Confusing and big error when misusing function instead of function call in iterator chain

[WaffleLapkin]

I've accidentally used a function (String::from) instead of a function call (String::from("...")) and the error was hard to read. Maybe it can be improved. (playground)

Code

fn test(media: &str) {
    std::iter::once((
        String::from, // <-- I've meant `String::from("...")`
        serde_json::Value::String(String::from("document")),
    ))
    .chain(
        std::iter::once((
            String::from("media"),
            serde_json::Value::String(media.to_owned()),
        ))
        .chain(
            Some("caption")
                .iter()
                .map(serde_json::to_value)
                .map(Result::unwrap)
                .map(|v| (String::from("caption"), v))
                .chain(
                    Some("parse_mode")
                        .iter()
                        .map(serde_json::to_value)
                        .map(Result::unwrap)
                        .map(|v| (String::from("parse_mode"), v)),
                ),
        ),
    )
    .collect::<Vec<_>>();
}

Error

(this error is from 1.51.0-nightly (2021-01-30 04caa632dd10c2bf64b6), previous compiler versions produce similar errors)

error[E0271]: type mismatch resolving `<std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:18:26: 18:58]>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:24:34: 24:69]>>> as IntoIterator>::Item == (fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)`
 --> src/lib.rs:8:10
  |
8 |         .chain(
  |          ^^^^^ expected struct `std::string::String`, found fn item
  |
  = note: expected type `(std::string::String, Value)`
            found tuple `(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)`

error[E0599]: the method `collect` exists for struct `std::iter::Chain<std::iter::Once<(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)>, std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:18:26: 18:58]>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:24:34: 24:69]>>>>`, but its trait bounds were not satisfied
  --> src/lib.rs:28:10
   |
28 |           .collect(),
   |            ^^^^^^^ method cannot be called on `std::iter::Chain<std::iter::Once<(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)>, std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:18:26: 18:58]>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:24:34: 24:69]>>>>` due to unsatisfied trait bounds
   |
   = note: the following trait bounds were not satisfied:
           `<std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:18:26: 18:58]>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:24:34: 24:69]>>> as Iterator>::Item = (fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)`
           which is required by `std::iter::Chain<std::iter::Once<(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)>, std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:18:26: 18:58]>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:24:34: 24:69]>>>>: Iterator`
           `std::iter::Chain<std::iter::Once<(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)>, std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:18:26: 18:58]>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:24:34: 24:69]>>>>: Iterator`
           which is required by `&mut std::iter::Chain<std::iter::Once<(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)>, std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:18:26: 18:58]>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> std::result::Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(std::result::Result<Value, serde_json::Error>) -> Value {std::result::Result::<Value, serde_json::Error>::unwrap}>, [closure@src/lib.rs:24:34: 24:69]>>>>: Iterator`

[henryboisdequin]

@rustbot label +C-enhancement

[estebank]

Triage: we've gotten more verbose, but not any clearer:

error[E0271]: type mismatch resolving `<Chain<..., ...> as IntoIterator>::Item == (..., ...)`
   --> src/lib.rs:7:9
    |
  6 |       .chain(
    |        ----- required by a bound introduced by this call
  7 | /         std::iter::once((
  8 | |             String::from("media"),
  9 | |             serde_json::Value::String(media.to_owned()),
 10 | |         ))
...   |
 23 | |                 ),
 24 | |         ),
    | |_________^ expected `(..., ...)`, found `(String, Value)`
    |
    = note: expected tuple `(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)`
               found tuple `(std::string::String, Value)`
note: the method call chain might not have had the expected associated types
   --> src/lib.rs:11:10
    |
  7 | /         std::iter::once((
  8 | |             String::from("media"),
  9 | |             serde_json::Value::String(media.to_owned()),
 10 | |         ))
    | |__________- this expression has type `Once<(String, Value)>`
 11 |           .chain(
    |  __________^
 12 | |             Some("caption")
 13 | |                 .iter()
 14 | |                 .map(serde_json::to_value)
...   |
 23 | |                 ),
 24 | |         ),
    | |_________^ `IntoIterator::Item` is `(String, Value)` here
note: required by a bound in `std::iter::Iterator::chain`
   --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:510:25
    |
507 |     fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
    |        ----- required by a bound in this associated function
...
510 |         U: IntoIterator<Item = Self::Item>,
    |                         ^^^^^^^^^^^^^^^^^ required by this bound in `Iterator::chain`
    = note: the full name for the type has been written to '/playground/target/debug/deps/playground-6525e1ccf992fedb.long-type-14644903136661441254.txt'
    = note: consider using `--verbose` to print the full type name to the console

