clippy::mut_from_ref: This lint checks for functions that take immutable references and return mutable ones

fn foo(&Foo) -> &mut Bar { .. }

#[allow(clippy::mut_from_ref)]
fn my_function() { /* ... */ }

[lints.clippy]
mut_from_ref = "allow"

cargo clippy -- -A clippy::mut-from-ref

Clippy's own documentation

What it does

This lint checks for functions that take immutable references and return mutable ones. This will not trigger if no unsafe code exists as there are multiple safe functions which will do this transformation

To be on the conservative side, if there’s at least one mutable reference with the output lifetime, this lint will not trigger.

Why is this bad?

Creating a mutable reference which can be repeatably derived from an immutable reference is unsound as it allows creating multiple live mutable references to the same object.

This error actually lead to an interim Rust release 1.15.1.

Known problems

This pattern is used by memory allocators to allow allocating multiple objects while returning mutable references to each one. So long as different mutable references are returned each time such a function may be safe.

Example

fn foo(&Foo) -> &mut Bar { .. }

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

Related lints

• min_maxChecks for expressions where std::cmp::min and max are used to clamp values, but switched so that the result is constant
• mistyped_literal_suffixesWarns for mistyped suffix in literals
• modulo_oneChecks for getting the remainder of integer division by one or minus one
• never_loopthis loop never actually loops
• non_octal_unix_permissionsChecks for non-octal values used to set Unix file permissions
• nonsensical_open_optionsChecks for duplicate open options as well as combinations that make no sense

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker