Macro format_ident

macro_rules! format_ident {
    ($fmt:expr) => { ... };
    ($fmt:expr, $($rest:tt)*) => { ... };
}

Formatting macro for constructing Idents.

Syntax

Syntax is copied from the format! macro, supporting both positional and named arguments.

Only a limited set of formatting traits are supported. The current mapping of format types to traits is:

• {} ⇒ IdentFragment
• {:o} ⇒ Octal
• {:x} ⇒ LowerHex
• {:X} ⇒ UpperHex
• {:b} ⇒ Binary

See std::fmt for more information.

IdentFragment

Unlike format!, this macro uses the IdentFragment formatting trait by default. This trait is like Display, with a few differences:

• IdentFragment is only implemented for a limited set of types, such as unsigned integers and strings.
• Ident arguments will have their r# prefixes stripped, if present.

Hygiene

The Span of the first Ident argument is used as the span of the final identifier, falling back to Span::call_site when no identifiers are provided.

// If `ident` is an Ident, the span of `my_ident` will be inherited from it.
let my_ident = format_ident!("My{}{}", ident, "IsCool");
assert_eq!(my_ident, "MyIdentIsCool");

Alternatively, the span can be overridden by passing the span named argument.

let my_span = /* ... */;
format_ident!("MyIdent", span = my_span);

Panics

This method will panic if the resulting formatted string is not a valid identifier.

Examples

Composing raw and non-raw identifiers:

let my_ident = format_ident!("My{}", "Ident");
assert_eq!(my_ident, "MyIdent");

let raw = format_ident!("r#Raw");
assert_eq!(raw, "r#Raw");

let my_ident_raw = format_ident!("{}Is{}", my_ident, raw);
assert_eq!(my_ident_raw, "MyIdentIsRaw");

Integer formatting options:

let num: u32 = 10;

let decimal = format_ident!("Id_{}", num);
assert_eq!(decimal, "Id_10");

let octal = format_ident!("Id_{:o}", num);
assert_eq!(octal, "Id_12");

let binary = format_ident!("Id_{:b}", num);
assert_eq!(binary, "Id_1010");

let lower_hex = format_ident!("Id_{:x}", num);
assert_eq!(lower_hex, "Id_a");

let upper_hex = format_ident!("Id_{:X}", num);
assert_eq!(upper_hex, "Id_A");