm` and `fuel-types` crates to the npm.
-
 
 ## [Version 0.35.0]
 

fuel-vm/src/interpreter/flow.rs

@@ -208,13 +208,15 @@ impl RetCtx<'_> {
             let ggas = registers[RegId::GGAS];
             let ret = registers[RegId::RET];
             let retl = registers[RegId::RETL];
+            let hp = registers[RegId::HP];
 
             registers.copy_from_slice(frame.registers());
 
             registers[RegId::CGAS] = cgas;
             registers[RegId::GGAS] = ggas;
             registers[RegId::RET] = ret;
             registers[RegId::RETL] = retl;
+            registers[RegId::HP] = hp;
 
             let fp = registers[RegId::FP];
             set_frame_pointer(context, registers.fp_mut(), fp);

fuel-vm/src/interpreter/flow/ret_tests.rs

@@ -4,12 +4,7 @@ use super::*;
 
 #[test]
 fn test_return() {
-    let mut frame_reg: [Word; VM_REGISTER_COUNT] =
-        std::iter::successors(Some(0), |x| Some(x + 1))
-            .take(VM_REGISTER_COUNT)
-            .collect::<Vec<_>>()
-            .try_into()
-            .unwrap();
+    let mut frame_reg: [Word; VM_REGISTER_COUNT] = std::array::from_fn(|i| i as Word);
     frame_reg[RegId::CGAS] = 100;
     let mut expected = frame_reg;
     let frame = CallFrame::new(
@@ -26,11 +21,13 @@ fn test_return() {
     registers[RegId::GGAS] = 100;
     registers[RegId::RET] = 101;
     registers[RegId::RETL] = 102;
+    registers[RegId::HP] = 1234;
 
     expected[RegId::CGAS] = 199;
     expected[RegId::GGAS] = 100;
     expected[RegId::RET] = 101;
     expected[RegId::RETL] = 102;
+    expected[RegId::HP] = 1234;
     expected[RegId::PC] += 4;
     let mut context = Context::Call {
         block_height: Default::default(),