From 176b647ccdfb1c89921783e3dd37d1b295fb6c3b Mon Sep 17 00:00:00 2001
From: Gilad Naaman <gilad@naaman.io>
Date: Mon, 4 Nov 2019 12:53:24 +0200
Subject: [PATCH] Added impl for constant offset_of

---
 .travis.yml      |  8 ++++++++
 Cargo.toml       |  4 ++++
 README.md        | 52 ++++++++++++++++++++++++++++++++++++++----------
 src/lib.rs       |  3 +++
 src/offset_of.rs | 51 +++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 107 insertions(+), 11 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index 1c7e80c..2090519 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,6 +16,14 @@ matrix:
     script:
       - sh ci/miri.sh
 
+  - env: CONST_OFFSET
+    rust: nightly
+    os: linux
+    script:  # `--lib` added to prevent doctests from being compiled.
+             # This is due to `unstable_const` requiring extra `feature(...)` directives
+             # which the doctests do not have.
+      - cargo test --verbose --features unstable_const --lib
+
   - env: RUSTFMT
     rust: 1.36.0
     install:
diff --git a/Cargo.toml b/Cargo.toml
index dbdfb3d..a7cf2e2 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,3 +14,7 @@ rustc_version = "0.2.3"
 
 [dev-dependencies]
 doc-comment = "0.3"
+
+[features]
+default = []
+unstable_const = []
diff --git a/README.md b/README.md
index d51cfd5..5d2ad77 100644
--- a/README.md
+++ b/README.md
@@ -34,20 +34,50 @@ extern crate memoffset;
 
 #[repr(C, packed)]
 struct Foo {
-	a: u32,
-	b: u32,
-	c: [u8; 5],
-	d: u32,
+    a: u32,
+    b: u32,
+    c: [u8; 5],
+    d: u32,
 }
 
 fn main() {
-	assert_eq!(offset_of!(Foo, b), 4);
-	assert_eq!(offset_of!(Foo, d), 4+4+5);
+    assert_eq!(offset_of!(Foo, b), 4);
+    assert_eq!(offset_of!(Foo, d), 4+4+5);
 
-	assert_eq!(span_of!(Foo, a),        0..4);
-	assert_eq!(span_of!(Foo, a ..  c),  0..8);
-	assert_eq!(span_of!(Foo, a ..= c),  0..13);
-	assert_eq!(span_of!(Foo, ..= d),    0..17);
-	assert_eq!(span_of!(Foo, b ..),     4..17);
+    assert_eq!(span_of!(Foo, a),        0..4);
+    assert_eq!(span_of!(Foo, a ..  c),  0..8);
+    assert_eq!(span_of!(Foo, a ..= c),  0..13);
+    assert_eq!(span_of!(Foo, ..= d),    0..17);
+    assert_eq!(span_of!(Foo, b ..),     4..17);
 }
 ```
+
+## Usage in constants ##
+`memoffset` has **experimental** support for compile-time `offset_of!` on a nightly compiler.
+
+In order to use it, you must enable the `unstable_const` crate feature and several compiler features.
+
+Cargo.toml:
+```toml
+[dependencies.memoffset]
+version = "0.5"
+features = ["unstable_const"]
+```
+
+Your crate root: (`lib.rs`/`main.rs`)
+```rust,ignore
+#![feature(const_transmute)]
+#![feature(const_ptr_offset_from)]
+#![feature(ptr_offset_from)]
+```
+
+and then:
+
+```rust,ignore
+struct Foo {
+    a: u32,
+    b: u32,
+}
+
+let foo = [0; offset_of!(Foo, b)] 
+```
\ No newline at end of file
diff --git a/src/lib.rs b/src/lib.rs
index 79ff21b..8d7d4f9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -57,6 +57,9 @@
 //! ```
 
 #![no_std]
+#![cfg_attr(feature = "unstable_const", feature(const_ptr_offset_from))]
+#![cfg_attr(feature = "unstable_const", feature(const_transmute))]
+#![cfg_attr(feature = "unstable_const", feature(ptr_offset_from))]
 
 #[macro_use]
 #[cfg(memoffset_doctests)]
diff --git a/src/offset_of.rs b/src/offset_of.rs
index 0346dce..0308274 100644
--- a/src/offset_of.rs
+++ b/src/offset_of.rs
@@ -76,6 +76,7 @@ macro_rules! _memoffset__field_check {
 ///     assert_eq!(offset_of!(Foo, b), 4);
 /// }
 /// ```
+#[cfg(not(feature = "unstable_const"))]
 #[macro_export(local_inner_macros)]
 macro_rules! offset_of {
     ($parent:path, $field:tt) => {{
@@ -94,6 +95,32 @@ macro_rules! offset_of {
     }};
 }
 
+#[cfg(feature = "unstable_const")]
+#[macro_export(local_inner_macros)]
+macro_rules! offset_of {
+    ($parent:path, $field:tt) => {{
+        _memoffset__field_check!($parent, $field);
+
+        // Get a base pointer.
+        // No UB here, and the pointer does not dangle, either.
+        let uninit = $crate::mem::MaybeUninit::<$parent>::uninit();
+        unsafe {
+            // This, on the other hand, *is* UB, since we're creating a reference
+            // to uninitialized data.
+            // Unfortunately it's the best we can do at the moment.
+            let base_ref = $crate::mem::transmute::<_, &$parent>(&uninit);
+            let base_u8_ptr = base_ref as *const _ as *const u8;
+
+            // This is another reference to uninitialized data.
+            // Crucially, we know that this will not trigger a deref coercion because
+            // of the `field_check!` we did above.
+            let field_u8_ptr = &base_ref.$field as *const _ as *const u8;
+            let offset = field_u8_ptr.offset_from(base_u8_ptr) as usize;
+            offset
+        }
+    }};
+}
+
 #[cfg(test)]
 mod tests {
     #[test]
@@ -145,4 +172,28 @@ mod tests {
 
         assert_eq!(offset_of!(sub::Foo, x), 0);
     }
+
+    #[test]
+    fn inside_generic_method() {
+        struct Pair<T, U>(T, U);
+
+        fn foo<T, U>(_: Pair<T, U>) -> usize {
+            offset_of!(Pair<T, U>, 1)
+        }
+
+        assert_eq!(foo(Pair(0, 0)), 4);
+    }
+
+    #[cfg(feature = "unstable_const")]
+    #[test]
+    fn const_offset() {
+        #[repr(C)]
+        struct Foo {
+            a: u32,
+            b: [u8; 2],
+            c: i64,
+        }
+
+        assert_eq!([0; offset_of!(Foo, b)].len(), 4);
+    }
 }