Auto merge of #51964 - matthewjasper:unused-mut-mir-generation, r=nikomatsakis

[NLL] Fix various unused mut errors

Closes #51801
Closes #50897
Closes #51830
Closes #51904
cc #51918 - keeping this one open in case there are any more issues

This PR contains multiple changes. List of changes with examples of what they fix:

* Change mir generation so that the parameter variable doesn't get a name when a `ref` pattern is used as an argument
```rust
fn f(ref y: i32) {} // doesn't trigger lint
```
* Change mir generation so that by-move closure captures don't get first moved into a temporary.
```rust
let mut x = 0; // doesn't trigger lint
move || {
    x = 1;
};
```
* Treat generator upvars the same as closure upvars
```rust
let mut x = 0; // This mut is now necessary and is not linted against.
move || {
    x = 1;
    yield;
};
```

r? @nikomatsakis

src/test/mir-opt/end_region_7.rs

@@ -34,38 +34,31 @@ fn foo<F>(f: F) where F: FnOnce() -> i32 {
 //     ...
 //     let mut _2: ();
 //     let mut _3: [closure@NodeId(22) d:D];
-//     let mut _4: D;
 //     bb0: {
 //         StorageLive(_1);
 //         _1 = D::{{constructor}}(const 0i32,);
 //         StorageLive(_3);
-//         StorageLive(_4);
-//         _4 = move _1;
-//         _3 = [closure@NodeId(22)] { d: move _4 };
-//         drop(_4) -> [return: bb4, unwind: bb3];
+//         _3 = [closure@NodeId(22)] { d: move _1 };
+//         _2 = const foo(move _3) -> [return: bb2, unwind: bb4];
 //     }
 //     bb1: {
 //         resume;
 //     }
 //     bb2: {
-//         drop(_1) -> bb1;
+//         drop(_3) -> [return: bb5, unwind: bb3];
 //     }
 //     bb3: {
-//         drop(_3) -> bb2;
+//         drop(_1) -> bb1;
 //     }
 //     bb4: {
-//         StorageDead(_4);
-//         _2 = const foo(move _3) -> [return: bb5, unwind: bb3];
+//         drop(_3) -> bb3;
 //     }
 //     bb5: {
-//         drop(_3) -> [return: bb6, unwind: bb2];
-//     }
-//     bb6: {
 //         StorageDead(_3);
 //         _0 = ();
-//         drop(_1) -> [return: bb7, unwind: bb1];
+//         drop(_1) -> [return: bb6, unwind: bb1];
 //     }
-//     bb7: {
+//     bb6: {
 //         StorageDead(_1);
 //         return;
 //     }

src/test/mir-opt/end_region_8.rs

@@ -37,17 +37,13 @@ fn foo<F>(f: F) where F: FnOnce() -> i32 {
 //    ...
 //    let mut _3: ();
 //    let mut _4: [closure@NodeId(22) r:&'19s D];
-//    let mut _5: &'21_1rs D;
 //    bb0: {
 //        StorageLive(_1);
 //        _1 = D::{{constructor}}(const 0i32,);
 //        StorageLive(_2);
 //        _2 = &'21_1rs _1;
 //        StorageLive(_4);
-//        StorageLive(_5);
-//        _5 = _2;
-//        _4 = [closure@NodeId(22)] { r: move _5 };
-//        StorageDead(_5);
+//        _4 = [closure@NodeId(22)] { r: _2 };
 //        _3 = const foo(move _4) -> [return: bb2, unwind: bb3];
 //    }
 //    bb1: {

src/test/ui/nll/capture-mut-ref.rs

@@ -0,0 +1,25 @@
+// Copyright 2018 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.
+
+// Check that capturing a mutable reference by move and assigning to its
+// referent doesn't make the unused mut lint think that it is mutable.
+
+#![feature(nll)]
+#![deny(unused_mut)]
+
+fn mutable_upvar() {
+    let mut x = &mut 0;
+    //~^ ERROR
+    move || {
+        *x = 1;
+    };
+}
+
+fn main() {}

src/test/ui/nll/capture-mut-ref.stderr

@@ -0,0 +1,16 @@
+error: variable does not need to be mutable
+  --> $DIR/capture-mut-ref.rs:18:9
+   |
+LL |     let mut x = &mut 0;
+   |         ----^
+   |         |
+   |         help: remove this `mut`
+   |
+note: lint level defined here
+  --> $DIR/capture-mut-ref.rs:15:9
+   |
+LL | #![deny(unused_mut)]
+   |         ^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/nll/extra-unused-mut.rs

@@ -0,0 +1,64 @@
+// Copyright 2018 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.
+
+// extra unused mut lint tests for #51918
+
+// run-pass
+
+#![feature(generators, nll)]
+#![deny(unused_mut)]
+
+fn ref_argument(ref _y: i32) {}
+
+// #51801
+fn mutable_upvar() {
+    let mut x = 0;
+    move || {
+        x = 1;
+    };
+}
+
+// #50897
+fn generator_mutable_upvar() {
+    let mut x = 0;
+    move || {
+        x = 1;
+        yield;
+    };
+}
+
+// #51830
+fn ref_closure_argument() {
+    let _ = Some(0).as_ref().map(|ref _a| true);
+}
+
+struct Expr {
+    attrs: Vec<u32>,
+}
+
+// #51904
+fn parse_dot_or_call_expr_with(mut attrs: Vec<u32>) {
+    let x = Expr { attrs: vec![] };
+    Some(Some(x)).map(|expr|
+        expr.map(|mut expr| {
+            attrs.push(666);
+            expr.attrs = attrs;
+            expr
+        })
+    );
+}
+
+fn main() {
+    ref_argument(0);
+    mutable_upvar();
+    generator_mutable_upvar();
+    ref_closure_argument();
+    parse_dot_or_call_expr_with(Vec::new());
+}

src/test/ui/nll/generator-upvar-mutability.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+// Check that generators respect the muatability of their upvars.
+
+#![feature(generators, nll)]
+
+fn mutate_upvar() {
+    let x = 0;
+    move || {
+        x = 1;
+        //~^ ERROR
+        yield;
+    };
+}
+
+fn main() {}

src/test/ui/nll/generator-upvar-mutability.stderr

@@ -0,0 +1,9 @@
+error[E0594]: cannot assign to immutable item `x`
+  --> $DIR/generator-upvar-mutability.rs:18:9
+   |
+LL |         x = 1;
+   |         ^^^^^ cannot assign
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.