Static MACRO_EXTENDED_TEMPORARY_SCOPES 

pub static MACRO_EXTENDED_TEMPORARY_SCOPES: &Lint

The macro_extended_temporary_scopes lint detects borrowed temporary values in arguments to pin! and formatting macros which have longer lifetimes than intended due to a bug in the compiler. For more information on temporary scopes and lifetime extension, see the Rust Reference.

Example

fn main() {
    println!("{:?}{}", (), if cond() { &build_string() } else { "" });
}

{{produces}}

Recommended fix

To extend the lifetimes of temporaries borrowed in macro arguments, create separate definitions for them with let statements.

fn main() {
    let string = if cond() { &build_string() } else { "" };
    println!("{:?}{}", (), string);
}

Explanation

Due to a compiler bug, pin! and formatting macros were able to extend the lifetimes of temporaries borrowed in their arguments past their usual scopes. The bug is fixed in future Rust versions, so we issue this future-incompatibility warning for code that may stop compiling or may change in behavior thereafter.