i think the memory model is fixed now? at least until arrays....

crates/generate/src/types.rs

@@ -105,12 +105,16 @@ fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenS
 
         impl ::memory::GuestTypeCopy for #ident {}
         impl ::memory::GuestTypeClone for #ident {
-            fn from_guest(location: &::memory::GuestPtr<#ident>) -> Result<#ident, ::memory::GuestError> {
+            fn read_from_guest(location: &::memory::GuestPtr<#ident>) -> Result<#ident, ::memory::GuestError> {
                 use ::std::convert::TryFrom;
                 let raw: #repr = unsafe { (location.as_raw() as *const #repr).read() };
                 let val = #ident::try_from(raw)?;
                 Ok(val)
             }
+            fn write_to_guest(&self, location: &::memory::GuestPtrMut<#ident>) {
+                let val: #repr = #repr::from(*self);
+                unsafe { (location.as_raw() as *mut #repr).write(val) };
+            }
         }
 
     }

crates/memory/src/guest_type.rs

@@ -1,4 +1,4 @@
-use crate::{GuestError, GuestPtr};
+use crate::{GuestError, GuestPtr, GuestPtrMut};
 
 pub trait GuestType: Sized {
     // These are morally the same as Rust ::std::mem::size_of / align_of, but they return
@@ -12,11 +12,12 @@ pub trait GuestType: Sized {
 
 pub trait GuestTypeCopy: GuestType + Copy {}
 pub trait GuestTypeClone: GuestType + Clone {
-    fn from_guest<'a>(location: &GuestPtr<'a, Self>) -> Result<Self, GuestError>;
+    fn read_from_guest<'a>(location: &GuestPtr<'a, Self>) -> Result<Self, GuestError>;
+    fn write_to_guest<'a>(&self, location: &GuestPtrMut<'a, Self>);
 }
-pub trait GuestTypeRef<'a>: GuestType {
-    type Ref;
-    fn from_guest(location: &GuestPtr<'a, Self>) -> Result<Self::Ref, GuestError>;
+pub trait GuestTypePtr<'a>: GuestType {
+    fn read_from_guest(location: &GuestPtr<'a, Self>) -> Result<Self, GuestError>;
+    fn write_to_guest(&self, location: &GuestPtrMut<'a, Self>);
 }
 
 macro_rules! builtin_type {

crates/memory/src/lib.rs

@@ -5,6 +5,6 @@ mod memory;
 mod region;
 
 pub use error::GuestError;
-pub use guest_type::{GuestErrorType, GuestType, GuestTypeClone, GuestTypeCopy, GuestTypeRef};
+pub use guest_type::{GuestErrorType, GuestType, GuestTypeClone, GuestTypeCopy, GuestTypePtr};
 pub use memory::{GuestMemory, GuestPtr, GuestPtrMut, GuestRef, GuestRefMut};
 pub use region::Region;

crates/memory/src/memory.rs

@@ -3,7 +3,7 @@ use std::marker::PhantomData;
 use std::rc::Rc;
 
 use crate::borrow::{BorrowHandle, GuestBorrows};
-use crate::{GuestError, GuestType, GuestTypeCopy, GuestTypeRef, Region};
+use crate::{GuestError, GuestType, GuestTypeClone, GuestTypeCopy, GuestTypePtr, Region};
 
 pub struct GuestMemory<'a> {
     ptr: *mut u8,
@@ -67,6 +67,9 @@ impl<'a, T: GuestType> GuestPtr<'a, T> {
     pub fn as_raw(&self) -> *const u8 {
         (self.mem.ptr as usize + self.region.start as usize) as *const u8
     }
+}
+
+impl<'a, T: GuestTypeCopy> GuestPtr<'a, T> {
     pub fn as_ref(&self) -> Result<GuestRef<'a, T>, GuestError> {
         T::validate(&self)?;
         let handle = {
@@ -84,6 +87,18 @@ impl<'a, T: GuestType> GuestPtr<'a, T> {
     }
 }
 
