Add more impls of PartialEq and PartialOrd for strings

Currently, some combinations of `&String` and `&str` don't support
comparison operators. For instance, this:

```rust
fn main() {
    let s1 = String::from("hello");
    let s2 = "world";
    _ = s1 < s2;
}
```

will fail with:
```
error[E0308]: mismatched types
 --> src/main.rs:4:14
  |
4 |     _ = s1 < s2;
  |         --   ^^- help: try using a conversion method: `.to_string()`
  |         |    |
  |         |    expected `String`, found `&str`
  |         expected because this is `String`
```

Other combinations only work because the compiler implicitly relies on
impls on different reference types, and that makes such combinations
fragile, breaking if any other impls of `PartialOrd` show up.

Add some additional impls to make such cases work, and to improve
robustness when adding other impls in the future.

Author: joshtriplett

library/alloc/src/string.rs

@@ -42,6 +42,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use core::cmp::Ordering;
 use core::error::Error;
 use core::iter::FusedIterator;
 #[cfg(not(no_global_oom_handling))]
@@ -2530,12 +2531,49 @@ macro_rules! impl_eq {
 
 impl_eq! { String, str }
 impl_eq! { String, &'a str }
+impl_eq! { &String, str }
 #[cfg(not(no_global_oom_handling))]
 impl_eq! { Cow<'a, str>, str }
 #[cfg(not(no_global_oom_handling))]
 impl_eq! { Cow<'a, str>, &'b str }
 #[cfg(not(no_global_oom_handling))]
 impl_eq! { Cow<'a, str>, String }
+#[cfg(not(no_global_oom_handling))]
+impl_eq! { Cow<'a, str>, &String }
+
+macro_rules! impl_ord {
+    ($lhs:ty, $rhs: ty) => {
+        #[stable(feature = "rust1", since = "1.0.0")]
+        #[allow(unused_lifetimes)]
+        impl<'a, 'b> PartialOrd<$rhs> for $lhs {
+            #[inline]
+            fn partial_cmp(&self, other: &$rhs) -> Option<Ordering> {
+                PartialOrd::partial_cmp(&self[..], &other[..])
+            }
+        }
+
+        #[stable(feature = "rust1", since = "1.0.0")]
+        #[allow(unused_lifetimes)]
+        impl<'a, 'b> PartialOrd<$lhs> for $rhs {
+            #[inline]
+            fn partial_cmp(&self, other: &$lhs) -> Option<Ordering> {
+                PartialOrd::partial_cmp(&self[..], &other[..])
+            }
+        }
+    };
+}
+
+impl_ord! { String, str }
+impl_ord! { String, &'a str }
+impl_ord! { &String, str }
+#[cfg(not(no_global_oom_handling))]
+impl_ord! { Cow<'a, str>, str }
+#[cfg(not(no_global_oom_handling))]
+impl_ord! { Cow<'a, str>, &'b str }
+#[cfg(not(no_global_oom_handling))]
+impl_ord! { Cow<'a, str>, String }
+#[cfg(not(no_global_oom_handling))]
+impl_ord! { Cow<'a, str>, &String }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Default for String {

library/core/src/str/traits.rs

@@ -30,6 +30,22 @@ impl PartialEq for str {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq<&str> for str {
+    #[inline]
+    fn eq(&self, other: &&str) -> bool {
+        self.as_bytes() == other.as_bytes()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq<str> for &str {
+    #[inline]
+    fn eq(&self, other: &str) -> bool {
+        self.as_bytes() == other.as_bytes()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Eq for str {}
 
@@ -48,6 +64,22 @@ impl PartialOrd for str {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialOrd<&str> for str {
+    #[inline]
+    fn partial_cmp(&self, other: &&str) -> Option<Ordering> {
+        Some(self.cmp(*other))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialOrd<str> for &str {
+    #[inline]
+    fn partial_cmp(&self, other: &str) -> Option<Ordering> {
+        Some(self.cmp(&other))
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I> ops::Index<I> for str
 where

tests/ui/binop/binary-op-suggest-deref.rs

@@ -67,9 +67,7 @@ fn baz() {
     let string_ref = &owned;
     let partial = "foobar";
     _ = string_ref == partial[..3];
-    //~^ERROR can't compare `&String` with `str` [E0277]
     _ = partial[..3] == string_ref;
-    //~^ERROR can't compare `str` with `&String` [E0277]
 }
 
 fn qux() {

tests/ui/binop/binary-op-suggest-deref.stderr

@@ -271,32 +271,8 @@ note: an implementation of `PartialEq<&&{integer}>` might be missing for `Foo`
 LL |     struct Foo;
    |     ^^^^^^^^^^ must implement `PartialEq<&&{integer}>`
 
-error[E0277]: can't compare `&String` with `str`
-  --> $DIR/binary-op-suggest-deref.rs:69:20
-   |
-LL |     _ = string_ref == partial[..3];
-   |                    ^^ no implementation for `&String == str`
-   |
-   = help: the trait `PartialEq<str>` is not implemented for `&String`
-help: consider dereferencing here
-   |
-LL |     _ = *string_ref == partial[..3];
-   |         +
-
-error[E0277]: can't compare `str` with `&String`
-  --> $DIR/binary-op-suggest-deref.rs:71:22
-   |
-LL |     _ = partial[..3] == string_ref;
-   |                      ^^ no implementation for `str == &String`
-   |
-   = help: the trait `PartialEq<&String>` is not implemented for `str`
-help: consider dereferencing here
-   |
-LL |     _ = partial[..3] == *string_ref;
-   |                         +
-
 error[E0277]: no implementation for `i32 & str`
-  --> $DIR/binary-op-suggest-deref.rs:78:17
+  --> $DIR/binary-op-suggest-deref.rs:76:17
    |
 LL |     let _ = FOO & (*"Sized".to_string().into_boxed_str());
    |                 ^ no implementation for `i32 & str`
@@ -309,14 +285,14 @@ LL |     let _ = FOO & (*"Sized".to_string().into_boxed_str());
              `i32` implements `BitAnd`
 
 error[E0277]: the size for values of type `str` cannot be known at compilation time
-  --> $DIR/binary-op-suggest-deref.rs:78:17
+  --> $DIR/binary-op-suggest-deref.rs:76:17
    |
 LL |     let _ = FOO & (*"Sized".to_string().into_boxed_str());
    |                 ^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `str`
 
-error: aborting due to 24 previous errors
+error: aborting due to 22 previous errors
 
 Some errors have detailed explanations: E0277, E0308, E0369.
 For more information about an error, try `rustc --explain E0277`.

tests/ui/inference/issue-72616.stderr

@@ -8,12 +8,15 @@ LL |         if String::from("a") == "a".try_into().unwrap() {}
    |
    = note: cannot satisfy `String: PartialEq<_>`
    = help: the following types implement trait `PartialEq<Rhs>`:
+             `&String` implements `PartialEq<Cow<'_, str>>`
+             `&String` implements `PartialEq<String>`
+             `&String` implements `PartialEq<str>`
+             `String` implements `PartialEq<&String>`
              `String` implements `PartialEq<&str>`
              `String` implements `PartialEq<ByteStr>`
              `String` implements `PartialEq<ByteString>`
              `String` implements `PartialEq<Cow<'_, str>>`
-             `String` implements `PartialEq<str>`
-             `String` implements `PartialEq`
+           and 2 others
 help: try using a fully qualified path to specify the expected types
    |
 LL |         if String::from("a") == <&str as TryInto<T>>::try_into("a").unwrap() {}