expand and better explain alignment check tests

Author: RalfJung

tests/compile-fail/unaligned_ptr_cast1.rs renamed to tests/compile-fail/unaligned_ptr1.rs

@@ -4,6 +4,6 @@
 fn main() {
     let x = [2u16, 3, 4]; // Make it big enough so we don't get an out-of-bounds error.
     let x = &x[0] as *const _ as *const u32;
-    // This must fail because alignment is violated
+    // This must fail because alignment is violated: the allocation's base is not sufficiently aligned.
     let _x = unsafe { *x }; //~ ERROR tried to access memory with alignment 2, but alignment 4 is required
 }

tests/compile-fail/unaligned_ptr2.rs

@@ -0,0 +1,10 @@
+// This should fail even without validation.
+// compile-flags: -Zmiri-disable-validation
+
+fn main() {
+    let x = [2u32, 3]; // Make it big enough so we don't get an out-of-bounds error.
+    let x = (x.as_ptr() as *const u8).wrapping_offset(3) as *const u32;
+    // This must fail because alignment is violated: the offset is not sufficiently aligned.
+    // Also make the offset not a power of 2, that used to ICE.
+    let _x = unsafe { *x }; //~ ERROR tried to access memory with alignment 1, but alignment 4 is required
+}

tests/compile-fail/unaligned_ptr_cast2.rs renamed to tests/compile-fail/unaligned_ptr3.rs

@@ -3,7 +3,7 @@
 
 fn main() {
     let x = [2u16, 3, 4, 5]; // Make it big enough so we don't get an out-of-bounds error.
-    let x = &x[0] as *const _ as *const *const u8;
+    let x = &x[0] as *const _ as *const *const u8; // cast to ptr-to-ptr, so that we load a ptr
     // This must fail because alignment is violated. Test specifically for loading pointers,
     // which have special code in miri's memory.
     let _x = unsafe { *x };

tests/compile-fail/unaligned_ptr_cast_zst.rs renamed to tests/compile-fail/unaligned_ptr_zst.rs

@@ -1,3 +1,6 @@
+// This should fail even without validation
+// compile-flags: -Zmiri-disable-validation
+
 fn main() {
     let x = &2u16;
     let x = x as *const _ as *const [u32; 0];