Language Design: Rust’s Almost-Rules

(Inspired by Almost Rules.)

Syntax

: is followed by a type

• except inside struct initializers, where it is followed by a value
• except function result types, which are preceded by ->

generics use <>

• except in expression contexts, which uses ::<>

invocations use ()

• except where {} or [] is used, because “they convey important information”
  • except for macro invocations, where (), {}, [] are equivalent and interchangeable

T {} initializes a struct

• except inside an if, where { starts a branch (see Rust’s Struct Initializer Syntax Was a Mistake)

types are uppercase

• except “primitive” types
  • except the primitive types array, slice, tuple and unit

Rust has no varargs

• except for extern functions
• except for macros

patterns introduce bindings

• except in macro pattern matching, where the same syntax matches identifiers verbatim

Semantics

types with a total order implement Eq and Ord

• except f64 and f32, which do not

struct initializers use temporary lifetime extension

• except tuple structs
  • except when using curly braces to initialize tuple structs