Rollup merge of #41838 - z1mvader:fix_fn_args_coerce_closure, r=nikomatsakis

Fixed argument inference for closures when coercing into 'fn'

This fixes https://github.com/rust-lang/rust/issues/41755. The tests  `compile-fail/closure-no-fn.rs` and `compile-fail/issue-40000.rs` were modified. A new test `run-pass/closure_to_fn_coercion-expected-types.rs` was added

r? @nikomatsakis

src/librustc_typeck/check/closure.rs

@@ -126,6 +126,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 (sig, kind)
             }
             ty::TyInfer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
+            ty::TyFnPtr(sig) => (Some(sig.skip_binder().clone()), Some(ty::ClosureKind::Fn)),
             _ => (None, None),
         }
     }

src/test/compile-fail/closure-no-fn-1.rs

@@ -15,10 +15,4 @@ fn main() {
     let mut a = 0u8;
     let foo: fn(u8) -> u8 = |v: u8| { a += v; a };
     //~^ ERROR mismatched types
-    let b = 0u8;
-    let bar: fn() -> u8 = || { b };
-    //~^ ERROR mismatched types
-    let baz: fn() -> u8 = || { b } as fn() -> u8;
-    //~^ ERROR mismatched types
-    //~^^ ERROR non-scalar cast
 }

src/test/compile-fail/closure-no-fn-2.rs

@@ -0,0 +1,18 @@
+// Copyright 2017 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.
+
+// Ensure that capturing closures are never coerced to fns
+// Especially interesting as non-capturing closures can be.
+
+fn main() {
+    let b = 0u8;
+    let bar: fn() -> u8 = || { b };
+    //~^ ERROR mismatched types
+}

src/test/compile-fail/closure-no-fn-3.rs

@@ -0,0 +1,18 @@
+// Copyright 2017 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.
+
+// Ensure that capturing closures are never coerced to fns
+// Especially interesting as non-capturing closures can be.
+
+fn main() {
+    let b = 0u8;
+    let baz: fn() -> u8 = (|| { b }) as fn() -> u8;
+    //~^ ERROR non-scalar cast
+}

src/test/compile-fail/issue-40000.rs

@@ -11,8 +11,7 @@
 #![feature(closure_to_fn_coercion)]
 
 fn main() {
-    let bar: fn(&mut u32) = |_| {}; //~ ERROR mismatched types
-    //~| expected concrete lifetime, found bound lifetime parameter
+    let bar: fn(&mut u32) = |_| {};
 
     fn foo(x: Box<Fn(&i32)>) {}
     let bar = Box::new(|x: &i32| {}) as Box<Fn(_)>;

src/test/run-pass/closure_to_fn_coercion-expected-types.rs

@@ -0,0 +1,17 @@
+// Copyright 2012 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.
+// Ensure that we deduce expected argument types when a `fn()` type is expected (#41755)
+
+#![feature(closure_to_fn_coercion)]
+fn foo(f: fn(Vec<u32>) -> usize) { }
+
+fn main() {
+    foo(|x| x.len())
+}