Auto merge of #55003 - zackmdavis:must_use_assoc_fn, r=varkor

`#[must_use]` for associated functions is supposed to actually work

In the comments of (closed, defunct) pull request #54884, @Centril [noted that](https://github.com/rust-lang/rust/pull/54884#issuecomment-427626495) must-use annotations didn't work on an associated function (what other communities might call a "static method"). Subsequent logging revealed that in this case we have a `Def::Method`, whereas the lint pass was only matching on `Def::Fn`. (One could argue that those def-names are thereby misleading—must-use for `self`-ful methods have always worked—but documenting or reworking that can be left to another day.)

r? @varkor

src/liballoc/rc.rs

@@ -867,7 +867,7 @@ impl<T: ?Sized> Clone for Rc<T> {
     ///
     /// let five = Rc::new(5);
     ///
-    /// Rc::clone(&five);
+    /// let _ = Rc::clone(&five);
     /// ```
     #[inline]
     fn clone(&self) -> Rc<T> {
@@ -1304,7 +1304,7 @@ impl<T: ?Sized> Clone for Weak<T> {
     ///
     /// let weak_five = Rc::downgrade(&Rc::new(5));
     ///
-    /// Weak::clone(&weak_five);
+    /// let _ = Weak::clone(&weak_five);
     /// ```
     #[inline]
     fn clone(&self) -> Weak<T> {

src/liballoc/sync.rs

@@ -713,7 +713,7 @@ impl<T: ?Sized> Clone for Arc<T> {
     ///
     /// let five = Arc::new(5);
     ///
-    /// Arc::clone(&five);
+    /// let _ = Arc::clone(&five);
     /// ```
     #[inline]
     fn clone(&self) -> Arc<T> {
@@ -1135,7 +1135,7 @@ impl<T: ?Sized> Clone for Weak<T> {
     ///
     /// let weak_five = Arc::downgrade(&Arc::new(5));
     ///
-    /// Weak::clone(&weak_five);
+    /// let _ = Weak::clone(&weak_five);
     /// ```
     #[inline]
     fn clone(&self) -> Weak<T> {

src/librustc_lint/unused.rs

@@ -80,10 +80,11 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
                 match callee.node {
                     hir::ExprKind::Path(ref qpath) => {
                         let def = cx.tables.qpath_def(qpath, callee.hir_id);
-                        if let Def::Fn(_) = def {
-                            Some(def)
-                        } else {  // `Def::Local` if it was a closure, for which we
-                            None  // do not currently support must-use linting
+                        match def {
+                            Def::Fn(_) | Def::Method(_) => Some(def),
+                            // `Def::Local` if it was a closure, for which we
+                            // do not currently support must-use linting
+                            _ => None
                         }
                     },
                     _ => None

src/test/run-pass/resolve-pseudo-shadowing.rs

@@ -12,7 +12,7 @@
 
 fn check<Clone>(_c: Clone) {
     fn check2() {
-        <() as std::clone::Clone>::clone(&());
+        let _ = <() as std::clone::Clone>::clone(&());
     }
     check2();
 }

src/test/ui/fn_must_use.rs

@@ -22,6 +22,11 @@ impl MyStruct {
     fn need_to_use_this_method_value(&self) -> usize {
         self.n
     }
+
+    #[must_use]
+    fn need_to_use_this_associated_function_value() -> isize {
+        -1
+    }
 }
 
 trait EvenNature {
@@ -66,6 +71,9 @@ fn main() {
     m.is_even(); // trait method!
     //~^ WARN unused return value
 
+    MyStruct::need_to_use_this_associated_function_value();
+    //~^ WARN unused return value
+
     m.replace(3); // won't warn (annotation needs to be in trait definition)
 
     // comparison methods are `must_use`

src/test/ui/fn_must_use.stderr

@@ -1,5 +1,5 @@
 warning: unused return value of `need_to_use_this_value` which must be used
-  --> $DIR/fn_must_use.rs:60:5
+  --> $DIR/fn_must_use.rs:65:5
    |
 LL |     need_to_use_this_value(); //~ WARN unused return value
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -12,39 +12,45 @@ LL | #![warn(unused_must_use)]
    = note: it's important
 
 warning: unused return value of `MyStruct::need_to_use_this_method_value` which must be used
-  --> $DIR/fn_must_use.rs:65:5
+  --> $DIR/fn_must_use.rs:70:5
    |
 LL |     m.need_to_use_this_method_value(); //~ WARN unused return value
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: unused return value of `EvenNature::is_even` which must be used
-  --> $DIR/fn_must_use.rs:66:5
+  --> $DIR/fn_must_use.rs:71:5
    |
 LL |     m.is_even(); // trait method!
    |     ^^^^^^^^^^^^
    |
    = note: no side effects
 
+warning: unused return value of `MyStruct::need_to_use_this_associated_function_value` which must be used
+  --> $DIR/fn_must_use.rs:74:5
+   |
+LL |     MyStruct::need_to_use_this_associated_function_value();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 warning: unused return value of `std::cmp::PartialEq::eq` which must be used
-  --> $DIR/fn_must_use.rs:72:5
+  --> $DIR/fn_must_use.rs:80:5
    |
 LL |     2.eq(&3); //~ WARN unused return value
    |     ^^^^^^^^^
 
 warning: unused return value of `std::cmp::PartialEq::eq` which must be used
-  --> $DIR/fn_must_use.rs:73:5
+  --> $DIR/fn_must_use.rs:81:5
    |
 LL |     m.eq(&n); //~ WARN unused return value
    |     ^^^^^^^^^
 
 warning: unused comparison which must be used
-  --> $DIR/fn_must_use.rs:76:5
+  --> $DIR/fn_must_use.rs:84:5
    |
 LL |     2 == 3; //~ WARN unused comparison
    |     ^^^^^^
 
 warning: unused comparison which must be used
-  --> $DIR/fn_must_use.rs:77:5
+  --> $DIR/fn_must_use.rs:85:5
    |
 LL |     m == n; //~ WARN unused comparison
    |     ^^^^^^