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

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

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

library/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.

src/test/ui/on-unimplemented/sum.rs

@@ -0,0 +1,9 @@
+// <https://github.com/rust-lang/rust/issues/105184>
+
+fn main() {
+    vec![(), ()].iter().sum::<i32>();
+    //~^ ERROR
+
+    vec![(), ()].iter().product::<i32>();
+    //~^ ERROR
+}

src/test/ui/on-unimplemented/sum.stderr

@@ -0,0 +1,39 @@
+error[E0277]: a value of type `i32` cannot be made by summing an iterator over elements of type `&()`
+  --> $DIR/sum.rs:4:5
+   |
+LL |     vec![(), ()].iter().sum::<i32>();
+   |     ^^^^^^^^^^^^^^^^^^^ --- required by a bound introduced by this call
+   |     |
+   |     value of type `i32` cannot be made by summing a `std::iter::Iterator<Item=&()>`
+   |
+   = help: the trait `Sum<&()>` is not implemented for `i32`
+   = help: the following other types implement trait `Sum<A>`:
+             <i32 as Sum<&'a i32>>
+             <i32 as Sum>
+note: required by a bound in `std::iter::Iterator::sum`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+   |
+LL |         S: Sum<Self::Item>,
+   |            ^^^^^^^^^^^^^^^ required by this bound in `std::iter::Iterator::sum`
+
+error[E0277]: a value of type `i32` cannot be made by multiplying all elements of type `&()` from an iterator
+  --> $DIR/sum.rs:7:5
+   |
+LL |     vec![(), ()].iter().product::<i32>();
+   |     ^^^^^^^^^^^^^^^^^^^ ------- required by a bound introduced by this call
+   |     |
+   |     value of type `i32` cannot be made by multiplying all elements from a `std::iter::Iterator<Item=&()>`
+   |
+   = help: the trait `Product<&()>` is not implemented for `i32`
+   = help: the following other types implement trait `Product<A>`:
+             <i32 as Product<&'a i32>>
+             <i32 as Product>
+note: required by a bound in `std::iter::Iterator::product`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+   |
+LL |         P: Product<Self::Item>,
+   |            ^^^^^^^^^^^^^^^^^^^ required by this bound in `std::iter::Iterator::product`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.