New lint: Prefer `.unwrap_or(0)` over `.unwrap_or_default()`

What it does

This lint disallows usages of the methods using Default trait when there is an alternative, explicit type for a select number of types which are deemed to have an "obvious" default implementation as well as being easy to type

Methods such as:

• Option::unwrap_or_default -> Option::unwrap_or
• Result::unwrap_or_default -> Result::unwrap_or
• Entry::or_default -> Entry::or_insert

Types affected:

• numbers: 0or 0.0
• &str: ""
• bool: false
• char: '\0'
• Option<T>: None

I would avoid implementing this for even slightly more involved types like the NonZero family as:

• more typing involved
• has to bring the respective type into scope

category: complexity
lint name: or_default_for_simple_type

Advantage

• Reduces cognitive complexity. You have to think about the type less
• You don't have to know the type. It is useful when reviewing code on github
• Is more concise to type
• Everyone knows what the defaults for these types are

Drawbacks

• Some people may prefer using Default trait

Example

let a: Option<u8> = None;
let b = a.unwrap_or_default();

Could be written as:

let a: Option<u8> = None;
let b = a.unwrap_or(0);

.