From 7dc779f328afe24c4f613fb49dfee0c2dd90f4ed Mon Sep 17 00:00:00 2001
From: Andrew Gaspar <andrew.gaspar@outlook.com>
Date: Fri, 23 Oct 2020 01:16:25 -0600
Subject: [PATCH] Support tuples in offset_of (#48)

Support tuple types via new offset_of_tuple / raw_field_tuple macros
---
 README.md        |  1 +
 build.rs         |  3 +++
 src/offset_of.rs | 64 +++++++++++++++++++++++++++++++++++++++++++++---
 src/raw_field.rs | 31 +++++++++++++++++++++++
 4 files changed, 96 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index b035674..e73a44b 100644
--- a/README.md
+++ b/README.md
@@ -6,6 +6,7 @@ C-Like `offset_of` functionality for Rust structs.
 
 Introduces the following macros:
  * `offset_of!` for obtaining the offset of a member of a struct.
+ * `offset_of_tuple!` for obtaining the offset of a member of a tuple. (Requires Rust 1.20+)
  * `span_of!` for obtaining the range that a field, or fields, span.
 
 `memoffset` works under `no_std` environments.
diff --git a/build.rs b/build.rs
index 5749207..c83bff2 100644
--- a/build.rs
+++ b/build.rs
@@ -4,6 +4,9 @@ fn main() {
     let ac = autocfg::new();
 
     // Check for a minimum version for a few features
+    if ac.probe_rustc_version(1, 20) {
+        println!("cargo:rustc-cfg=tuple_ty");
+    }
     if ac.probe_rustc_version(1, 31) {
         println!("cargo:rustc-cfg=allow_clippy");
     }
diff --git a/src/offset_of.rs b/src/offset_of.rs
index bb2bce1..2ffb79c 100644
--- a/src/offset_of.rs
+++ b/src/offset_of.rs
@@ -24,7 +24,7 @@
 #[macro_export]
 #[doc(hidden)]
 macro_rules! _memoffset__let_base_ptr {
-    ($name:ident, $type:path) => {
+    ($name:ident, $type:ty) => {
         // No UB here, and the pointer does not dangle, either.
         // But we have to make sure that `uninit` lives long enough,
         // so it has to be in the same scope as `$name`. That's why
@@ -38,7 +38,7 @@ macro_rules! _memoffset__let_base_ptr {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! _memoffset__let_base_ptr {
-    ($name:ident, $type:path) => {
+    ($name:ident, $type:ty) => {
         // No UB right here, but we will later dereference this pointer to
         // offset into a field, and that is UB because the pointer is dangling.
         let $name = $crate::mem::align_of::<$type>() as *const $type;
@@ -66,7 +66,7 @@ macro_rules! _memoffset_offset_from {
     };
 }
 
-/// Calculates the offset of the specified field from the start of the struct.
+/// Calculates the offset of the specified field from the start of the named struct.
 ///
 /// ## Examples
 /// ```
@@ -97,6 +97,30 @@ macro_rules! offset_of {
     }};
 }
 
+/// Calculates the offset of the specified field from the start of the tuple.
+///
+/// ## Examples
+/// ```
+/// #[macro_use]
+/// extern crate memoffset;
+///
+/// fn main() {
+///     assert!(offset_of_tuple!((u8, u32), 1) >= 0, "Tuples do not have a defined layout");
+/// }
+/// ```
+#[cfg(tuple_ty)]
+#[macro_export(local_inner_macros)]
+macro_rules! offset_of_tuple {
+    ($parent:ty, $field:tt) => {{
+        // Get a base pointer (non-dangling if rustc supports `MaybeUninit`).
+        _memoffset__let_base_ptr!(base_ptr, $parent);
+        // Get field pointer.
+        let field_ptr = raw_field_tuple!(base_ptr, $parent, $field);
+        // Compute offset.
+        _memoffset_offset_from!(field_ptr, base_ptr)
+    }};
+}
+
 #[cfg(test)]
 mod tests {
     #[test]
@@ -160,6 +184,19 @@ mod tests {
         assert_eq!(foo(Pair(0, 0)), 4);
     }
 
+    #[cfg(tuple_ty)]
+    #[test]
+    fn test_tuple_offset() {
+        let f = (0i32, 0.0f32, 0u8);
+        let f_ptr = &f as *const _;
+        let f1_ptr = &f.1 as *const _;
+
+        assert_eq!(
+            f1_ptr as usize - f_ptr as usize,
+            offset_of_tuple!((i32, f32, u8), 1)
+        );
+    }
+
     #[test]
     fn test_raw_field() {
         #[repr(C)]
@@ -180,6 +217,27 @@ mod tests {
         assert_eq!(f_ptr as usize + 8, raw_field!(f_ptr, Foo, c) as usize);
     }
 
+    #[cfg(tuple_ty)]
+    #[test]
+    fn test_raw_field_tuple() {
+        let t = (0u32, 0u8, false);
+        let t_ptr = &t as *const _;
+        let t_addr = t_ptr as usize;
+
+        assert_eq!(
+            &t.0 as *const _ as usize - t_addr,
+            raw_field_tuple!(t_ptr, (u32, u8, bool), 0) as usize - t_addr
+        );
+        assert_eq!(
+            &t.1 as *const _ as usize - t_addr,
+            raw_field_tuple!(t_ptr, (u32, u8, bool), 1) as usize - t_addr
+        );
+        assert_eq!(
+            &t.2 as *const _ as usize - t_addr,
+            raw_field_tuple!(t_ptr, (u32, u8, bool), 2) as usize - t_addr
+        );
+    }
+
     #[cfg(feature = "unstable_const")]
     #[test]
     fn const_offset() {
diff --git a/src/raw_field.rs b/src/raw_field.rs
index 51a4c41..16d01bb 100644
--- a/src/raw_field.rs
+++ b/src/raw_field.rs
@@ -63,6 +63,16 @@ macro_rules! _memoffset__field_check {
     };
 }
 
+/// Deref-coercion protection macro.
+#[macro_export]
+#[doc(hidden)]
+macro_rules! _memoffset__field_check_tuple {
+    ($type:ty, $field:tt) => {
+        // Make sure the type argument is a tuple
+        let (_, ..): $type;
+    };
+}
+
 /// Computes a const raw pointer to the given field of the given base pointer
 /// to the given parent type.
 ///
@@ -82,3 +92,24 @@ macro_rules! raw_field {
         }
     }};
 }
+
+/// Computes a const raw pointer to the given field of the given base pointer
+/// to the given parent tuple typle.
+///
+/// The `base` pointer *must not* be dangling, but it *may* point to
+/// uninitialized memory.
+#[cfg(tuple_ty)]
+#[macro_export(local_inner_macros)]
+macro_rules! raw_field_tuple {
+    ($base:expr, $parent:ty, $field:tt) => {{
+        _memoffset__field_check_tuple!($parent, $field);
+
+        // Get the field address.
+        // Crucially, we know that this will not trigger a deref coercion because
+        // of the field check we did above.
+        #[allow(unused_unsafe)] // for when the macro is used in an unsafe block
+        unsafe {
+            _memoffset__raw_const!((*($base as *const $parent)).$field)
+        }
+    }};
+}