Rollup merge of rust-lang#105265 - aDotInTheVoid:sum-product-on-unimplemented, r=estebank

Add `rustc_on_unimplemented` to `Sum` and `Product` trait.

Helps with rust-lang#105184, but I don't think it fully fixes it.

Author: matthiaskrgr

core/src/iter/traits/accum.rs

@@ -10,6 +10,10 @@ use crate::num::Wrapping;
 /// [`sum()`]: Iterator::sum
 /// [`FromIterator`]: iter::FromIterator
 #[stable(feature = "iter_arith_traits", since = "1.12.0")]
+#[rustc_on_unimplemented(
+    message = "a value of type `{Self}` cannot be made by summing an iterator over elements of type `{A}`",
+    label = "value of type `{Self}` cannot be made by summing a `std::iter::Iterator<Item={A}>`"
+)]
 pub trait Sum<A = Self>: Sized {
     /// Method which takes an iterator and generates `Self` from the elements by
     /// "summing up" the items.
@@ -27,6 +31,10 @@ pub trait Sum<A = Self>: Sized {
 /// [`product()`]: Iterator::product
 /// [`FromIterator`]: iter::FromIterator
 #[stable(feature = "iter_arith_traits", since = "1.12.0")]
+#[rustc_on_unimplemented(
+    message = "a value of type `{Self}` cannot be made by multiplying all elements of type `{A}` from an iterator",
+    label = "value of type `{Self}` cannot be made by multiplying all elements from a `std::iter::Iterator<Item={A}>`"
+)]
 pub trait Product<A = Self>: Sized {
     /// Method which takes an iterator and generates `Self` from the elements by
     /// multiplying the items.