Auto merge of #3001 - RalfJung:align, r=RalfJung

add some interesting tests for alignment corner cases

`strange_enum_discriminant_offset` example found in rust-lang/rust#53998.

Author: bors

tests/fail/unaligned_pointers/field_requires_parent_struct_alignment.rs

@@ -0,0 +1,24 @@
+/// This tests that when a field sits at offset 0 in a 4-aligned struct, accessing the field
+/// requires alignment 4 even if the field type has lower alignment requirements.
+
+#[repr(C)]
+pub struct S {
+    x: u8,
+    y: u32,
+}
+
+unsafe fn foo(x: *const S) -> u8 {
+    unsafe { (*x).x } //~ERROR: accessing memory with alignment 1, but alignment 4 is required
+}
+
+fn main() {
+    unsafe {
+        let mem = [0u64; 16];
+        let odd_ptr = std::ptr::addr_of!(mem).cast::<u8>().add(1);
+        // `odd_ptr` is now not aligned enough for `S`.
+        // If accessing field `x` can exploit that it is at offset 0
+        // in a 4-aligned struct, that field access requires alignment 4,
+        // thus making this UB.
+        foo(odd_ptr.cast());
+    }
+}

tests/fail/unaligned_pointers/field_requires_parent_struct_alignment.stderr

@@ -0,0 +1,20 @@
+error: Undefined Behavior: accessing memory with alignment ALIGN, but alignment ALIGN is required
+  --> $DIR/field_requires_parent_struct_alignment.rs:LL:CC
+   |
+LL |     unsafe { (*x).x }
+   |              ^^^^^^ accessing memory with alignment ALIGN, but alignment ALIGN is required
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `foo` at $DIR/field_requires_parent_struct_alignment.rs:LL:CC
+note: inside `main`
+  --> $DIR/field_requires_parent_struct_alignment.rs:LL:CC
+   |
+LL |         foo(odd_ptr.cast());
+   |         ^^^^^^^^^^^^^^^^^^^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

tests/pass/align_strange_enum_discriminant_offset.rs

@@ -0,0 +1,23 @@
+#![allow(unused)]
+
+#[repr(u16)]
+enum DeviceKind {
+    Nil = 0,
+}
+
+#[repr(C, packed)]
+struct DeviceInfo {
+    endianness: u8,
+    device_kind: DeviceKind,
+}
+
+fn main() {
+    // The layout of `Option<(DeviceInfo, u64)>` is funny: it uses the
+    // `DeviceKind` enum as niche, so that is offset 1, but the niche type is u16!
+    // So despite the type having alignment 8 and the field type alignment 2,
+    // the actual alignment is 1.
+    let x = None::<(DeviceInfo, u8)>;
+    let y = None::<(DeviceInfo, u16)>;
+    let z = None::<(DeviceInfo, u64)>;
+    format!("{} {} {}", x.is_some(), y.is_some(), y.is_some());
+}