Auto merge of #45864 - nikomatsakis:issue-30046-infer-fn-once-in-closures, r=eddyb

adjust closure kind based on the guarantor's upvar note

Fixes #30046.

r? @eddyb

3 files changed, 77 insertions, 0 deletions

diff --git a/src/test/run-pass/unboxed-closures-move-from-projection-issue-30046.rs b/src/test/run-pass/unboxed-closures-move-from-projection-issue-30046.rs
new file mode 100644
index 00000000000..d902ebc9dc9
--- /dev/null
+++ b/src/test/run-pass/unboxed-closures-move-from-projection-issue-30046.rs
@@ -0,0 +1,35 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(unused)]
+
+fn foo<F>(f: F)
+    where F: FnOnce()
+{
+}
+
+fn main() {
+    // Test that this closure is inferred to `FnOnce`
+    // because it moves from `y.as<Option::Some>.0`:
+    let x = Some(vec![1, 2, 3]);
+    foo(|| {
+        match x {
+            Some(y) => { }
+            None => { }
+        }
+    });
+
+    // Test that this closure is inferred to `FnOnce`
+    // because it moves from `y.0`:
+    let y = (vec![1, 2, 3], 0);
+    foo(|| {
+        let x = y.0;
+    });
+}
diff --git a/src/test/ui/unboxed-closures-infer-fn-once-move-from-projection.rs b/src/test/ui/unboxed-closures-infer-fn-once-move-from-projection.rs
new file mode 100644
index 00000000000..14ef3b5f178
--- /dev/null
+++ b/src/test/ui/unboxed-closures-infer-fn-once-move-from-projection.rs
@@ -0,0 +1,26 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(unused)]
+
+fn foo<F>(f: F)
+    where F: Fn()
+{
+}
+
+fn main() {
+    // Test that this closure is inferred to `FnOnce` because it moves
+    // from `y.0`. This affects the error output (the error is that
+    // the closure implements `FnOnce`, not that it moves from inside
+    // a `Fn` closure.)
+    let y = (vec![1, 2, 3], 0);
+    let c = || drop(y.0);
+    foo(c);
+}
diff --git a/src/test/ui/unboxed-closures-infer-fn-once-move-from-projection.stderr b/src/test/ui/unboxed-closures-infer-fn-once-move-from-projection.stderr
new file mode 100644
index 00000000000..d968c409396
--- /dev/null
+++ b/src/test/ui/unboxed-closures-infer-fn-once-move-from-projection.stderr
@@ -0,0 +1,16 @@
+error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`
+  --> $DIR/unboxed-closures-infer-fn-once-move-from-projection.rs:24:13
+   |
+24 |     let c = || drop(y.0);
+   |             ^^^^^^^^^^^^
+25 |     foo(c);
+   |     --- the requirement to implement `Fn` derives from here
+   |
+note: closure is `FnOnce` because it moves the variable `y` out of its environment
+  --> $DIR/unboxed-closures-infer-fn-once-move-from-projection.rs:24:21
+   |
+24 |     let c = || drop(y.0);
+   |                     ^
+
+error: aborting due to previous error
+