Suggest setting associated types on type errors

[estebank]

The following code is missing a constraint for TryInto<Error=TryIntoError>:

use std::convert::TryInto;

struct TryIntoError;
struct X;

impl TryInto<usize> for X {
    type Error = TryIntoError;

    fn try_into(self) -> Result<usize, TryIntoError> {
        Ok(0)
    }
}

trait T: TryInto<usize> {
    fn foo(self) -> Result<usize, TryIntoError> {
        TryInto::try_into(self)
    }
}

which causes the following error:

error[E0308]: mismatched types
  --> src/main.rs:16:9
   |
15 |     fn foo(self) -> Result<usize, TryIntoError> {
   |                     --------------------------- expected `std::result::Result<usize, TryIntoError>` because of return type
16 |         TryInto::try_into(self)
   |         ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `TryIntoError`, found associated type
   |
   = note: expected type `std::result::Result<_, TryIntoError>`
              found type `std::result::Result<_, <Self as std::convert::TryInto<usize>>::Error>`

This should be closer to

error[E0308]: mismatched types
  --> src/main.rs:16:9
   |
14 |   trait T: TryInto<usize> {
   |            -------------- help: explicitely set associated type `Error`: `TryInto<usize, TryIntoError>`
15 |     fn foo(self) -> Result<usize, TryIntoError> {
   |                     --------------------------- expected `std::result::Result<usize, TryIntoError>` because of return type
16 |         TryInto::try_into(self)
   |         ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `TryIntoError`, found associated type
   |
   = note: expected type `std::result::Result<_, TryIntoError>`
              found type `std::result::Result<_, <Self as std::convert::TryInto<usize>>::Error>`

or

error[E0308]: mismatched types
  --> src/main.rs:16:9
   |
15 |     fn foo(self) -> Result<usize, TryIntoError> {
   |                     --------------------------- expected `std::result::Result<usize, TryIntoError>` because of return type
16 |         TryInto::try_into(self)
   |         ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `TryIntoError`, found associated type
   |
   = note: expected type `std::result::Result<_, TryIntoError>`
              found type `std::result::Result<_, <Self as std::convert::TryInto<usize>>::Error>`
help: explicitely set associated type `Error`
   |
14 |   trait T: TryInto<usize, TryIntoError> {
   |                         ^^^^^^^^^^^^^^
help: alternatively, use `TryInto::Error`
   |
15 |     fn foo(self) -> Result<usize, Self::Error> {
   |                                   ^^^^^^^^^^^

CC reporter @minggfeng

[estebank]

Related case from https://twitter.com/ShekoHex/status/1165248106767814657:

use std::net::ToSocketAddrs;
pub struct Config<A>
where
    A: ToSocketAddrs,
{
    addr: A,
}

impl<A> Default for Config<A>
where
    A: ToSocketAddrs,
{
    fn default() -> Self {
        Self {
            addr: "127.0.0.1:1883",
        }
    }
}

fn start<A: ToSocketAddrs>(addr: A) {}

error[E0308]: mismatched types
  --> src/lib.rs:15:19
   |
15 |             addr: "127.0.0.1:1883",
   |                   ^^^^^^^^^^^^^^^^ expected type parameter, found reference
   |
   = note: expected type `A`
              found type `&'static str`

[estebank]

Case from #56183:

pub struct MyType<T> {
    val: T,
}

impl<T: ToString> MyType<T> {
    pub fn new(val: T) -> Self {
        Self {
            val
        }
    }
    
    pub fn new_from_self(&self, val: u8) -> MyType<u8> {
        MyType {
            val
        }
    }

    pub fn new_from_self_generic(&self, val: u8) -> Self {
        Self {
            val
        }
    }
}

[estebank]

Triage, current output for these cases:

error[E0308]: mismatched types
  --> file12.rs:16:9
   |
15 |     fn foo(self) -> Result<usize, TryIntoError> {
   |                     --------------------------- expected `std::result::Result<usize, TryIntoError>` because of return type
16 |         TryInto::try_into(self)
   |         ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `TryIntoError`, found associated type
   |
   = note: expected enum `std::result::Result<_, TryIntoError>`
              found enum `std::result::Result<_, <Self as std::convert::TryInto<usize>>::Error>`
   = note: consider constraining the associated type `<Self as std::convert::TryInto<usize>>::Error` to `TryIntoError`
   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html

error[E0308]: mismatched types
  --> file12.rs:15:19
   |
9  | impl<A> Default for Config<A>
   |      - this type parameter
...
15 |             addr: "127.0.0.1:1883",
   |                   ^^^^^^^^^^^^^^^^ expected type parameter `A`, found `&str`
   |
   = note: expected type parameter `A`
                   found reference `&'static str`
   = help: type parameters must be constrained to match other types
   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters

error[E0308]: mismatched types
  --> file12.rs:20:13
   |
5  | impl<T: ToString> MyType<T> {
   |      - this type parameter
...
20 |             val
   |             ^^^ expected type parameter `T`, found `u8`
   |
   = note: expected type parameter `T`
                        found type `u8`
   = help: type parameters must be constrained to match other types
   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters