Remove use of offset_of! from wasmtime-environ.

wasmtime-environ is meant to support cross compilation, so it shouldn't
have dependencies on target layout of structs. This moves the layout
back into wasmtime-execute, and adds a system of asserts for checking
that wasmtime-environ's offsets stay in sync.

lib/environ/src/vmoffsets.rs

@@ -0,0 +1,145 @@
+/// This class computes offsets to fields within `VMContext` and other
+/// related structs that JIT code accesses directly.
+pub struct VMOffsets {
+    pointer_size: u8,
+}
+
+impl VMOffsets {
+    /// Return a new `VMOffsets` instance, for a given pointer size.
+    pub fn new(pointer_size: u8) -> Self {
+        Self { pointer_size }
+    }
+}
+
+/// Offsets for `wasmtime_execute::VMMemory`.
+impl VMOffsets {
+    /// The offset of the `base` field.
+    pub fn vmmemory_base(&self) -> u8 {
+        0 * self.pointer_size
+    }
+
+    /// The offset of the `current_length` field.
+    pub fn vmmemory_current_length(&self) -> u8 {
+        1 * self.pointer_size
+    }
+
+    /// Return the size of `VMMemory`.
+    pub fn size_of_vmmemory(&self) -> u8 {
+        2 * self.pointer_size
+    }
+}
+
+/// Offsets for `wasmtime_execute::VMGlobal`.
+impl VMOffsets {
+    /// Return the size of `VMGlobal`.
+    pub fn size_of_vmglobal(&self) -> u8 {
+        8
+    }
+}
+
+/// Offsets for `wasmtime_execute::VMTable`.
+impl VMOffsets {
+    /// The offset of the `base` field.
+    pub fn vmtable_base(&self) -> u8 {
+        0 * self.pointer_size
+    }
+
+    /// The offset of the `current_elements` field.
+    pub fn vmtable_current_elements(&self) -> u8 {
+        1 * self.pointer_size
+    }
+
+    /// Return the size of `VMTable`.
+    pub fn size_of_vmtable(&self) -> u8 {
+        2 * self.pointer_size
+    }
+}
+
+/// Offsets for `wasmtime_execute::VMContext`.
+impl VMOffsets {
+    /// The offset of the `memories` field.
+    pub fn vmctx_memories(&self) -> u8 {
+        0 * self.pointer_size
+    }
+
+    /// The offset of the `globals` field.
+    pub fn vmctx_globals(&self) -> u8 {
+        1 * self.pointer_size
+    }
+
+    /// The offset of the `tables` field.
+    pub fn vmctx_tables(&self) -> u8 {
+        2 * self.pointer_size
+    }
+
+    /// The offset of the `instance` field.
+    #[allow(dead_code)]
+    pub fn vmctx_instance(&self) -> u8 {
+        3 * self.pointer_size
+    }
+
+    /// Return the size of `VMContext`.
+    #[allow(dead_code)]
+    pub fn size_of_vmctx(&self) -> u8 {
+        4 * self.pointer_size
+    }
+
+    /// Return the offset from the `memories` pointer to `VMMemory` index `index`.
+    pub fn index_vmmemory(&self, index: u32) -> i32 {
+        cast::i32(
+            index
+                .checked_mul(u32::from(self.size_of_vmmemory()))
+                .unwrap(),
+        ).unwrap()
+    }
+
+    /// Return the offset from the `globals` pointer to `VMGlobal` index `index`.
+    pub fn index_vmglobal(&self, index: u32) -> i32 {
+        cast::i32(
+            index
+                .checked_mul(u32::from(self.size_of_vmglobal()))
+                .unwrap(),
+        ).unwrap()
+    }
+
+    /// Return the offset from the `tables` pointer to `VMTable` index `index`.
+    pub fn index_vmtable(&self, index: u32) -> i32 {
+        cast::i32(
+            index
+                .checked_mul(u32::from(self.size_of_vmtable()))
+                .unwrap(),
+        ).unwrap()
+    }
+
+    /// Return the offset from the `memories` pointer to the `base` field in
+    /// `VMMemory` index `index`.
+    pub fn index_vmmemory_base(&self, index: u32) -> i32 {
+        self.index_vmmemory(index)
+            .checked_add(i32::from(self.vmmemory_base()))
+            .unwrap()
+    }
+
+    /// Return the offset from the `memories` pointer to the `current_length` field in
+    /// `VMMemory` index `index`.
+    pub fn index_vmmemory_current_length(&self, index: u32) -> i32 {
+        self.index_vmmemory(index)
+            .checked_add(i32::from(self.vmmemory_current_length()))
+            .unwrap()
+    }
+
+    /// Return the offset from the `tables` pointer to the `base` field in
+    /// `VMTable` index `index`.
+    pub fn index_vmtable_base(&self, index: u32) -> i32 {
+        self.index_vmtable(index)
+            .checked_add(i32::from(self.vmtable_base()))
+            .unwrap()
+    }
+
+    /// Return the offset from the `tables` pointer to the `current_elements` field in
+    /// `VMTable` index `index`.
+    pub fn index_vmtable_current_elements(&self, index: u32) -> i32 {
+        self.index_vmtable(index)
+            .checked_add(i32::from(self.vmtable_current_elements()))
+            .unwrap()
+    }
+}