Determining Preconditions

Where do you find the safety preconditions?

# // Copyright 2026 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn main() {
    let b: *mut i32 = std::ptr::null_mut();
    println!("{:?}", b.as_mut());
}

Attempt to compile the program to trigger the compiler error ("error[E0133]: call to unsafe function ...").

Ask: “Where would you look if you wanted to know the preconditions for a function? Here we need to understand when it's safe to convert from a null pointer to a mutable reference.”

Locations to look:

• A function's API documentation, especially its safety section
• The source code and its internal safety comments
• Module documentation
• Rust Reference

Consult the documentation for the as_mut method.

Highlight Safety section.

Safety

When calling this method, you have to ensure that either the pointer is null or the pointer is convertible to a reference.

Click the "convertible to a reference" hyperlink to the "Pointer to reference conversion"

Track down the rules for converting a pointer to a reference, i.e., whether it is "dereferenceable".

Consider the implications of this excerpt (Rust 1.90.0) "You must enforce Rust’s aliasing rules. The exact aliasing rules are not decided yet, ..."