Add `size_of`, `size_of_val`, `align_of`, and `align_of_val` to the prelude

Many, many projects use `size_of` to get the size of a type. However,
it's also often equally easy to hardcode a size (e.g. `8` instead of
`size_of::<u64>()`). Minimizing friction in the use of `size_of` helps
ensure that people use it and make code more self-documenting.

The name `size_of` is unambiguous: the name alone, without any prefix or
path, is self-explanatory and unmistakeable for any other functionality.
Adding it to the prelude cannot produce any name conflicts, as any local
definition will silently shadow the one from the prelude. Thus, we don't
need to wait for a new edition prelude to add it.

Add `size_of_val`, `align_of`, and `align_of_val` as well, with similar
justification: widely useful, self-explanatory, unmistakeable for
anything else, won't produce conflicts.

4 files changed, 8 insertions, 2 deletions

diff --git a/library/std/src/io/error/repr_bitpacked.rs b/library/std/src/io/error/repr_bitpacked.rs
index 6f8d5e37775..c9d3934ad70 100644
--- a/library/std/src/io/error/repr_bitpacked.rs
+++ b/library/std/src/io/error/repr_bitpacked.rs
@@ -104,7 +104,6 @@
 
 use super::{Custom, ErrorData, ErrorKind, RawOsError, SimpleMessage};
 use core::marker::PhantomData;
-use core::mem::{align_of, size_of};
 use core::ptr::{self, NonNull};
 
 // The 2 least-significant bits are used as tag.
diff --git a/library/std/src/os/unix/net/ancillary.rs b/library/std/src/os/unix/net/ancillary.rs
index 0597fdcbd72..fe8e2be9372 100644
--- a/library/std/src/os/unix/net/ancillary.rs
+++ b/library/std/src/os/unix/net/ancillary.rs
@@ -3,7 +3,7 @@
 use super::{sockaddr_un, SocketAddr};
 use crate::io::{self, IoSlice, IoSliceMut};
 use crate::marker::PhantomData;
-use crate::mem::{size_of, zeroed};
+use crate::mem::zeroed;
 use crate::os::unix::io::RawFd;
 use crate::path::Path;
 use crate::ptr::{eq, read_unaligned};
diff --git a/library/std/src/prelude/common.rs b/library/std/src/prelude/common.rs
index 01936734d75..ceee3e33c3e 100644
--- a/library/std/src/prelude/common.rs
+++ b/library/std/src/prelude/common.rs
@@ -14,6 +14,9 @@ pub use crate::ops::{Drop, Fn, FnMut, FnOnce};
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(no_inline)]
 pub use crate::mem::drop;
+#[stable(feature = "size_of_prelude", since = "CURRENT_RUSTC_VERSION")]
+#[doc(no_inline)]
+pub use crate::mem::{align_of, align_of_val, size_of, size_of_val};
 
 // Re-exported types and traits
 #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/library/std/src/prelude/mod.rs b/library/std/src/prelude/mod.rs
index 0bdbab716ad..2d4639342bf 100644
--- a/library/std/src/prelude/mod.rs
+++ b/library/std/src/prelude/mod.rs
@@ -36,6 +36,10 @@
 //!   operations for both destructors and overloading `()`.
 //! * <code>[std::mem]::[drop]</code>, a convenience function for explicitly
 //!   dropping a value.
+//! * <code>[std::mem]::{[size_of], [size_of_val]}</code>, to get the size of
+//!   a type or value.
+//! * <code>[std::mem]::{[align_of], [align_of_val]}</code>, to get the
+//!   alignment of a type or value.
 //! * <code>[std::boxed]::[Box]</code>, a way to allocate values on the heap.
 //! * <code>[std::borrow]::[ToOwned]</code>, the conversion trait that defines
 //!   [`to_owned`], the generic method for creating an owned type from a