CODEWITHSUNDEEP
typesreferencerust
Claudia
Claudia

Reputation: 1269

Hash map macro refuses to type-check, failing with a misleading (and seemingly buggy) error message?

I've got these two related macros here:

#[macro_export]
macro_rules! hash_map {
    ( $( $key:expr => $value:expr ),* ) => {
        {
            use ::std::iter::FromIterator;
            ::std::collections::HashMap::from_iter(&[$(($key, $value)),*])
        }
    };
}

#[macro_export]
macro_rules! hash_set {
    ( $( $x:expr ),* ) => {
        {
            use ::std::iter::FromIterator;
            ::std::collections::HashSet::from_iter(&[$($x),*])
        }
    };
}

I'm primarily using it in tests, like this:

assert!(foo.one == hash_map!{});
assert!(foo.two == hash_set![String::from("some value")]);

But for some reason, I'm getting a very misleading and nonsensical error message out of it:

error[E0271]: type mismatch resolving `<&[_; 0] as std::iter::IntoIterator>::Item == (_, _)`
   --> src/common.rs:M:N
    |
6   |             ::std::collections::HashMap::from_iter(&[$(($key, $value)),*])
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected reference, found tuple
    | 
   ::: src/foo.rs:M:N
    |
154 |         assert!(foo.one == hash_map!{});
    |                                         ----------- in this macro invocation
    |
    = note: expected type `&_`
               found type `(_, _)`
    = note: required by `std::iter::FromIterator::from_iter`

error[E0308]: mismatched types
   --> src/common.rs:M:N
    |
16  |             ::std::collections::HashSet::from_iter(&[$($x),*])
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::string::String`, found reference
    | 
   ::: src/foo.rs:M:N
    |
150 |     fn it_can_do_things() {
    |                                - help: try adding a return type: `-> std::collections::HashSet<&std::string::String, _>`...
155 |         assert!(foo.two == hash_set![String::from("some value")]);
    |                                  -------------------------------------------- in this macro invocation
    |
    = note: expected type `std::collections::HashSet<std::string::String, std::collections::hash_map::RandomState>`
               found type `std::collections::HashSet<std::string::String, std::collections::hash_map::RandomState>`
               found type `std::collections::HashSet<&std::string::String, _>`

I've tried making the tuple a reference itself for the first, but that didn't help.

error[E0271]: type mismatch resolving `<&[_; 0] as std::iter::IntoIterator>::Item == (_, _)`
   --> src/common.rs:M:N
    |
6   |             ::std::collections::HashMap::from_iter(&[$(&($key, $value)),*])
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected reference, found tuple
    | 
   ::: src/foo.rs:M:N
    |
154 |         assert!(foo.one == hash_map!{});
    |                                         ----------- in this macro invocation
    |
    = note: expected type `&_`
               found type `(_, _)`
    = note: required by `std::iter::FromIterator::from_iter`

My goal is to try to get the hash map to reserve and add everything in a single batch, but I just can't get it to work correctly. Note that the hash map/set I'm creating owns its values.

Upvotes: 2

Views: 546

Answers (1)

Boiethios
Boiethios

Reputation: 42849

Minimal reproduction:

fn main() {
    use std::collections::HashMap;
    use std::iter::FromIterator;

    let _ = HashMap::from_iter([("key", "value")].iter());
}

You must iterate over owned tuples, not borrowed, hence the strange error message:

fn main() {
    use std::collections::HashMap;
    use std::iter::FromIterator;

    let _: HashMap<_, _> = HashMap::from_iter([("key", "value")].iter().cloned());
}

You can also use collect as said in the documentation of FromIterator:

FromIterator's from_iter is rarely called explicitly, and is instead used through Iterator's collect method. See collect's documentation for more examples.

fn main() {
    use std::collections::HashMap;

    let _: HashMap<_, _> = [("key", "value")].iter().cloned().collect();
    // or
    let _: HashMap<_, _> = vec![("key", "value")].into_iter().collect();
}

Note that the error message is more understandable with collect:

error[E0277]: the trait bound `std::collections::HashMap<_, _>: std::iter::FromIterator<&(&str, &str)>` is not satisfied
 --> src/main.rs:4:54
  |
4 |     let _: HashMap<_, _> = [("key", "value")].iter().collect();
  |                                                      ^^^^^^^ a collection of type `std::collections::HashMap<_, _>` cannot be built from an iterator over elements of type `&(&str, &str)`
  |
  = help: the trait `std::iter::FromIterator<&(&str, &str)>` is not implemented for `std::collections::HashMap<_, _>`

The last line is the more helpful: the iterator must iterate over tuples and not references to tuples:

impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S>

Upvotes: 5

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