Merge rust-bitcoin/rust-bitcoin#2972: Use index size rather than pointer size to enforce convertibility of `u32` to `usize`

c427d8b213 bitcoin: Compile time assert on index size (Tobin C. Harding)
49a6acc1a0 internals: Remove double parenthesis in const_assert (Tobin C. Harding)
2300b285ef units: Remove compile time pointer width check (Tobin C. Harding)

Pull request description:

  3 patches in preparation for other size related work, this PR does not touch the `ToU64` issue which will be handled separately.

  - Patch 1: Don't check pointer width in `units` because its not consensus code
  - Patch 2: Modify internal macro `const_assert`
  - Patch 3: Use index size to enforce not building on a 16 bit machine

ACKs for top commit:
  Kixunil:
    ACK c427d8b213 though I think the last commit was kinda a waste of time and it should have been adding the trait instead or leave it for later.
  apoelstra:
    ACK c427d8b213 successfully ran local tests; unsure if we want to merg this or wait for #3215

Tree-SHA512: 823df5b6a5af3265bce2422c00d287f45816faeb5f965685650ac974a1bd441cf548e25ac2962591732ff221bee91a55703da936382eb166c014ca5d4129edf8

bitcoin/src/lib.rs

@@ -37,12 +37,19 @@
 #![allow(clippy::manual_range_contains)] // More readable than clippy's format.
 #![allow(clippy::needless_borrows_for_generic_args)] // https://github.com/rust-lang/rust-clippy/issues/12454
 
-// Disable 16-bit support at least for now as we can't guarantee it yet.
+use core::mem;
-#[cfg(target_pointer_width = "16")]
+
-compile_error!(
+// We only support machines with index size of 4 bytes or more.
-    "rust-bitcoin currently only supports architectures with pointers wider than 16 bits, let us
+//
-    know if you want 16-bit support. Note that we do NOT guarantee that we will implement it!"
+// Bitcoin consensus code relies on being able to have containers with more than 65536 (2^16)
-);
+// entries in them so we cannot support consensus logic on machines that only have 16-bit memory
+// addresses.
+//
+// We specifically do not use `target_pointer_width` because of the possibility that pointer width
+// does not equal index size.
+//
+// ref: https://github.com/rust-bitcoin/rust-bitcoin/pull/2929#discussion_r1661848565
+internals::const_assert!(mem::size_of::<usize>() >= 4);
 
 #[cfg(bench)]
 extern crate test;

internals/src/macros.rs

@@ -139,9 +139,9 @@ macro_rules! debug_from_display {
 /// Asserts a boolean expression at compile time.
 #[macro_export]
 macro_rules! const_assert {
-    ($x:expr) => {{
+    ($x:expr) => {
         const _: [(); 0 - !$x as usize] = [];
-    }};
+    };
 }
 
 /// Derives `From<core::convert::Infallible>` for the given type.

units/src/lib.rs

@@ -14,13 +14,6 @@
 #![allow(clippy::needless_borrows_for_generic_args)] // https://github.com/rust-lang/rust-clippy/issues/12454
 #![no_std]
 
-// Disable 16-bit support at least for now as we can't guarantee it yet.
-#[cfg(target_pointer_width = "16")]
-compile_error!(
-    "rust-bitcoin currently only supports architectures with pointers wider than 16 bits, let us
-    know if you want 16-bit support. Note that we do NOT guarantee that we will implement it!"
-);
-
 #[cfg(feature = "alloc")]
 extern crate alloc;