mark comparison trait methods as #[must_use]

Note that this doesn't actually give us warnings on `a == b;` and the like, as
some observers may have hoped.

This is in the matter of #43302.

src/libcore/cmp.rs

@@ -110,11 +110,13 @@ use self::Ordering::*;
 pub trait PartialEq<Rhs: ?Sized = Self> {
     /// This method tests for `self` and `other` values to be equal, and is used
     /// by `==`.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn eq(&self, other: &Rhs) -> bool;
 
     /// This method tests for `!=`.
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn ne(&self, other: &Rhs) -> bool { !self.eq(other) }
 }
@@ -625,6 +627,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
     /// let result = std::f64::NAN.partial_cmp(&1.0);
     /// assert_eq!(result, None);
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn partial_cmp(&self, other: &Rhs) -> Option<Ordering>;
 
@@ -640,6 +643,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
     /// assert_eq!(result, false);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn lt(&self, other: &Rhs) -> bool {
         match self.partial_cmp(other) {
@@ -661,6 +665,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
     /// assert_eq!(result, true);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn le(&self, other: &Rhs) -> bool {
         match self.partial_cmp(other) {
@@ -681,6 +686,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
     /// assert_eq!(result, false);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn gt(&self, other: &Rhs) -> bool {
         match self.partial_cmp(other) {
@@ -702,6 +708,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
     /// assert_eq!(result, true);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn ge(&self, other: &Rhs) -> bool {
         match self.partial_cmp(other) {