don't emit unused_results lint for tuples of trivial types

Author: WaffleLapkin

compiler/rustc_lint/src/unused/must_use.rs

@@ -108,13 +108,6 @@ impl IsTyMustUse {
             _ => self,
         }
     }
-
-    fn yes(self) -> Option<MustUsePath> {
-        match self {
-            Self::Yes(must_use_path) => Some(must_use_path),
-            _ => None,
-        }
-    }
 }
 
 /// A path through a type to a `must_use` source. Contains useful info for the lint.
@@ -254,16 +247,23 @@ pub fn is_ty_must_use<'tcx>(
             // Default to `expr`.
             let elem_exprs = elem_exprs.iter().chain(iter::repeat(expr));
 
-            let nested_must_use = tys
-                .iter()
-                .zip(elem_exprs)
-                .enumerate()
-                .filter_map(|(i, (ty, expr))| {
-                    is_ty_must_use(cx, ty, expr, simplify_uninhabited).yes().map(|path| (i, path))
-                })
-                .collect::<Vec<_>>();
+            let mut nested_must_use = Vec::new();
+            let all_trivial = tys.iter().zip(elem_exprs).enumerate().all(|(i, (ty, expr))| {
+                let must_use = is_ty_must_use(cx, ty, expr, simplify_uninhabited);
+
+                let trivial = matches!(must_use, IsTyMustUse::Trivial);
+                if let IsTyMustUse::Yes(path) = must_use {
+                    nested_must_use.push((i, path));
+                }
+
+                trivial
+            });
 
-            if !nested_must_use.is_empty() {
+            if all_trivial {
+                // If all tuple elements are trivial, mark the whole tuple as such.
+                // i.e. don't emit `unused_results` for types such as `((), ())`
+                IsTyMustUse::Trivial
+            } else if !nested_must_use.is_empty() {
                 IsTyMustUse::Yes(MustUsePath::TupleElement(nested_must_use))
             } else {
                 IsTyMustUse::No

tests/ui/lint/unused/unused-result.rs

@@ -44,11 +44,12 @@ fn main() {
     let _ = foo::<MustUse>();
     let _ = foo::<MustUseMsg>();
 
+    // "trivial" types
     ();
-    ((), ()); //~ ERROR: unused result of type
+    ((), ());
     Ok::<(), Nothing>(());
     ControlFlow::<Nothing>::Continue(());
-    ((), Ok::<(), Nothing>(()), ((((), ())), ((),))); //~ ERROR: unused result of type
+    ((), Ok::<(), Nothing>(()), ((((), ())), ((),)));
     foo::<Nothing>();
 
     ((), 1); //~ ERROR: unused result of type `((), i32)`

tests/ui/lint/unused/unused-result.stderr

@@ -61,29 +61,17 @@ help: use `let _ = ...` to ignore the resulting value
 LL |     let _ = foo::<MustUseMsg>();
    |     +++++++
 
-error: unused result of type `((), ())`
-  --> $DIR/unused-result.rs:48:5
-   |
-LL |     ((), ());
-   |     ^^^^^^^^^
-
-error: unused result of type `((), Result<(), Nothing>, (((), ()), ((),)))`
-  --> $DIR/unused-result.rs:51:5
-   |
-LL |     ((), Ok::<(), Nothing>(()), ((((), ())), ((),)));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error: unused result of type `((), i32)`
-  --> $DIR/unused-result.rs:54:5
+  --> $DIR/unused-result.rs:55:5
    |
 LL |     ((), 1);
    |     ^^^^^^^^
 
 error: unused result of type `(i32, ())`
-  --> $DIR/unused-result.rs:55:5
+  --> $DIR/unused-result.rs:56:5
    |
 LL |     (1, ());
    |     ^^^^^^^^
 
-error: aborting due to 9 previous errors
+error: aborting due to 7 previous errors