Auto merge of #98655 - nnethercote:dont-derive-PartialEq-ne, r=dtolnay

Don't derive `PartialEq::ne`.

Currently we skip deriving `PartialEq::ne` for C-like (fieldless) enums
and empty structs, thus reyling on the default `ne`. This behaviour is
unnecessarily conservative, because the `PartialEq` docs say this:

> Implementations must ensure that eq and ne are consistent with each other:
>
> `a != b` if and only if `!(a == b)` (ensured by the default
> implementation).

This means that the default implementation (`!(a == b)`) is always good
enough. So this commit changes things such that `ne` is never derived.

The motivation for this change is that not deriving `ne` reduces compile
times and binary sizes.

Observable behaviour may change if a user has defined a type `A` with an
inconsistent `PartialEq` and then defines a type `B` that contains an
`A` and also derives `PartialEq`. Such code is already buggy and
preserving bug-for-bug compatibility isn't necessary.

Two side-effects of the change:
- There is only one error message produced for types where `PartialEq`
  cannot be derived, instead of two.
- For coverage reports, some warnings about generated `ne` methods not
  being executed have disappeared.

Both side-effects seem fine, and possibly preferable.

15 files changed, 7 insertions, 192 deletions

diff --git a/src/test/run-make/coverage-reports/expected_show_coverage.doctest.txt b/src/test/run-make/coverage-reports/expected_show_coverage.doctest.txt
index 7ae0e978808..732de652627 100644
--- a/src/test/run-make/coverage-reports/expected_show_coverage.doctest.txt
+++ b/src/test/run-make/coverage-reports/expected_show_coverage.doctest.txt
@@ -23,12 +23,6 @@
    22|      1|//! ```
    23|      2|//! #[derive(Debug, PartialEq)]
                        ^1
-  ------------------
-  | Unexecuted instantiation: <rust_out::main::_doctest_main____coverage_doctest_rs_22_0::SomeError as core::cmp::PartialEq>::ne
-  ------------------
-  | <rust_out::main::_doctest_main____coverage_doctest_rs_22_0::SomeError as core::cmp::PartialEq>::eq:
-  |   23|      2|//! #[derive(Debug, PartialEq)]
-  ------------------
    24|      1|//! struct SomeError {
    25|      1|//!     msg: String,
    26|      1|//! }
diff --git a/src/test/run-make/coverage-reports/expected_show_coverage.issue-83601.txt b/src/test/run-make/coverage-reports/expected_show_coverage.issue-83601.txt
index de32c88b725..25c74ab2e70 100644
--- a/src/test/run-make/coverage-reports/expected_show_coverage.issue-83601.txt
+++ b/src/test/run-make/coverage-reports/expected_show_coverage.issue-83601.txt
@@ -2,12 +2,6 @@
     2|       |
     3|      3|#[derive(Debug, PartialEq, Eq)]
                               ^2
-  ------------------
-  | <issue_83601::Foo as core::cmp::PartialEq>::eq:
-  |    3|      2|#[derive(Debug, PartialEq, Eq)]
-  ------------------
-  | Unexecuted instantiation: <issue_83601::Foo as core::cmp::PartialEq>::ne
-  ------------------
     4|       |struct Foo(u32);
     5|       |
     6|      1|fn main() {
diff --git a/src/test/run-make/coverage-reports/expected_show_coverage.issue-84561.txt b/src/test/run-make/coverage-reports/expected_show_coverage.issue-84561.txt
index f24f7c69404..4a60432c14c 100644
--- a/src/test/run-make/coverage-reports/expected_show_coverage.issue-84561.txt
+++ b/src/test/run-make/coverage-reports/expected_show_coverage.issue-84561.txt
@@ -2,12 +2,6 @@
     2|       |
     3|       |// expect-exit-status-101
     4|     21|#[derive(PartialEq, Eq)]
-  ------------------
-  | <issue_84561::Foo as core::cmp::PartialEq>::eq:
-  |    4|     21|#[derive(PartialEq, Eq)]
-  ------------------
-  | Unexecuted instantiation: <issue_84561::Foo as core::cmp::PartialEq>::ne
-  ------------------
     5|       |struct Foo(u32);
     6|      1|fn test3() {
     7|      1|    let is_true = std::env::args().len() == 1;
diff --git a/src/test/run-make/coverage-reports/expected_show_coverage.partial_eq.txt b/src/test/run-make/coverage-reports/expected_show_coverage.partial_eq.txt
index fc266653349..a77175af663 100644
--- a/src/test/run-make/coverage-reports/expected_show_coverage.partial_eq.txt
+++ b/src/test/run-make/coverage-reports/expected_show_coverage.partial_eq.txt
@@ -3,11 +3,6 @@
     3|       |
     4|      2|#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
                        ^0            ^0      ^0     ^1       ^1 ^0^0
-  ------------------
-  | Unexecuted instantiation: <partial_eq::Version as core::cmp::PartialEq>::ne
-  ------------------
-  | Unexecuted instantiation: <partial_eq::Version as core::cmp::PartialEq>::eq
-  ------------------
     5|       |pub struct Version {
     6|       |    major: usize,
     7|       |    minor: usize,
diff --git a/src/test/ui/derives/derives-span-PartialEq-enum-struct-variant.rs b/src/test/ui/derives/derives-span-PartialEq-enum-struct-variant.rs
index d66faa086de..67a27729db7 100644
--- a/src/test/ui/derives/derives-span-PartialEq-enum-struct-variant.rs
+++ b/src/test/ui/derives/derives-span-PartialEq-enum-struct-variant.rs
@@ -7,7 +7,6 @@ struct Error;
 enum Enum {
    A {
      x: Error //~ ERROR
-//~^ ERROR
    }
 }
 
