Macro polonius_the_crab::polonius_loop

macro_rules! polonius_loop {
    (
    | $var:ident $(,)? | -> $Ret:ty
        $body:block
    $(,)?
) => { ... };
}

Convenience support for the loop { … polonius!(…) } pattern.

Example

#![forbid(unsafe_code)]
use {
   ::polonius_the_crab::{
       prelude::*,
   },
   ::std::{
       collections::HashMap,
   },
};

enum Value {
   Alive(i32),
   Daed,
}

/// Notice how this example, *despite its usage of the fancy `.entry()` API
/// of `HashMap`s*, still needs `polonius_the_crab` to express this logic!
fn get_first_alive_from_base_or_insert (
   mut map: &'_ mut HashMap<usize, Value>,
   base: usize,
   default_value: i32,
) -> &'_ i32
{
   let mut idx = base;
   // (loop {
   polonius_loop!(|map| -> &'polonius i32 {
       use ::std::collections::hash_map::*;
       // return(
       polonius_return!(
           match map.entry(idx) {
               | Entry::Occupied(entry) => match entry.into_mut() {
                   // Found a value!
                   | &mut Value::Alive(ref val) => val,
                   // "tombstone", keep searching
                   | &mut Value::Daed => {
                       idx += 1;
                       // continue;
                       polonius_continue!();
                   },
               },
               | Entry::Vacant(slot) => match slot.insert(Value::Alive(default_value)) {
                   | &mut Value::Alive(ref val) => val,
                   | &mut Value::Daed => unreachable!(),
               },
           }
       );
   })
}

Error message without polonius

error[E0499]: cannot borrow `*map` as mutable more than once at a time
  --> src/lib.rs:222:18
   |
22 | mut map: &'_ mut HashMap<usize, Value>,
   |          - let's call the lifetime of this reference `'1`
...
33 |     match map.entry(idx) {
   |           ^^^ `*map` was mutably borrowed here in the previous iteration of the loop
...
45 |         | &mut Value::Alive(ref val) => val,
   |                                         --- returning this value requires that `*map` be borrowed for `'1`

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