Rollup merge of rust-lang#89770 - jkugelman:must-use-from-and-into, r=joshtriplett

Add #[must_use] to From::from and Into::into

Risk of churn: **High**
Magic 8-Ball says: **Outlook not so good**

I figured I'd put this out there. If we don't do it now maybe we save it for a rainy day.

Parent issue: rust-lang#89692

r? `@joshtriplett`

Author: matthiaskrgr

library/core/src/convert/mod.rs

@@ -273,6 +273,7 @@ pub trait AsMut<T: ?Sized> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Into<T>: Sized {
     /// Performs the conversion.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn into(self) -> T;
 }
@@ -367,6 +368,7 @@ pub trait Into<T>: Sized {
 pub trait From<T>: Sized {
     /// Performs the conversion.
     #[lang = "from"]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn from(_: T) -> Self;
 }

src/test/ui/never_type/never-value-fallback-issue-66757.nofallback.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `E: From<()>` is not satisfied
-  --> $DIR/never-value-fallback-issue-66757.rs:27:5
+  --> $DIR/never-value-fallback-issue-66757.rs:28:5
    |
 LL |     <E as From<_>>::from(never);
    |     ^^^^^^^^^^^^^^^^^^^^ the trait `From<()>` is not implemented for `E`

src/test/ui/never_type/never-value-fallback-issue-66757.rs

@@ -22,6 +22,7 @@ impl From<!> for E {
 
 #[allow(unreachable_code)]
 #[allow(dead_code)]
+#[allow(unused_must_use)]
 fn foo(never: !) {
     <E as From<!>>::from(never);  // Ok
     <E as From<_>>::from(never);  //[nofallback]~ ERROR trait bound `E: From<()>` is not satisfied