diff --git a/src/test/ui/derives/derives-span-PartialEq-enum-struct-variant.stderr b/src/test/ui/derives/derives-span-PartialEq-enum-struct-variant.stderr
index f655600b760..9953154fd4b 100644
--- a/src/test/ui/derives/derives-span-PartialEq-enum-struct-variant.stderr
+++ b/src/test/ui/derives/derives-span-PartialEq-enum-struct-variant.stderr
@@ -18,26 +18,6 @@ help: consider annotating `Error` with `#[derive(PartialEq)]`
 LL | #[derive(PartialEq)]
    |
 
-error[E0369]: binary operation `!=` cannot be applied to type `Error`
-  --> $DIR/derives-span-PartialEq-enum-struct-variant.rs:9:6
-   |
-LL | #[derive(PartialEq)]
-   |          --------- in this derive macro expansion
-...
-LL |      x: Error
-   |      ^^^^^^^^
-   |
-note: an implementation of `PartialEq<_>` might be missing for `Error`
-  --> $DIR/derives-span-PartialEq-enum-struct-variant.rs:4:1
-   |
-LL | struct Error;
-   | ^^^^^^^^^^^^ must implement `PartialEq<_>`
-   = note: this error originates in the derive macro `PartialEq` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: consider annotating `Error` with `#[derive(PartialEq)]`
-   |
-LL | #[derive(PartialEq)]
-   |
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0369`.
diff --git a/src/test/ui/derives/derives-span-PartialEq-enum.rs b/src/test/ui/derives/derives-span-PartialEq-enum.rs
index 66edf460b31..0becc7e0d10 100644
--- a/src/test/ui/derives/derives-span-PartialEq-enum.rs
+++ b/src/test/ui/derives/derives-span-PartialEq-enum.rs
@@ -7,8 +7,7 @@ struct Error;
 enum Enum {
    A(
      Error //~ ERROR
-//~^ ERROR
-     )
+   )
 }
 
 fn main() {}
diff --git a/src/test/ui/derives/derives-span-PartialEq-enum.stderr b/src/test/ui/derives/derives-span-PartialEq-enum.stderr
index ce4b00bfbf5..7c130452301 100644
--- a/src/test/ui/derives/derives-span-PartialEq-enum.stderr
+++ b/src/test/ui/derives/derives-span-PartialEq-enum.stderr
@@ -18,26 +18,6 @@ help: consider annotating `Error` with `#[derive(PartialEq)]`
 LL | #[derive(PartialEq)]
    |
 
