Auto merge of #51463 - estebank:error-codes, r=nikomatsakis

Various changes to existing diagnostics

* [Add code to `invalid ABI` error, add span label, move list to help to make message shorter](23ae5af):
```
error[E0697]: invalid ABI: found `路濫狼á́́`
  --> $DIR/unicode.rs:11:8
   |
LL | extern "路濫狼á́́" fn foo() {} //~ ERROR invalid ABI
   |        ^^^^^^^^^ invalid ABI
   |
   = help: valid ABIs: cdecl, stdcall, fastcall, vectorcall, thiscall, aapcs, win64, sysv64, ptx-kernel, msp430-interrupt, x86-interrupt, Rust, C, system, rust-intrinsic, rust-call, platform-intrinsic, unadjusted
```
* [Add code to incorrect `pub` restriction error](e96fdea)
* [Add message to `rustc_on_unimplemented` attributes in core to have them set a custom message _and_ label](2cc7e5e):
```
error[E0277]: `W` does not have a constant size known at compile-time
  --> $DIR/unsized-enum2.rs:33:8
   |
LL |     VA(W),
   |        ^ `W` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `W`
   = help: consider adding a `where W: std::marker::Sized` bound
   = note: no field of an enum variant may have a dynamically sized type
```
```
error[E0277]: `Foo` cannot be sent between threads safely
  --> $DIR/E0277-2.rs:26:5
   |
LL |     is_send::<Foo>();
   |     ^^^^^^^^^^^^^^ `Foo` cannot be sent between threads safely
   |
   = help: the trait `std::marker::Send` is not implemented for `Foo`
```
```
error[E0277]: can't compare `{integer}` with `std::string::String`
  --> $DIR/binops.rs:16:7
   |
LL |     5 < String::new();
   |       ^ no implementation for `{integer} < std::string::String` and `{integer} > std::string::String`
   |
   = help: the trait `std::cmp::PartialOrd<std::string::String>` is not implemented for `{integer}`
```
```
error[E0277]: can't compare `{integer}` with `std::result::Result<{integer}, _>`
  --> $DIR/binops.rs:17:7
   |
LL |     6 == Ok(1);
   |       ^^ no implementation for `{integer} == std::result::Result<{integer}, _>`
   |
   = help: the trait `std::cmp::PartialEq<std::result::Result<{integer}, _>>` is not implemented for `{integer}`
```
```
error[E0277]: a collection of type `i32` cannot be built from an iterator over elements of type `i32`
  --> $DIR/type-check-defaults.rs:16:19
   |
LL | struct WellFormed<Z = Foo<i32, i32>>(Z);
   |                   ^ a collection of type `i32` cannot be built from `std::iter::Iterator<Item=i32>`
   |
   = help: the trait `std::iter::FromIterator<i32>` is not implemented for `i32`
note: required by `Foo`
  --> $DIR/type-check-defaults.rs:15:1
   |
LL | struct Foo<T, U: FromIterator<T>>(T, U);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
* [Add link to book for `Sized` errors](1244dc7):
```
error[E0277]: `std::fmt::Debug + std::marker::Sync + 'static` does not have a constant size known at compile-time
  --> $DIR/const-unsized.rs:13:29
   |
LL | const CONST_0: Debug+Sync = *(&0 as &(Debug+Sync));
   |                             ^^^^^^^^^^^^^^^^^^^^^^ `std::fmt::Debug + std::marker::Sync + 'static` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `std::fmt::Debug + std::marker::Sync + 'static`
   = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types--sized>
   = note: constant expressions must have a statically known size
```
* [Point to previous line for single expected token not found](4816516) (if the current token is in a different line)

Author: bors

src/liballoc/vec.rs

@@ -1693,7 +1693,10 @@ impl<T: Hash> Hash for Vec<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented = "vector indices are of type `usize` or ranges of `usize`"]
+#[rustc_on_unimplemented(
+    message="vector indices are of type `usize` or ranges of `usize`",
+    label="vector indices are of type `usize` or ranges of `usize`",
+)]
 impl<T, I> Index<I> for Vec<T>
 where
     I: ::core::slice::SliceIndex<[T]>,
@@ -1707,7 +1710,10 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented = "vector indices are of type `usize` or ranges of `usize`"]
+#[rustc_on_unimplemented(
+    message="vector indices are of type `usize` or ranges of `usize`",
+    label="vector indices are of type `usize` or ranges of `usize`",
+)]
 impl<T, I> IndexMut<I> for Vec<T>
 where
     I: ::core::slice::SliceIndex<[T]>,

src/libcore/cmp.rs

@@ -108,7 +108,10 @@ use self::Ordering::*;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(alias = "==")]
 #[doc(alias = "!=")]
-#[rustc_on_unimplemented = "can't compare `{Self}` with `{Rhs}`"]
+#[rustc_on_unimplemented(
+    message="can't compare `{Self}` with `{Rhs}`",
+    label="no implementation for `{Self} == {Rhs}`",
+)]
 pub trait PartialEq<Rhs: ?Sized = Self> {
     /// This method tests for `self` and `other` values to be equal, and is used
     /// by `==`.
@@ -611,7 +614,10 @@ impl PartialOrd for Ordering {
 #[doc(alias = "<")]
 #[doc(alias = "<=")]
 #[doc(alias = ">=")]
-#[rustc_on_unimplemented = "can't compare `{Self}` with `{Rhs}`"]
+#[rustc_on_unimplemented(
+    message="can't compare `{Self}` with `{Rhs}`",
+    label="no implementation for `{Self} < {Rhs}` and `{Self} > {Rhs}`",
+)]
 pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
     /// This method returns an ordering between `self` and `other` values if one exists.
     ///