rust - What is the {integer} or {float} in a compiler error message?

Question

It is surprisingly hard to find this in the docs. This might even be a two part question:

1. Are {integer} and {float} some sort of language alias for a specific primitive type?

2. What does it mean for a type name to be enclosed in curly braces in a compilation/syntax error message?

Example:

error: no method named pow found for type {integer} in the current scope

Answer 1

{integer} in error messages is a placeholder for any of the integer types ({i,u}{8,16,32,64,128}). (Source)

Integer literals in Rust are type inferred based on their usage. For example, in the following code, the type of 123 is u8 in the first instance and u64 in the second:

let a: u8 = 123;
let a: u64 = 123;

{integer} is used to represent any integer type in error messages when the compiler hasn't figured out a concrete type of the value.