diff --git a/crates/generate/src/funcs.rs b/crates/generate/src/funcs.rs
index de2c7413da..f624e15146 100644
--- a/crates/generate/src/funcs.rs
+++ b/crates/generate/src/funcs.rs
@@ -97,7 +97,7 @@ pub fn define_func(names: &Names, func: &witx::InterfaceFunc) -> TokenStream {
             Err(e) => { return #err_val; },
         };
         #(#marshal_rets)*
-        let success:#err_type = ::memory::GuestError::success();
+        let success:#err_type = ::memory::GuestErrorType::success();
         #abi_ret::from(success)
     })
 }
@@ -111,7 +111,7 @@ fn marshal_arg(
     let interface_typename = names.type_ref(&tref);
     let name = names.func_param(&param.name);
 
-    let error_handling = |method| -> TokenStream {
+    let error_handling: TokenStream = {
         if let Some(tref) = error_type {
             let abi_ret = match tref.type_().passed_by() {
                 witx::TypePassedBy::Value(atom) => names.atom_type(atom),
@@ -119,7 +119,7 @@ fn marshal_arg(
             };
             let err_typename = names.type_ref(&tref);
             quote! {
-                let err: #err_typename = ::memory::GuestError::#method(e, ctx);
+                let err: #err_typename = ::memory::GuestErrorType::from_error(e, ctx);
                 return #abi_ret::from(err);
             }
         } else {
@@ -128,15 +128,13 @@ fn marshal_arg(
             }
         }
     };
-    let value_error_handling = error_handling(quote!(from_value_error));
-    let memory_error_handling = error_handling(quote!(from_memory_error));
 
     let try_into_conversion = quote! {
         use ::std::convert::TryInto;
         let #name: #interface_typename = match #name.try_into() {
             Ok(a) => a,
             Err(e) => {
-                #value_error_handling
+                #error_handling
             }
         };
     };
@@ -151,7 +149,7 @@ fn marshal_arg(
                 let #name: #interface_typename = match (#name as i32).try_into() {
                     Ok(a) => a,
                     Err(e) => {
-                        #value_error_handling
+                        #error_handling
                     }
                 }
             },
@@ -172,7 +170,7 @@ fn marshal_arg(
                 let #name = match memory.ptr_mut::<#pointee_type>(#name as u32) {
                     Ok(p) => p,
                     Err(e) => {
-                        #memory_error_handling
+                        #error_handling
                     }
                 };
             }
@@ -183,7 +181,7 @@ fn marshal_arg(
                 let #name = match memory.ptr::<#pointee_type>(#name as u32) {
                     Ok(p) => p,
                     Err(e) => {
-                        #memory_error_handling
+                        #error_handling
                     }
                 };
             }
diff --git a/crates/generate/src/types.rs b/crates/generate/src/types.rs
index a03b4003a2..d385ae3715 100644
--- a/crates/generate/src/types.rs
+++ b/crates/generate/src/types.rs
@@ -55,18 +55,18 @@ fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenS
         }
 
         impl ::std::convert::TryFrom<#repr> for #ident {
-            type Error = ::memory::GuestValueError;
-            fn try_from(value: #repr) -> Result<#ident, ::memory::GuestValueError> {
+            type Error = ::memory::GuestError;
+            fn try_from(value: #repr) -> Result<#ident, ::memory::GuestError> {
                 match value as usize {
                     #(#tryfrom_repr_cases),*,
-                    _ => Err(::memory::GuestValueError::InvalidEnum(stringify!(#ident))),
+                    _ => Err(::memory::GuestError::InvalidEnumValue(stringify!(#ident))),
                 }
             }
         }
 
         impl ::std::convert::TryFrom<#abi_repr> for #ident {
-            type Error = ::memory::GuestValueError;
-            fn try_from(value: #abi_repr) -> Result<#ident, ::memory::GuestValueError> {
+            type Error = ::memory::GuestError;
+            fn try_from(value: #abi_repr) -> Result<#ident, ::memory::GuestError> {
                 #ident::try_from(value as #repr)
             }
         }
@@ -95,7 +95,7 @@ fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenS
         }
 
