Add unstable #[may_ignore] attribute to cancel #[must_use]

compiler/rustc_feature/src/active.rs

@@ -620,6 +620,9 @@ declare_features! (
     /// Allows capturing disjoint fields in a closure/generator (RFC 2229).
     (active, capture_disjoint_fields, "1.49.0", Some(53488), None),
 
+    /// Allow `#[may_ignore]` attribute.
+    (active, may_ignore, "1.50.0", Some(99999999), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_feature/src/builtin_attrs.rs

@@ -207,6 +207,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ungated!(forbid, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
     ungated!(deny, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
     ungated!(must_use, AssumedUsed, template!(Word, NameValueStr: "reason")),
+    gated!(may_ignore, AssumedUsed, template!(Word), experimental!(may_ignore)),
     // FIXME(#14407)
     ungated!(
         deprecated, Normal,

compiler/rustc_lint/src/unused.rs

@@ -101,11 +101,6 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
             return;
         }
 
-        let ty = cx.typeck_results().expr_ty(&expr);
-        let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, s.span, "", "", 1);
-
-        let mut fn_warned = false;
-        let mut op_warned = false;
         let maybe_def_id = match expr.kind {
             hir::ExprKind::Call(ref callee, _) => {
                 match callee.kind {
@@ -123,6 +118,21 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
             hir::ExprKind::MethodCall(..) => cx.typeck_results().type_dependent_def_id(expr.hir_id),
             _ => None,
         };
+
+        if let Some(def_id) = maybe_def_id {
+            for attr in cx.tcx.get_attrs(def_id).iter() {
+                if cx.sess().check_name(attr, sym::may_ignore) {
+                    // Don't warn if the function was marked as #[may_ignore].
+                    return;
+                }
+            }
+        }
+
+        let ty = cx.typeck_results().expr_ty(&expr);
+        let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, s.span, "", "", 1);
+
+        let mut fn_warned = false;
+        let mut op_warned = false;
         if let Some(def_id) = maybe_def_id {
             fn_warned = check_must_use_def(cx, def_id, s.span, "return value of ", "");
         } else if type_permits_lack_of_use {

compiler/rustc_span/src/symbol.rs

@@ -681,6 +681,7 @@ symbols! {
         maxnumf32,
         maxnumf64,
         may_dangle,
+        may_ignore,
         maybe_uninit,
         maybe_uninit_uninit,
         maybe_uninit_zeroed,

src/test/ui/feature-gate-may_ignore.rs

@@ -0,0 +1,4 @@
+#[may_ignore]
+//~^ ERROR the `#[may_ignore]` attribute is an experimental feature [E0658]
+fn main() {
+}

src/test/ui/feature-gate-may_ignore.stderr

@@ -0,0 +1,12 @@
+error[E0658]: the `#[may_ignore]` attribute is an experimental feature
+  --> $DIR/feature-gate-may_ignore.rs:1:1
+   |
+LL | #[may_ignore]
+   | ^^^^^^^^^^^^^
+   |
+   = note: see issue #99999999 <https://github.com/rust-lang/rust/issues/99999999> for more information
+   = help: add `#![feature(may_ignore)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/may_ignore.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+#![feature(may_ignore)]
+#![warn(unused_must_use)]
+
+fn warn() -> Result<i32, i32> {
+    Err(1)
+}
+
+#[may_ignore]
+fn no_warn() -> Result<i32, i32> {
+    Err(2)
+}
+
+fn main() {
+    warn(); //~ WARN [unused_must_use]
+    no_warn();
+}

src/test/ui/may_ignore.stderr

@@ -0,0 +1,15 @@
+warning: unused `std::result::Result` that must be used
+  --> $DIR/may_ignore.rs:16:5
+   |
+LL |     warn();
+   |     ^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/may_ignore.rs:4:9
+   |
+LL | #![warn(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+   = note: this `Result` may be an `Err` variant, which should be handled
+
+warning: 1 warning emitted
+