error[E0599]: the method `collect` exists for struct `Chain<Once<(fn(_) -> String {<String as From<_>>::from}, Value)>, ...>`, but its trait bounds were not satisfied
  --> src/lib.rs:26:6
   |
 2 | /     std::iter::once((
 3 | |         String::from, // <-- I've meant `String::from("...")`
 4 | |         serde_json::Value::String(String::from("document")),
 5 | |     ))
...  |
26 | |     .collect::<Vec<_>>();
   | |     -^^^^^^^ method cannot be called due to unsatisfied trait bounds
   | |_____|
   |
   |
  ::: /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/adapters/chain.rs:23:1
   |
23 |   pub struct Chain<A, B> {
   |   ---------------------- doesn't satisfy `<_ as Iterator>::Item = (fn(_) -> String {<String as From<_>>::from}, Value)` or `_: Iterator`
   |
   = note: the following trait bounds were not satisfied:
           `<std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(Result<Value, serde_json::Error>) -> Value {Result::<Value, serde_json::Error>::unwrap}>, {closure@src/lib.rs:16:22: 16:25}>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(Result<Value, serde_json::Error>) -> Value {Result::<Value, serde_json::Error>::unwrap}>, {closure@src/lib.rs:22:30: 22:33}>>> as Iterator>::Item = (fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)`
           which is required by `std::iter::Chain<std::iter::Once<(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)>, std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(Result<Value, serde_json::Error>) -> Value {Result::<Value, serde_json::Error>::unwrap}>, {closure@src/lib.rs:16:22: 16:25}>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(Result<Value, serde_json::Error>) -> Value {Result::<Value, serde_json::Error>::unwrap}>, {closure@src/lib.rs:22:30: 22:33}>>>>: Iterator`
           `std::iter::Chain<std::iter::Once<(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)>, std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(Result<Value, serde_json::Error>) -> Value {Result::<Value, serde_json::Error>::unwrap}>, {closure@src/lib.rs:16:22: 16:25}>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(Result<Value, serde_json::Error>) -> Value {Result::<Value, serde_json::Error>::unwrap}>, {closure@src/lib.rs:22:30: 22:33}>>>>: Iterator`
           which is required by `&mut std::iter::Chain<std::iter::Once<(fn(_) -> std::string::String {<std::string::String as From<_>>::from}, Value)>, std::iter::Chain<std::iter::Once<(std::string::String, Value)>, std::iter::Chain<std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(Result<Value, serde_json::Error>) -> Value {Result::<Value, serde_json::Error>::unwrap}>, {closure@src/lib.rs:16:22: 16:25}>, std::iter::Map<std::iter::Map<std::iter::Map<std::option::Iter<'_, &str>, fn(&&str) -> Result<Value, serde_json::Error> {to_value::<&&str>}>, fn(Result<Value, serde_json::Error>) -> Value {Result::<Value, serde_json::Error>::unwrap}>, {closure@src/lib.rs:22:30: 22:33}>>>>: Iterator`
   = note: the full name for the type has been written to '/playground/target/debug/deps/playground-6525e1ccf992fedb.long-type-658639706476016280.txt'
   = note: consider using `--verbose` to print the full type name to the console