When single impl can satisfy inference error, suggest type

[estebank]

When encountering an inference error where a return type must be known, like when calling Iterator::sum::<T>() without specifying T, look for all T that would satisfy Sum<S>. If only one, suggest it.

error[E0283]: type annotations needed
  --> $DIR/cannot-infer-iterator-sum-return-type.rs:4:9
   |
LL |     let sum = v
   |         ^^^
...
LL |         .sum(); // `sum::<T>` needs `T` to be specified
   |          --- type must be known at this point
   |
   = note: cannot satisfy `_: Sum<i32>`
help: the trait `Sum` is implemented for `i32`
  --> $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL
  ::: $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL
   |
   = note: in this macro invocation
note: required by a bound in `std::iter::Iterator::sum`
  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
   = note: this error originates in the macro `integer_sum_product` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider giving `sum` an explicit type, where the type for type parameter `S` is specified
   |
LL |     let sum: i32 = v
   |            +++++

Fix #100802.

[rustbot]

Some changes occurred in need_type_info.rs

cc @lcnr

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[estebank]

This gives us the

help: the trait `Sum` is implemented for `i32`
    --> library/core/src/iter/traits/accum.rs:48:9
     |
  48 |         impl Sum for $a {
     |         ^^^^^^^^^^^^^^^
...
 204 | integer_sum_product! { i8 i16 i32 i64 i128 isize u8 u16 u32 u64 u128 usize }
     | ---------------------------------------------------------------------------- in this macro invocation

instead of

     = help: the following types implement trait `Sum<A>`:
               `Duration` implements `Sum<&'a Duration>`
               `Duration` implements `Sum`
               `Option<T>` implements `Sum<Option<U>>`
               `Result<T, E>` implements `Sum<Result<U, E>>`
               `Saturating<u128>` implements `Sum<&'a Saturating<u128>>`
               `Saturating<u128>` implements `Sum`
               `Saturating<u16>` implements `Sum<&'a Saturating<u16>>`
               `Saturating<u16>` implements `Sum`
             and 88 others