         impl ::memory::GuestTypeCopy for #ident {
-            fn read_val<P: ::memory::GuestPtrRead<#ident>>(src: &P) -> Result<#ident, ::memory::GuestValueError> {
+            fn read_val<'a, P: ::memory::GuestPtrRead<'a, #ident>>(src: &P) -> Result<#ident, ::memory::GuestError> {
                 use ::std::convert::TryInto;
                 let val = unsafe { ::std::ptr::read_unaligned(src.ptr() as *const #repr) };
                 val.try_into()
diff --git a/crates/memory/src/error.rs b/crates/memory/src/error.rs
new file mode 100644
index 0000000000..7c5632ca05
--- /dev/null
+++ b/crates/memory/src/error.rs
@@ -0,0 +1,12 @@
+use crate::Region;
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum GuestError {
+    #[error("Invalid enum value {0}")]
+    InvalidEnumValue(&'static str),
+    #[error("Out of bounds: {0:?}")]
+    PtrOutOfBounds(Region),
+    #[error("Borrowed: {0:?}")]
+    PtrBorrowed(Region),
+}
diff --git a/crates/memory/src/guest_type.rs b/crates/memory/src/guest_type.rs
index 324ca5bf09..6233888540 100644
--- a/crates/memory/src/guest_type.rs
+++ b/crates/memory/src/guest_type.rs
@@ -1,24 +1,17 @@
-use crate::{GuestPtrMut, GuestPtrRead, MemoryError};
-use thiserror::Error;
+use crate::{GuestError, GuestPtrMut, GuestPtrRead};
 
 pub trait GuestType: Sized {
     fn size() -> u32;
     fn name() -> &'static str;
 }
 
-#[derive(Debug, Error)]
-pub enum GuestValueError {
-    #[error("Invalid enum {0}")]
-    InvalidEnum(&'static str),
-}
-
 pub trait GuestTypeCopy: GuestType + Copy {
-    fn read_val<P: GuestPtrRead<Self>>(src: &P) -> Result<Self, GuestValueError>;
+    fn read_val<'a, P: GuestPtrRead<'a, Self>>(src: &P) -> Result<Self, GuestError>;
     fn write_val(val: Self, dest: &GuestPtrMut<Self>);
 }
 
 pub trait GuestTypeClone: GuestType + Clone {
-    fn read_ref<P: GuestPtrRead<Self>>(src: &P, dest: &mut Self) -> Result<(), GuestValueError>;
+    fn read_ref<'a, P: GuestPtrRead<'a, Self>>(src: &P, dest: &mut Self) -> Result<(), GuestError>;
     fn write_ref(val: &Self, dest: &GuestPtrMut<Self>);
 }
 
@@ -26,7 +19,7 @@ impl<T> GuestTypeClone for T
 where
     T: GuestTypeCopy,
 {
-    fn read_ref<P: GuestPtrRead<Self>>(src: &P, dest: &mut T) -> Result<(), GuestValueError> {
+    fn read_ref<'a, P: GuestPtrRead<'a, Self>>(src: &P, dest: &mut T) -> Result<(), GuestError> {
         let val = GuestTypeCopy::read_val(src)?;
         *dest = val;
         Ok(())
@@ -49,7 +42,7 @@ macro_rules! builtin_copy {
         }
 
         impl GuestTypeCopy for $t {
-            fn read_val<P: GuestPtrRead<$t>>(src: &P) -> Result<$t, GuestValueError> {
+            fn read_val<'a, P: GuestPtrRead<'a, $t>>(src: &P) -> Result<$t, GuestError> {
                 Ok(unsafe {
                     ::std::ptr::read_unaligned(src.ptr() as *const $t)
                 })
@@ -67,9 +60,8 @@ macro_rules! builtin_copy {
 // These definitions correspond to all the witx BuiltinType variants that are Copy:
 builtin_copy!(u8, i8, u16, i16, u32, i32, u64, i64, f32, f64, usize, char);
 
-pub trait GuestError {
+pub trait GuestErrorType {
     type Context;
     fn success() -> Self;
-    fn from_memory_error(memory_error: MemoryError, ctx: &mut Self::Context) -> Self;
-    fn from_value_error(value_error: GuestValueError, ctx: &mut Self::Context) -> Self;
+    fn from_error(e: GuestError, ctx: &mut Self::Context) -> Self;
 }
diff --git a/crates/memory/src/lib.rs b/crates/memory/src/lib.rs
index 1099776f95..f8705d6c80 100644
--- a/crates/memory/src/lib.rs
+++ b/crates/memory/src/lib.rs
@@ -1,8 +1,10 @@
 mod borrow;
+mod error;
 mod guest_type;
 mod memory;
 mod region;
 
-pub use guest_type::{GuestError, GuestType, GuestTypeClone, GuestTypeCopy, GuestValueError};
-pub use memory::{GuestMemory, GuestPtr, GuestPtrMut, GuestPtrRead, MemoryError};
+pub use error::GuestError;
+pub use guest_type::{GuestErrorType, GuestType, GuestTypeClone, GuestTypeCopy};
+pub use memory::{GuestMemory, GuestPtr, GuestPtrMut, GuestPtrRead};
 pub use region::Region;
diff --git a/crates/memory/src/memory.rs b/crates/memory/src/memory.rs
index 64fa192953..3af1a0413d 100644
--- a/crates/memory/src/memory.rs
+++ b/crates/memory/src/memory.rs
@@ -1,11 +1,9 @@
 use std::cell::RefCell;
 use std::marker::PhantomData;
 use std::rc::Rc;
-use thiserror::Error;
 
 use crate::borrow::{BorrowHandle, GuestBorrows};
-use crate::guest_type::GuestType;
-use crate::region::Region;
+use crate::{GuestError, GuestType, Region};
 
 pub struct GuestMemory<'a> {
     ptr: *mut u8,
@@ -30,13 +28,13 @@ impl<'a> GuestMemory<'a> {
             && r.start < (self.len - r.len)
     }
 
-    pub fn ptr<T: GuestType>(&'a self, at: u32) -> Result<GuestPtr<'a, T>, MemoryError> {
+    pub fn ptr<T: GuestType>(&'a self, at: u32) -> Result<GuestPtr<'a, T>, GuestError> {
         let region = Region {
             start: at,
             len: T::size(),
         };
         if !self.contains(region) {
-            Err(MemoryError::OutOfBounds(region))?;
+            Err(GuestError::PtrOutOfBounds(region))?;
         }
         let mut borrows = self.borrows.borrow_mut();
         if let Some(handle) = borrows.borrow_immut(region) {
@@ -47,17 +45,17 @@ impl<'a> GuestMemory<'a> {
                 type_: PhantomData,
             })
         } else {
-            Err(MemoryError::Borrowed(region))
+            Err(GuestError::PtrBorrowed(region))
         }
     }
 
-    pub fn ptr_mut<T: GuestType>(&'a self, at: u32) -> Result<GuestPtrMut<'a, T>, MemoryError> {
+    pub fn ptr_mut<T: GuestType>(&'a self, at: u32) -> Result<GuestPtrMut<'a, T>, GuestError> {
         let region = Region {
             start: at,
             len: T::size(),
         };
         if !self.contains(region) {
-            Err(MemoryError::OutOfBounds(region))?;
+            Err(GuestError::PtrOutOfBounds(region))?;
         }
         let mut borrows = self.borrows.borrow_mut();
         if let Some(handle) = borrows.borrow_mut(region) {
@@ -68,13 +66,19 @@ impl<'a> GuestMemory<'a> {
                 type_: PhantomData,
             })
         } else {
-            Err(MemoryError::Borrowed(region))
+            Err(GuestError::PtrBorrowed(region))
         }
     }
 }
 
-pub trait GuestPtrRead<T> {
-    fn ptr(&self) -> *const u8;
+/// These methods should not be used by the end user - just by implementations of the
+/// GuestValueClone and GuestValueCopy traits!
+pub trait GuestPtrRead<'a, T> {
+    fn mem(&self) -> &'a GuestMemory<'a>;
+    fn region(&self) -> &Region;
+    fn ptr(&self) -> *const u8 {
+        (self.mem().ptr as usize + self.region().start as usize) as *const u8
+    }
 }
 
 pub struct GuestPtr<'a, T> {
@@ -84,9 +88,31 @@ pub struct GuestPtr<'a, T> {
     type_: PhantomData<T>,
 }
 
