Mark PartialEq as #[rustc_trivial_field_reads]

Author: GrigorenkoPV

library/core/src/cmp.rs

@@ -247,6 +247,7 @@ use crate::ops::ControlFlow;
     append_const_msg
 )]
 #[rustc_diagnostic_item = "PartialEq"]
+#[rustc_trivial_field_reads]
 #[const_trait]
 #[rustc_const_unstable(feature = "const_trait_impl", issue = "67792")]
 pub trait PartialEq<Rhs: PointeeSized = Self>: PointeeSized {

tests/ui/lint/dead-code/partial-eq.rs

@@ -0,0 +1,20 @@
+#![deny(dead_code)]
+
+#[derive(PartialEq)]
+struct MyStruct {
+    x: i32,
+    y: i32, //~ ERROR field `y` is never read
+}
+
+struct MyStruct2 {
+    x: i32,
+    y: i32, //~ ERROR field `y` is never read
+}
+
+pub fn main() {
+    let ms = MyStruct { x: 1, y: 2 };
+    let _ = ms.x;
+
+    let ms = MyStruct2 { x: 1, y: 2 };
+    let _ = ms.x;
+}

tests/ui/lint/dead-code/partial-eq.stderr

@@ -0,0 +1,27 @@
+error: field `y` is never read
+  --> $DIR/partial-eq.rs:6:5
+   |
+LL | struct MyStruct {
+   |        -------- field in this struct
+LL |     x: i32,
+LL |     y: i32,
+   |     ^
+   |
+   = note: `MyStruct` has a derived impl for the trait `PartialEq`, but this is intentionally ignored during dead code analysis
+note: the lint level is defined here
+  --> $DIR/partial-eq.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: field `y` is never read
+  --> $DIR/partial-eq.rs:11:5
+   |
+LL | struct MyStruct2 {
+   |        --------- field in this struct
+LL |     x: i32,
+LL |     y: i32,
+   |     ^
+
+error: aborting due to 2 previous errors
+