-error[E0369]: binary operation `!=` cannot be applied to type `Error`
-  --> $DIR/derives-span-PartialEq-enum.rs:9:6
-   |
-LL | #[derive(PartialEq)]
-   |          --------- in this derive macro expansion
-...
-LL |      Error
-   |      ^^^^^
-   |
-note: an implementation of `PartialEq<_>` might be missing for `Error`
-  --> $DIR/derives-span-PartialEq-enum.rs:4:1
-   |
-LL | struct Error;
-   | ^^^^^^^^^^^^ must implement `PartialEq<_>`
-   = note: this error originates in the derive macro `PartialEq` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: consider annotating `Error` with `#[derive(PartialEq)]`
-   |
-LL | #[derive(PartialEq)]
-   |
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0369`.
diff --git a/src/test/ui/derives/derives-span-PartialEq-struct.rs b/src/test/ui/derives/derives-span-PartialEq-struct.rs
index ce5c67af77f..c92ef5fadf7 100644
--- a/src/test/ui/derives/derives-span-PartialEq-struct.rs
+++ b/src/test/ui/derives/derives-span-PartialEq-struct.rs
@@ -6,7 +6,6 @@ struct Error;
 #[derive(PartialEq)]
 struct Struct {
     x: Error //~ ERROR
-//~^ ERROR
 }
 
 fn main() {}
diff --git a/src/test/ui/derives/derives-span-PartialEq-struct.stderr b/src/test/ui/derives/derives-span-PartialEq-struct.stderr
index c5c8f255fb3..ba3d6ced3f4 100644
--- a/src/test/ui/derives/derives-span-PartialEq-struct.stderr
+++ b/src/test/ui/derives/derives-span-PartialEq-struct.stderr
@@ -18,26 +18,6 @@ help: consider annotating `Error` with `#[derive(PartialEq)]`
 LL | #[derive(PartialEq)]
    |
 
-error[E0369]: binary operation `!=` cannot be applied to type `Error`
-  --> $DIR/derives-span-PartialEq-struct.rs:8:5
-   |
-LL | #[derive(PartialEq)]
-   |          --------- in this derive macro expansion
-LL | struct Struct {
-LL |     x: Error
-   |     ^^^^^^^^
-   |
-note: an implementation of `PartialEq<_>` might be missing for `Error`
-  --> $DIR/derives-span-PartialEq-struct.rs:4:1
-   |
-LL | struct Error;
-   | ^^^^^^^^^^^^ must implement `PartialEq<_>`
-   = note: this error originates in the derive macro `PartialEq` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: consider annotating `Error` with `#[derive(PartialEq)]`
-   |
-LL | #[derive(PartialEq)]
-   |
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0369`.
diff --git a/src/test/ui/derives/derives-span-PartialEq-tuple-struct.rs b/src/test/ui/derives/derives-span-PartialEq-tuple-struct.rs
index eaa62831136..10ac347aa0b 100644
--- a/src/test/ui/derives/derives-span-PartialEq-tuple-struct.rs
+++ b/src/test/ui/derives/derives-span-PartialEq-tuple-struct.rs
@@ -6,7 +6,6 @@ struct Error;
 #[derive(PartialEq)]
 struct Struct(
     Error //~ ERROR
-//~^ ERROR
 );
 
 fn main() {}
diff --git a/src/test/ui/derives/derives-span-PartialEq-tuple-struct.stderr b/src/test/ui/derives/derives-span-PartialEq-tuple-struct.stderr
index f5d609cb33f..ab0b56a9eef 100644
--- a/src/test/ui/derives/derives-span-PartialEq-tuple-struct.stderr
+++ b/src/test/ui/derives/derives-span-PartialEq-tuple-struct.stderr
@@ -18,26 +18,6 @@ help: consider annotating `Error` with `#[derive(PartialEq)]`
 LL | #[derive(PartialEq)]
    |
 
