remove the memcpy-on-equal-ptrs assumption

Author: RalfJung

library/core/src/lib.rs

@@ -24,9 +24,8 @@
 //!   which are generated by Rust codegen backends. Additionally, this library can make explicit
 //!   calls to `strlen`. Their signatures are the same as found in C, but there are extra
 //!   assumptions about their semantics: For `memcpy`, `memmove`, `memset`, `memcmp`, and `bcmp`, if
-//!   the `n` parameter is 0, the function is assumed to not be UB. Furthermore, for `memcpy`, if
-//!   source and target pointer are equal, the function is assumed to not be UB.
-//!   (Note that these are standard assumptions among compilers:
+//!   the `n` parameter is 0, the function is assumed to not be UB, even if the pointers are NULL or
+//!   dangling. (Note that making extra assumptions about these functions is common among compilers:
 //!   [clang](https://reviews.llvm.org/D86993) and [GCC](https://gcc.gnu.org/onlinedocs/gcc/Standards.html#C-Language) do the same.)
 //!   These functions are often provided by the system libc, but can also be provided by the
 //!   [compiler-builtins crate](https://crates.io/crates/compiler_builtins).