Implicit Conversions

Wherever any rule of this specification requires an expression to have some type, an expression whose type differs is accepted when a conversion from its type to the required type is specified on this page. The conversion is applied implicitly; conversions require no notation, and the rules that rely on them need not mention them.

Widening

An expression of type T implicitly converts to U when T widens to U within the same family:

• signed integers: i8 → i16 → i32 → i64
• unsigned integers: u8 → u16 → u32 → u64
• floating-point: f32 → f64

Conversions between families are not allowed; in particular, signed and unsigned integers, or integers and floating-point types, cannot be implicitly converted.

Handle Dereference

An expression of handle type &T implicitly converts to T by dereferencing the handle. Conversions compose: an expression of type &&T converts first to &T, and then to T.

Common Type

Two or more types have a common type when they are all identical, or when they are all numeric types within the same family; in the latter case, the common type is the widest of them.

When determining a common type, a candidate type may be replaced by any type it converts to; the replacement is repeated until the types are identical or all numeric within one family, and the resulting type is the common type. Thus, for example, &T and T have the common type T.