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doc: clarifying unsafe code vs undefined behavior #19105

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17 changes: 8 additions & 9 deletions src/doc/reference.md
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Expand Up @@ -1117,13 +1117,13 @@ Rust:
##### Unsafe functions

Unsafe functions are functions that are not safe in all contexts and/or for all
possible inputs. Such a function must be prefixed with the keyword `unsafe`.
possible inputs. Such a function must be prefixed with the keyword `unsafe` and
can only be called from an `unsafe` block or another `unsafe` function.

##### Unsafe blocks

A block of code can also be prefixed with the `unsafe` keyword, to permit
calling `unsafe` functions or dereferencing raw pointers within a safe
function.
A block of code can be prefixed with the `unsafe` keyword, to permit calling
`unsafe` functions or dereferencing raw pointers within a safe function.

When a programmer has sufficient conviction that a sequence of potentially
unsafe operations is actually safe, they can encapsulate that sequence (taken
Expand All @@ -1143,12 +1143,11 @@ represented with reference-counted pointers in safe code. By using `unsafe`
blocks to represent the reverse links as raw pointers, it can be implemented
with only boxes.

##### Behavior considered unsafe
##### Behavior considered undefined

This is a list of behavior which is forbidden in all Rust code. Type checking
provides the guarantee that these issues are never caused by safe code. An
`unsafe` block or function is responsible for never invoking this behaviour or
exposing an API making it possible for it to occur in safe code.
The following is a list of behavior which is forbidden in all Rust code,
including within `unsafe` blocks and `unsafe` functions. Type checking provides
the guarantee that these issues are never caused by safe code.

* Data races
* Dereferencing a null/dangling raw pointer
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