Queue attributes

#[proc_macro_derive(Queues, attributes(queues, queue))]

Applies to a fieldless enum only. The derive generates impl QueueSet and nothing else — in particular it does not add QueueSet’s supertraits, so the enum needs #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Queues)].

`#[queues(...)]` — on the enum

Optional. Both keys are optional.

Key	Type	Valid range	Default
`prefix`	string literal	non-empty; names become `"{prefix}.{name}"`	none
`crate`	string literal (module path)	a valid path; a bare name gets a leading `::`; `crate::`, `self::` and `super::` are used verbatim	auto: `::queuey::__core` if the caller depends on `queuey`, else `::queuey_core`

crate exists for re-exporting wrappers around queuey. Application code never needs it.

`#[queue(...)]` — on each variant

Optional. Every key is optional.

Key	Type	Valid range	Default
`name`	string literal	non-empty	snake_case of the variant
`prefetch`	integer	`1..=65535`	`16`
`durable`	bool	`true` / `false`	`true`
`message_ttl`	duration literal	greater than zero	none
`max_priority`	integer	`0..=255`; `0` disables priorities	`10` (`DEFAULT_MAX_PRIORITY`)
`retry(...)`	nested list	see Job attributes	`RetryPolicy::default()`, which is no retries

Snake-casing handles acronyms: SendEmails becomes send_emails and HTTPCalls becomes http_calls.

Duration literals

Every duration-valued key takes a string literal parsed at macro time: an integer followed by a unit, where the unit is one of ms, s, m, h or d. A bare integer means seconds. Whitespace is ignored, so " 5 s " and "5 ms" both parse. The value resolves to whole milliseconds.

Accepted: "500ms", "30s", "2 m", "1h", "7d", "30".

Rejected: "", " ", "s", "abc", "1x", "1.5s", "-1s", "1sm", "1 m s", "ms1", "+2s", anything overflowing u64 milliseconds, and zero everywhere.

Compile errors

Every mistake is an error pointing at the offending token.

Shape of the type

Mistake	Message
Not an enum	`#[derive(Queues)] can only be applied to enums`
No variants	`#[derive(Queues)] requires at least one variant`
A variant with fields	`queue variants must not have fields`
Generic enum	`generic queue enums are not supported`

Names

Mistake	Message
`prefix = ""`	`prefix` must not be empty
`name = ""`	`name` must not be empty
Two variants resolving to one name	duplicate queue name `myapp.emails`, with a note `myapp.emails` is first used here
A variant whose snake_case name is empty	`queue name is empty; give the variant a #[queue(name = "...")]`

Values

Mistake	Message
`prefetch = 0`	`prefetch = 0` means unlimited in AMQP; omit the attribute or use a positive value
`prefetch` negative or above 65535	`prefetch` must be an integer between 1 and 65535
`max_priority` out of range or not an integer	`max_priority` must be an integer in 0..=255
`message_ttl = "0s"`	`message_ttl` must be greater than zero: a zero TTL discards every message the moment it is published
An unparseable duration	invalid duration `30 fortnights`: expected an integer with an optional unit (`ms`, `s`, `m`, `h`, `d`), for example "500ms", "30s" or "2m"

Keys

Mistake	Message
A repeated key	duplicate key `prefetch`
An unknown key in `#[queue(...)]`	unknown key `concurrency` in `#[queue(...)]`, expected one of `name`, `prefetch`, `durable`, `message_ttl`, `max_priority`, `retry`
An unknown key in `#[queues(...)]`	expects `prefix` or `crate`

What is generated

pub trait QueueSet:
    Copy + Clone + Eq + std::hash::Hash + std::fmt::Debug + Send + Sync + 'static
{
    fn all() -> &'static [Self];
    fn name(&self) -> &'static str;
    fn config(&self) -> QueueConfig;
    fn from_name(name: &str) -> Option<Self>;
}

all() returns the variants in declaration order. from_name is provided by the trait as a linear scan of all().

pub struct QueueConfig {
    pub name: String,
    pub prefetch: u16,
    pub retry: RetryPolicy,
    pub durable: bool,
    pub message_ttl: Option<Duration>,
    pub max_priority: Option<u8>,
}

impl QueueConfig {
    pub fn new(name: impl Into<String>) -> Self;
    pub fn prefetch(mut self, prefetch: u16) -> Self;
    pub fn retry(mut self, retry: RetryPolicy) -> Self;
    pub fn durable(mut self, durable: bool) -> Self;
    pub fn message_ttl(mut self, ttl: Duration) -> Self;
    pub fn max_priority(mut self, levels: u8) -> Self;   // 0 => None
}

QueueConfig::new defaults to prefetch 16, RetryPolicy::default(), durable, no TTL, and Some(DEFAULT_MAX_PRIORITY).

pub const DEFAULT_MAX_PRIORITY: u8 = 10;

Ten levels, because RabbitMQ recommends at most ten.