Fix the downcast in create_mygc_mutator (#399)

Author: qinsoon

docs/tutorial/code/mygc_semispace/mutator.rs

@@ -1,5 +1,6 @@
 // ANCHOR: imports
 use super::MyGC; // Add
+use crate::Plan;
 use crate::plan::barriers::NoBarrier;
 use crate::plan::mutator_context::Mutator;
 use crate::plan::mutator_context::MutatorConfig;
@@ -59,19 +60,22 @@ lazy_static! {
 
 pub fn create_mygc_mutator<VM: VMBinding>(
     mutator_tls: VMMutatorThread,
-    plan: &'static MyGC<VM>,
+    plan: &'static dyn Plan<VM = VM>,
 ) -> Mutator<VM> {
+    // ANCHOR: plan_downcast
+    let mygc = plan.downcast_ref::<MyGC<VM>>().unwrap();
+    // ANCHOR_END: plan_downcast
     let config = MutatorConfig {
         allocator_mapping: &*ALLOCATOR_MAPPING,
         // Modify
         // ANCHOR: space_mapping
         space_mapping: box vec![
-            (AllocatorSelector::BumpPointer(0), plan.tospace()),
+            (AllocatorSelector::BumpPointer(0), mygc.tospace()),
             (
                 AllocatorSelector::BumpPointer(1),
-                plan.common.get_immortal(),
+                mygc.common.get_immortal(),
             ),
-            (AllocatorSelector::LargeObject(0), plan.common.get_los()),
+            (AllocatorSelector::LargeObject(0), mygc.common.get_los()),
         ],
         // ANCHOR_END: space_mapping
         prepare_func: &mygc_mutator_prepare, // Modify

docs/tutorial/src/mygc/ss/alloc.md

@@ -189,9 +189,17 @@ space in `space_mapping`. Note that the space allocation is formatted as a list
 of tuples. For example, the first bump pointer allocator (`BumpPointer(0)`) is 
 bound with `tospace`.
 
+Downcast the dynamic `Plan` type to `MyGC` so we can access specific spaces in `MyGC`.
+
+```rust
+{{#include ../../../code/mygc_semispace/mutator.rs:plan_downcast}}
+```
+
+Then, use `mygc` to access the spaces in `MyGC`.
+
    1. `BumpPointer(0)` should map to the tospace.
-   2. `BumpPointer(1)` should map to `plan.common.get_immortal()`.
-   3. `LargeObject(0)` should map to `plan.common.get_los()`.
+   2. `BumpPointer(1)` should map to `mygc.common.get_immortal()`.
+   3. `LargeObject(0)` should map to `mygc.common.get_los()`.
    4. None of the above should be dereferenced (ie, they should not have 
    the `&` prefix).