DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK in rustc_lint::builtin - Rust
Description: The `dependency_on_unit_never_type_fallback` lint detects cases where code compiles with never type fallback being `()`, but will stop compiling with fallback being `!`.

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## DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK

# Static DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK

Source

    pub static DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK: &'static Lint

Expand description

The `dependency_on_unit_never_type_fallback` lint detects cases where code compiles with never type fallback being `()`, but will stop compiling with fallback being `!`.

#### §Example

ⓘ

    fn main() {
        if true {
            // return has type `!` which, is some cases, causes never type fallback
            return
        } else {
            // the type produced by this call is not specified explicitly,
            // so it will be inferred from the previous branch
            Default::default()
        };
        // depending on the fallback, this may compile (because `()` implements `Default`),
        // or it may not (because `!` does not implement `Default`)
    }

{{produces}}

#### §Explanation

Due to historic reasons never type fallback was `()`, meaning that `!` got spontaneously coerced to `()`. There are plans to change that, but they may make the code such as above not compile. Instead of depending on the fallback, you should specify the type explicitly:

    if true {
        return
    } else {
        // type is explicitly specified, fallback can't hurt us no more
        <() as Default>::default()
    };

See Tracking Issue for making `!` fall back to `!`.