-impl<'a, T: GuestType> GuestPtrRead<T> for GuestPtr<'a, T> {
-    fn ptr(&self) -> *const u8 {
-        (self.mem.ptr as usize + self.region.start as usize) as *const u8
+impl<'a, T> GuestPtrRead<'a, T> for GuestPtr<'a, T> {
+    fn mem(&self) -> &'a GuestMemory<'a> {
+        self.mem
+    }
+    fn region(&self) -> &Region {
+        &self.region
+    }
+}
+
+impl<'a, T> GuestType for GuestPtr<'a, T> {
+    fn size() -> u32 {
+        4
+    }
+    fn name() -> &'static str {
+        "GuestPtr<...>"
+    }
+}
+
+impl<'a, T: GuestType> GuestPtr<'a, T> {
+    pub fn read_ptr<P: GuestPtrRead<'a, Self>>(src: &P) -> Result<Self, GuestError> {
+        let raw_ptr = unsafe { ::std::ptr::read_unaligned(src.ptr() as *const u32) };
+        src.mem().ptr(raw_ptr)
+    }
+    pub fn write_ptr(ptr: &Self, dest: &GuestPtrMut<Self>) {
+        unsafe { ::std::ptr::write_unaligned(dest.ptr_mut() as *mut u32, ptr.region.start) }
     }
 }
 
@@ -104,9 +130,12 @@ pub struct GuestPtrMut<'a, T> {
     type_: PhantomData<T>,
 }
 
-impl<'a, T: GuestType> GuestPtrRead<T> for GuestPtrMut<'a, T> {
-    fn ptr(&self) -> *const u8 {
-        (self.mem.ptr as usize + self.region.start as usize) as *const u8
+impl<'a, T> GuestPtrRead<'a, T> for GuestPtrMut<'a, T> {
+    fn mem(&self) -> &'a GuestMemory<'a> {
+        self.mem
+    }
+    fn region(&self) -> &Region {
+        &self.region
     }
 }
 
@@ -115,6 +144,7 @@ impl<'a, T> GuestPtrMut<'a, T> {
         (self.mem.ptr as usize + self.region.start as usize) as *mut u8
     }
 }
+
 impl<'a, T> Drop for GuestPtrMut<'a, T> {
     fn drop(&mut self) {
         let mut borrows = self.mem.borrows.borrow_mut();
@@ -122,10 +152,29 @@ impl<'a, T> Drop for GuestPtrMut<'a, T> {
     }
 }
 
-#[derive(Debug, Error)]
-pub enum MemoryError {
-    #[error("Out of bounds: {0:?}")]
-    OutOfBounds(Region),
-    #[error("Borrowed: {0:?}")]
-    Borrowed(Region),
+impl<'a, T> GuestType for GuestPtrMut<'a, T> {
+    fn size() -> u32 {
+        4
+    }
+    fn name() -> &'static str {
+        "GuestPtrMut<...>"
+    }
+}
+
+impl<'a, T: GuestType> GuestPtrMut<'a, T> {
+    pub fn read_ptr<P: GuestPtrRead<'a, Self>>(src: &P) -> Result<Self, GuestError> {
+        let raw_ptr = unsafe { ::std::ptr::read_unaligned(src.ptr() as *const u32) };
+        src.mem().ptr_mut(raw_ptr)
+    }
+    pub fn write_ptr(ptr: &Self, dest: &GuestPtrMut<Self>) {
+        unsafe { ::std::ptr::write_unaligned(dest.ptr_mut() as *mut u32, ptr.region.start) }
+    }
+
+    pub fn as_immut(self) -> GuestPtr<'a, T> {
+        let mem = self.mem;
+        let start = self.region.start;
+        drop(self);
+        mem.ptr(start)
+            .expect("can borrow just-dropped mutable region as immut")
+    }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 2f8f782965..fd287a0f0c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -3,8 +3,7 @@ pub mod test {
     generate::from_witx!("test.witx");
 
     pub struct WasiCtx {
-        mem_errors: Vec<::memory::MemoryError>,
-        value_errors: Vec<::memory::GuestValueError>,
+        guest_errors: Vec<::memory::GuestError>,
     }
 
     impl foo::Foo for WasiCtx {
@@ -17,43 +16,65 @@ pub mod test {
             excuse: types::Excuse,
             a_better_excuse_by_reference: ::memory::GuestPtrMut<types::Excuse>,
             a_lamer_excuse_by_reference: ::memory::GuestPtr<types::Excuse>,
+            two_layers_of_excuses: ::memory::GuestPtrMut<::memory::GuestPtr<types::Excuse>>,
         ) -> Result<(), types::Errno> {
             use memory::GuestTypeCopy;
+
+            // Read enum value from mutable:
             let a_better_excuse =
-                types::Excuse::read_val(&a_better_excuse_by_reference).map_err(|val_err| {
-                    eprintln!("a_better_excuse_by_reference value error: {:?}", val_err);
+                types::Excuse::read_val(&a_better_excuse_by_reference).map_err(|e| {
+                    eprintln!("a_better_excuse_by_reference error: {}", e);
                     types::Errno::InvalidArg
                 })?;
+
+            // Read enum value from immutable ptr:
             let a_lamer_excuse =
-                types::Excuse::read_val(&a_lamer_excuse_by_reference).map_err(|val_err| {
-                    eprintln!("a_lamer_excuse_by_reference value error: {:?}", val_err);
+                types::Excuse::read_val(&a_lamer_excuse_by_reference).map_err(|e| {
+                    eprintln!("a_lamer_excuse_by_reference error: {}", e);
                     types::Errno::InvalidArg
                 })?;
+
+            // Write enum to mutable ptr:
             types::Excuse::write_val(a_lamer_excuse, &a_better_excuse_by_reference);
 
+            // Read ptr value from mutable ptr:
+            let one_layer_down =
+                ::memory::GuestPtr::read_ptr(&two_layers_of_excuses).map_err(|e| {
+                    eprintln!("one_layer_down error: {}", e);
+                    types::Errno::InvalidArg
+                })?;
+
+            // Read enum value from that ptr:
+            let two_layers_down = types::Excuse::read_val(&one_layer_down).map_err(|e| {
+                eprintln!("two_layers_down error: {}", e);
+                types::Errno::InvalidArg
+            })?;
+
+            // Write ptr value to mutable ptr:
+            ::memory::GuestPtr::write_ptr(
+                &a_better_excuse_by_reference.as_immut(),
+                &two_layers_of_excuses,
+            );
+
             println!(
-                "BAZ: {:?} {:?} {:?}",
-                excuse, a_better_excuse, a_lamer_excuse
+                "BAZ: excuse: {:?}, better excuse: {:?}, lamer excuse: {:?}, two layers down: {:?}",
+                excuse, a_better_excuse, a_lamer_excuse, two_layers_down
             );
             Ok(())
         }
     }
 
     // Errno is used as a first return value in the functions above, therefore
-    // it must implement GuestError with type Context = WasiCtx.
+    // it must implement GuestErrorType with type Context = WasiCtx.
     // The context type should let you do logging or debugging or whatever you need
     // with these errors. We just push them to vecs.
-    impl ::memory::GuestError for types::Errno {
+    impl ::memory::GuestErrorType for types::Errno {
         type Context = WasiCtx;
         fn success() -> types::Errno {
             types::Errno::Ok
         }
-        fn from_memory_error(e: ::memory::MemoryError, ctx: &mut WasiCtx) -> types::Errno {
-            ctx.mem_errors.push(e);
-            types::Errno::InvalidArg
-        }
-        fn from_value_error(e: ::memory::GuestValueError, ctx: &mut WasiCtx) -> types::Errno {
-            ctx.value_errors.push(e);
+        fn from_error(e: ::memory::GuestError, ctx: &mut WasiCtx) -> types::Errno {
+            ctx.guest_errors.push(e);
             types::Errno::InvalidArg
         }
     }
diff --git a/test.witx b/test.witx
index d8e8848cc5..16964d8eea 100644
--- a/test.witx
+++ b/test.witx
@@ -23,5 +23,6 @@
     (param $an_excuse $excuse)
     (param $an_excuse_by_reference (@witx pointer $excuse))
     (param $a_lamer_excuse (@witx const_pointer $excuse))
+    (param $two_layers_of_excuses (@witx pointer (@witx const_pointer $excuse)))
     (result $error $errno))
 )