+impl<'a, T: GuestTypeClone> GuestPtr<'a, T> {
+    pub fn clone_from_guest(&self) -> Result<T, GuestError> {
+        T::read_from_guest(self)
+    }
+}
+
+impl<'a, T: GuestTypePtr<'a>> GuestPtr<'a, T> {
+    pub fn read_ptr_from_guest(&self) -> Result<T, GuestError> {
+        T::read_from_guest(self)
+    }
+}
+
 impl<'a, T> GuestType for GuestPtr<'a, T>
 where
     T: GuestType,
@@ -106,17 +121,24 @@ where
     }
 }
 
-impl<'a, T> GuestTypeRef<'a> for GuestPtr<'a, T>
+// Operations for reading and writing Ptrs to memory:
+impl<'a, T> GuestTypePtr<'a> for GuestPtr<'a, T>
 where
     T: GuestType,
 {
-    type Ref = GuestRef<'a, T>;
-    fn from_guest(location: &GuestPtr<'a, Self>) -> Result<Self::Ref, GuestError> {
+    fn read_from_guest(location: &GuestPtr<'a, Self>) -> Result<Self, GuestError> {
         // location is guaranteed to be in GuestMemory and aligned to 4
         let raw_ptr: u32 = unsafe { *(location.as_raw() as *const u32) };
         // GuestMemory can validate that the raw pointer contents are legal for T:
         let guest_ptr: GuestPtr<'a, T> = location.mem.ptr(raw_ptr)?;
-        Ok(guest_ptr.as_ref()?)
+        Ok(guest_ptr)
+    }
+    fn write_to_guest(&self, location: &GuestPtrMut<'a, Self>) {
+        // location is guaranteed to be in GuestMemory and aligned to 4
+        unsafe {
+            let raw_ptr: *mut u32 = location.as_raw() as *mut u32;
+            raw_ptr.write(self.region.start);
+        }
     }
 }
 
@@ -139,7 +161,9 @@ impl<'a, T: GuestType> GuestPtrMut<'a, T> {
     pub fn as_raw(&self) -> *const u8 {
         self.as_immut().as_raw()
     }
+}
 
+impl<'a, T: GuestTypeCopy> GuestPtrMut<'a, T> {
     pub fn as_ref(&self) -> Result<GuestRef<'a, T>, GuestError> {
         self.as_immut().as_ref()
     }
@@ -161,6 +185,25 @@ impl<'a, T: GuestType> GuestPtrMut<'a, T> {
     }
 }
 
+impl<'a, T: GuestTypePtr<'a>> GuestPtrMut<'a, T> {
+    pub fn read_ptr_from_guest(&self) -> Result<T, GuestError> {
+        T::read_from_guest(&self.as_immut())
+    }
+    pub fn write_ptr_to_guest(&self, ptr: &T) {
+        T::write_to_guest(ptr, &self);
+    }
+}
+
+impl<'a, T: GuestTypeClone> GuestPtrMut<'a, T> {
+    pub fn clone_from_guest(&self) -> Result<T, GuestError> {
+        T::read_from_guest(&self.as_immut())
+    }
+
+    pub fn clone_to_guest(&self, val: &T) {
+        T::write_to_guest(val, &self)
+    }
+}
+
 impl<'a, T> GuestType for GuestPtrMut<'a, T>
 where
     T: GuestType,
@@ -183,17 +226,24 @@ where
     }
 }
 
-impl<'a, T> GuestTypeRef<'a> for GuestPtrMut<'a, T>
+// Reading and writing GuestPtrMuts to memory:
+impl<'a, T> GuestTypePtr<'a> for GuestPtrMut<'a, T>
 where
     T: GuestType,
 {
-    type Ref = GuestRefMut<'a, T>;
-    fn from_guest(location: &GuestPtr<'a, Self>) -> Result<Self::Ref, GuestError> {
+    fn read_from_guest(location: &GuestPtr<'a, Self>) -> Result<Self, GuestError> {
         // location is guaranteed to be in GuestMemory and aligned to 4
         let raw_ptr: u32 = unsafe { *(location.as_raw() as *const u32) };
         // GuestMemory can validate that the raw pointer contents are legal for T:
         let guest_ptr_mut: GuestPtrMut<'a, T> = location.mem.ptr_mut(raw_ptr)?;
-        Ok(guest_ptr_mut.as_ref_mut()?)
+        Ok(guest_ptr_mut)
+    }
+    fn write_to_guest(&self, location: &GuestPtrMut<'a, Self>) {
+        // location is guaranteed to be in GuestMemory and aligned to 4
+        unsafe {
+            let raw_ptr: *mut u32 = location.as_raw() as *mut u32;
+            raw_ptr.write(self.region.start);
+        }
     }
 }
 