-error[E0369]: binary operation `!=` cannot be applied to type `Error`
-  --> $DIR/derives-span-PartialEq-tuple-struct.rs:8:5
-   |
-LL | #[derive(PartialEq)]
-   |          --------- in this derive macro expansion
-LL | struct Struct(
-LL |     Error
-   |     ^^^^^
-   |
-note: an implementation of `PartialEq<_>` might be missing for `Error`
-  --> $DIR/derives-span-PartialEq-tuple-struct.rs:4:1
-   |
-LL | struct Error;
-   | ^^^^^^^^^^^^ must implement `PartialEq<_>`
-   = note: this error originates in the derive macro `PartialEq` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: consider annotating `Error` with `#[derive(PartialEq)]`
-   |
-LL | #[derive(PartialEq)]
-   |
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0369`.
diff --git a/src/test/ui/derives/deriving-no-inner-impl-error-message.rs b/src/test/ui/derives/deriving-no-inner-impl-error-message.rs
index 39e41a59ef4..b3ce31bf0d3 100644
--- a/src/test/ui/derives/deriving-no-inner-impl-error-message.rs
+++ b/src/test/ui/derives/deriving-no-inner-impl-error-message.rs
@@ -3,7 +3,6 @@ struct NoCloneOrEq;
 #[derive(PartialEq)]
 struct E {
     x: NoCloneOrEq //~ ERROR binary operation `==` cannot be applied to type `NoCloneOrEq`
-         //~^ ERROR binary operation `!=` cannot be applied to type `NoCloneOrEq`
 }
 #[derive(Clone)]
 struct C {
diff --git a/src/test/ui/derives/deriving-no-inner-impl-error-message.stderr b/src/test/ui/derives/deriving-no-inner-impl-error-message.stderr
index 7875f072356..ef8c44caacf 100644
--- a/src/test/ui/derives/deriving-no-inner-impl-error-message.stderr
+++ b/src/test/ui/derives/deriving-no-inner-impl-error-message.stderr
@@ -18,28 +18,8 @@ help: consider annotating `NoCloneOrEq` with `#[derive(PartialEq)]`
 LL | #[derive(PartialEq)]
    |
 
