do not reference LLVM in our definition of UB

Author: RalfJung

src/behavior-considered-undefined.md

@@ -43,10 +43,11 @@ r[undefined.place-projection]
   [array/slice index expression][project-slice].
 
 r[undefined.alias]
-* Breaking the [pointer aliasing rules]. `Box<T>`, `&mut T` and `&T` follow
-  LLVM’s scoped [noalias] model, except if the `&T` contains an
-  [`UnsafeCell<U>`]. References and boxes must not be [dangling] while they are
-  live. The exact liveness duration is not specified, but some bounds exist:
+* Breaking the pointer aliasing rules. The exact aliasing rules are not determined yet, but here is a rough sketch of what the requirements look like:
+  `&T` must point to memory that is not mutated while they are live (except for data inside an [`UnsafeCell<U>`]),
+  and `&mut T` must point to memory that is not read or written by any pointer not derived from the reference and that no other reference points to while they are live.
+  `Box<T>` is treated similar to `&'static mut T` for the purpose of these rules.
+  The exact liveness duration is not specified, but some bounds exist:
   * For references, the liveness duration is upper-bounded by the syntactic
     lifetime assigned by the borrow checker; it cannot be live any *longer* than
     that lifetime.
@@ -56,9 +57,7 @@ r[undefined.alias]
     least as long as that function call, again except if the `&T` contains an
     [`UnsafeCell<U>`].
 
-  All this also applies when values of these
-  types are passed in a (nested) field of a compound type, but not behind
-  pointer indirections.
+  All this also applies when values of these types are passed in a (nested) field of a compound type, but not behind pointer indirections.
 
 r[undefined.immutable]
 * Mutating immutable bytes.
@@ -201,7 +200,7 @@ r[undefined.validity.never]
 
 r[undefined.validity.scalar]
 * An integer (`i*`/`u*`), floating point value (`f*`), or raw pointer must be
-  initialized, i.e., must not be obtained from [uninitialized memory][undef].
+  initialized, i.e., must not be obtained from uninitialized memory.
 
 r[undefined.validity.str]
 * A `str` value is treated like `[u8]`, i.e. it must be initialized.
@@ -248,10 +247,7 @@ reading uninitialized memory is permitted are inside `union`s and in "padding"
 
 [`bool`]: types/boolean.md
 [`const`]: items/constant-items.md
-[noalias]: http://llvm.org/docs/LangRef.html#noalias
-[pointer aliasing rules]: http://llvm.org/docs/LangRef.html#pointer-aliasing-rules
 [abi]: items/external-blocks.md#abi
-[undef]: http://llvm.org/docs/LangRef.html#undefined-values
 [`target_feature`]: attributes/codegen.md#the-target_feature-attribute
 [`UnsafeCell<U>`]: std::cell::UnsafeCell
 [Rustonomicon]: ../nomicon/index.html