@@ -204,6 +254,16 @@ pub struct GuestRef<'a, T> {
     type_: PhantomData<T>,
 }
 
+impl<'a, T> GuestRef<'a, T> {
+    pub fn as_ptr(&self) -> GuestPtr<'a, T> {
+        GuestPtr {
+            mem: self.mem,
+            region: self.region,
+            type_: self.type_,
+        }
+    }
+}
+
 impl<'a, T> ::std::ops::Deref for GuestRef<'a, T>
 where
     T: GuestTypeCopy,
@@ -218,20 +278,6 @@ where
     }
 }
 
-impl<'a, T> GuestRef<'a, T>
-where
-    T: GuestTypeRef<'a>,
-{
-    pub fn from_guest(&self) -> Result<T::Ref, GuestError> {
-        let ptr = GuestPtr {
-            mem: self.mem,
-            region: self.region,
-            type_: self.type_,
-        };
-        GuestTypeRef::from_guest(&ptr)
-    }
-}
-
 impl<'a, T> Drop for GuestRef<'a, T> {
     fn drop(&mut self) {
         let mut borrows = self.mem.borrows.borrow_mut();
@@ -246,6 +292,23 @@ pub struct GuestRefMut<'a, T> {
     type_: PhantomData<T>,
 }
 
+impl<'a, T> GuestRefMut<'a, T> {
+    pub fn as_ptr(&self) -> GuestPtr<'a, T> {
+        GuestPtr {
+            mem: self.mem,
+            region: self.region,
+            type_: self.type_,
+        }
+    }
+    pub fn as_ptr_mut(&self) -> GuestPtrMut<'a, T> {
+        GuestPtrMut {
+            mem: self.mem,
+            region: self.region,
+            type_: self.type_,
+        }
+    }
+}
+
 impl<'a, T> ::std::ops::Deref for GuestRefMut<'a, T>
 where
     T: GuestTypeCopy,
@@ -273,20 +336,6 @@ where
     }
 }
 
-impl<'a, T> GuestRefMut<'a, T>
-where
-    T: GuestTypeRef<'a>,
-{
-    pub fn from_guest(&self) -> Result<T::Ref, GuestError> {
-        let ptr = GuestPtr {
-            mem: self.mem,
-            region: self.region,
-            type_: self.type_,
-        };
-        GuestTypeRef::from_guest(&ptr)
-    }
-}
-
 impl<'a, T> Drop for GuestRefMut<'a, T> {
     fn drop(&mut self) {
         let mut borrows = self.mem.borrows.borrow_mut();

src/lib.rs

@@ -36,22 +36,20 @@ pub mod test {
             *a_better_excuse_ref = a_lamer_excuse;
 
             // Read ptr value from mutable ptr:
-            let mut one_layer_down: ::memory::GuestRefMut<::memory::GuestPtr<types::Excuse>> =
-                two_layers_of_excuses.as_ref_mut().map_err(|e| {
+            let one_layer_down: ::memory::GuestPtr<types::Excuse> =
+                two_layers_of_excuses.read_ptr_from_guest().map_err(|e| {
                     eprintln!("one_layer_down error: {}", e);
                     types::Errno::InvalidArg
                 })?;
 
             // Read enum value from that ptr:
-            let two_layers_down: ::memory::GuestRef<types::Excuse> =
-                one_layer_down.from_guest().map_err(|e| {
+            let two_layers_down: types::Excuse = *one_layer_down.as_ref().map_err(|e| {
                 eprintln!("two_layers_down error: {}", e);
                 types::Errno::InvalidArg
             })?;
 
             // Write ptr value to mutable ptr:
-            // FIXME this is still impossible...
-            // two_layers_of_excuses.write_guest(&a_better_excuse_by_reference)
+            two_layers_of_excuses.write_ptr_to_guest(&a_better_excuse_by_reference.as_immut());
 
             println!(
                 "BAZ: excuse: {:?}, better excuse: {:?}, lamer excuse: {:?}, two layers down: {:?}",