-error[E0369]: binary operation `!=` cannot be applied to type `NoCloneOrEq`
-  --> $DIR/deriving-no-inner-impl-error-message.rs:5:5
-   |
-LL | #[derive(PartialEq)]
-   |          --------- in this derive macro expansion
-LL | struct E {
-LL |     x: NoCloneOrEq
-   |     ^^^^^^^^^^^^^^
-   |
-note: an implementation of `PartialEq<_>` might be missing for `NoCloneOrEq`
-  --> $DIR/deriving-no-inner-impl-error-message.rs:1:1
-   |
-LL | struct NoCloneOrEq;
-   | ^^^^^^^^^^^^^^^^^^ must implement `PartialEq<_>`
-   = note: this error originates in the derive macro `PartialEq` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: consider annotating `NoCloneOrEq` with `#[derive(PartialEq)]`
-   |
-LL | #[derive(PartialEq)]
-   |
-
 error[E0277]: the trait bound `NoCloneOrEq: Clone` is not satisfied
-  --> $DIR/deriving-no-inner-impl-error-message.rs:10:5
+  --> $DIR/deriving-no-inner-impl-error-message.rs:9:5
    |
 LL | #[derive(Clone)]
    |          ----- in this derive macro expansion
@@ -53,7 +33,7 @@ help: consider annotating `NoCloneOrEq` with `#[derive(Clone)]`
 LL | #[derive(Clone)]
    |
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0277, E0369.
 For more information about an error, try `rustc --explain E0277`.
diff --git a/src/test/ui/deriving/deriving-all-codegen.stdout b/src/test/ui/deriving/deriving-all-codegen.stdout
index 21fe663f067..65f7dec8408 100644
--- a/src/test/ui/deriving/deriving-all-codegen.stdout
+++ b/src/test/ui/deriving/deriving-all-codegen.stdout
@@ -122,10 +122,6 @@ impl ::core::cmp::PartialEq for Point {
     fn eq(&self, other: &Point) -> bool {
         self.x == other.x && self.y == other.y
     }
-    #[inline]
-    fn ne(&self, other: &Point) -> bool {
-        self.x != other.x || self.y != other.y
-    }
 }
 impl ::core::marker::StructuralEq for Point {}
 #[automatically_derived]
@@ -239,13 +235,6 @@ impl ::core::cmp::PartialEq for Big {
                     self.b6 == other.b6 && self.b7 == other.b7 &&
             self.b8 == other.b8
     }
-    #[inline]
-    fn ne(&self, other: &Big) -> bool {
-        self.b1 != other.b1 || self.b2 != other.b2 || self.b3 != other.b3 ||
-                            self.b4 != other.b4 || self.b5 != other.b5 ||
-                    self.b6 != other.b6 || self.b7 != other.b7 ||
-            self.b8 != other.b8
-    }
 }
 impl ::core::marker::StructuralEq for Big {}
 #[automatically_derived]
@@ -361,8 +350,6 @@ impl ::core::marker::StructuralPartialEq for Unsized {}
 impl ::core::cmp::PartialEq for Unsized {
     #[inline]
     fn eq(&self, other: &Unsized) -> bool { self.0 == other.0 }
-    #[inline]
-    fn ne(&self, other: &Unsized) -> bool { self.0 != other.0 }
 }
 impl ::core::marker::StructuralEq for Unsized {}
 #[automatically_derived]
@@ -428,8 +415,6 @@ impl ::core::marker::StructuralPartialEq for PackedCopy {}
 impl ::core::cmp::PartialEq for PackedCopy {
     #[inline]
     fn eq(&self, other: &PackedCopy) -> bool { { self.0 } == { other.0 } }
-    #[inline]
-    fn ne(&self, other: &PackedCopy) -> bool { { self.0 } != { other.0 } }
 }
 impl ::core::marker::StructuralEq for PackedCopy {}
 #[automatically_derived]
@@ -503,12 +488,6 @@ impl ::core::cmp::PartialEq for PackedNonCopy {
         let Self(ref __self_1_0) = *other;
         *__self_0_0 == *__self_1_0
     }
-    #[inline]
-    fn ne(&self, other: &PackedNonCopy) -> bool {
-        let Self(ref __self_0_0) = *self;
-        let Self(ref __self_1_0) = *other;
-        *__self_0_0 != *__self_1_0
-    }
 }
 impl ::core::marker::StructuralEq for PackedNonCopy {}
 #[automatically_derived]
@@ -638,13 +617,6 @@ impl ::core::cmp::PartialEq for Enum1 {
                 *__self_0 == *__arg1_0,
         }
     }
-    #[inline]
-    fn ne(&self, other: &Enum1) -> bool {
-        match (self, other) {
-            (Enum1::Single { x: __self_0 }, Enum1::Single { x: __arg1_0 }) =>
-                *__self_0 != *__arg1_0,
-        }
-    }
 }
 impl ::core::marker::StructuralEq for Enum1 {}
 #[automatically_derived]
@@ -883,20 +855,6 @@ impl ::core::cmp::PartialEq for Mixed {
                 _ => true,
             }
     }
-    #[inline]
-    fn ne(&self, other: &Mixed) -> bool {
-        let __self_tag = ::core::intrinsics::discriminant_value(self);
-        let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        __self_tag != __arg1_tag ||
-            match (self, other) {
-                (Mixed::R(__self_0), Mixed::R(__arg1_0)) =>
-                    *__self_0 != *__arg1_0,
-                (Mixed::S { d1: __self_0, d2: __self_1 }, Mixed::S {
-                    d1: __arg1_0, d2: __arg1_1 }) =>
-                    *__self_0 != *__arg1_0 || *__self_1 != *__arg1_1,
-                _ => false,
-            }
-    }
 }
 impl ::core::marker::StructuralEq for Mixed {}
 #[automatically_derived]
@@ -1023,21 +981,6 @@ impl ::core::cmp::PartialEq for Fielded {
                 _ => unsafe { ::core::intrinsics::unreachable() }
             }
     }
-    #[inline]
-    fn ne(&self, other: &Fielded) -> bool {
-        let __self_tag = ::core::intrinsics::discriminant_value(self);
-        let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        __self_tag != __arg1_tag ||
-            match (self, other) {
-                (Fielded::X(__self_0), Fielded::X(__arg1_0)) =>
-                    *__self_0 != *__arg1_0,
-                (Fielded::Y(__self_0), Fielded::Y(__arg1_0)) =>
-                    *__self_0 != *__arg1_0,
-                (Fielded::Z(__self_0), Fielded::Z(__arg1_0)) =>
-                    *__self_0 != *__arg1_0,
-                _ => unsafe { ::core::intrinsics::unreachable() }
-            }
-    }
 }
 impl ::core::marker::StructuralEq for Fielded {}
 #[automatically_derived]