Auto merge of #50149 - aaronaaeng:master, r=estebank

Added warning for unused arithmetic expressions

The compiler now displays a warning when a binary arithmetic operation is evaluated but not used.  This resolves #50124  by following the instructions outlined in the issue.  The changes are as follows:

- Added new pattern matching for unused arithmetic expressions in `src/librustc_lint/unused.rs`
- Added `#[must_use]` attributes to the binary operation methods in `src/libcore/internal_macros.rs`
- Added `#[must_use]` attributes to the non-assigning binary operators in `src/libcore/ops/arith.rs`

3 files changed, 13 insertions, 0 deletions

diff --git a/src/libcore/ops/arith.rs b/src/libcore/ops/arith.rs
index a1f6030428f..a1bc5463f73 100644
--- a/src/libcore/ops/arith.rs
+++ b/src/libcore/ops/arith.rs
@@ -94,6 +94,7 @@ pub trait Add<RHS=Self> {
     type Output;
 
     /// Performs the `+` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn add(self, rhs: RHS) -> Self::Output;
 }
@@ -191,6 +192,7 @@ pub trait Sub<RHS=Self> {
     type Output;
 
     /// Performs the `-` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn sub(self, rhs: RHS) -> Self::Output;
 }
@@ -310,6 +312,7 @@ pub trait Mul<RHS=Self> {
     type Output;
 
     /// Performs the `*` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn mul(self, rhs: RHS) -> Self::Output;
 }
@@ -433,6 +436,7 @@ pub trait Div<RHS=Self> {
     type Output;
 
     /// Performs the `/` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn div(self, rhs: RHS) -> Self::Output;
 }
@@ -517,6 +521,7 @@ pub trait Rem<RHS=Self> {
     type Output = Self;
 
     /// Performs the `%` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn rem(self, rhs: RHS) -> Self::Output;
 }
@@ -601,6 +606,7 @@ pub trait Neg {
     type Output;
 
     /// Performs the unary `-` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn neg(self) -> Self::Output;
 }
diff --git a/src/libcore/ops/bit.rs b/src/libcore/ops/bit.rs
index 02b6f62db6e..3900f365b0a 100644
--- a/src/libcore/ops/bit.rs
+++ b/src/libcore/ops/bit.rs
@@ -46,6 +46,7 @@ pub trait Not {
     type Output;
 
     /// Performs the unary `!` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn not(self) -> Self::Output;
 }
@@ -129,6 +130,7 @@ pub trait BitAnd<RHS=Self> {
     type Output;
 
     /// Performs the `&` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn bitand(self, rhs: RHS) -> Self::Output;
 }
@@ -212,6 +214,7 @@ pub trait BitOr<RHS=Self> {
     type Output;
 
     /// Performs the `|` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn bitor(self, rhs: RHS) -> Self::Output;
 }
@@ -298,6 +301,7 @@ pub trait BitXor<RHS=Self> {
     type Output;
 
     /// Performs the `^` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn bitxor(self, rhs: RHS) -> Self::Output;
 }
@@ -385,6 +389,7 @@ pub trait Shl<RHS=Self> {
     type Output;
 
     /// Performs the `<<` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn shl(self, rhs: RHS) -> Self::Output;
 }
@@ -493,6 +498,7 @@ pub trait Shr<RHS=Self> {
     type Output;
 
     /// Performs the `>>` operation.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn shr(self, rhs: RHS) -> Self::Output;
 }
diff --git a/src/libcore/ops/deref.rs b/src/libcore/ops/deref.rs
index 332d154170f..54eecc82e19 100644
--- a/src/libcore/ops/deref.rs
+++ b/src/libcore/ops/deref.rs
@@ -77,6 +77,7 @@ pub trait Deref {
     type Target: ?Sized;
 
     /// Dereferences the value.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn deref(&self) -> &Self::Target;
 }