add tests for assigning fields without initializing var

We did not seem to have any!

src/test/ui/nll/assign_mutable_fields.rs

+// 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.
+
+// Currently, we permit you to assign to individual fields of a mut
+// var, but we do not permit you to use the complete var afterwards.
+// We hope to fix this at some point.
+//
+// FIXME(#21232)
+
+#![feature(nll)]
+
+fn assign_both_fields_and_use() {
+    let mut x: (u32, u32);
+    x.0 = 1;
+    x.1 = 22;
+    drop(x.0);
+    drop(x.1);
+}
+
+fn assign_both_fields_the_use_var() {
+    let mut x: (u32, u32);
+    x.0 = 1;
+    x.1 = 22;
+    drop(x); //~ ERROR
+}
+
+fn main() { }

src/test/ui/nll/assign_mutable_fields.stderr

+error[E0381]: use of possibly uninitialized variable: `x`
+  --> $DIR/assign_mutable_fields.rs:31:10
+   |
+LL |     drop(x); //~ ERROR
+   |          ^ use of possibly uninitialized `x`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0381`.

src/test/ui/nll/reassignment_immutable_fields.rs

+// 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.
+
+// This test is currently disallowed, but we hope someday to support it.
+//
+// FIXME(#21232)
+
+#![feature(nll)]
+
+fn assign_both_fields_and_use() {
+    let x: (u32, u32);
+    x.0 = 1; //~ ERROR
+    x.1 = 22; //~ ERROR
+    drop(x.0);
+    drop(x.1);
+}
+
+fn assign_both_fields_the_use_var() {
+    let x: (u32, u32);
+    x.0 = 1; //~ ERROR
+    x.1 = 22; //~ ERROR
+    drop(x); //~ ERROR
+}
+
+fn main() { }

src/test/ui/nll/reassignment_immutable_fields.stderr

+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields.rs:19:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.1`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields.rs:20:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+LL |     x.1 = 22; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields.rs:27:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.1`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields.rs:28:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+LL |     x.1 = 22; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error[E0381]: use of possibly uninitialized variable: `x`
+  --> $DIR/reassignment_immutable_fields.rs:29:10
+   |
+LL |     drop(x); //~ ERROR
+   |          ^ use of possibly uninitialized `x`
+
+error: aborting due to 5 previous errors
+
+Some errors occurred: E0381, E0594.
+For more information about an error, try `rustc --explain E0381`.

src/test/ui/nll/reassignment_immutable_fields_overlapping.rs

+// 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.
+
+// This should never be allowed -- `foo.a` and `foo.b` are
+// overlapping, so since `x` is not `mut` we should not permit
+// reassignment.
+
+#![feature(nll)]
+
+union Foo {
+    a: u32,
+    b: u32,
+}
+
+unsafe fn overlapping_fields() {
+    let x: Foo;
+    x.a = 1;  //~ ERROR
+    x.b = 22; //~ ERROR
+}
+
+fn main() { }

src/test/ui/nll/reassignment_immutable_fields_overlapping.stderr

+error[E0594]: cannot assign to `x.a`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_overlapping.rs:24:5
+   |
+LL |     let x: Foo;
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.a = 1;  //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.b`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_overlapping.rs:25:5
+   |
+LL |     let x: Foo;
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.a = 1;  //~ ERROR
+LL |     x.b = 22; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/nll/reassignment_immutable_fields_twice.rs

+// 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.
+
+// This should never be allowed -- since `x` is not `mut`, so `x.0`
+// cannot be assigned twice.
+
+#![feature(nll)]
+
+fn var_then_field() {
+    let x: (u32, u32);
+    x = (22, 44);
+    x.0 = 1; //~ ERROR
+}
+
+fn same_field_twice() {
+    let x: (u32, u32);
+    x.0 = 1; //~ ERROR
+    x.0 = 22; //~ ERROR
+    x.1 = 44; //~ ERROR
+}
+
+fn main() { }

src/test/ui/nll/reassignment_immutable_fields_twice.stderr

+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_twice.rs:19:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x = (22, 44);
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_twice.rs:24:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_twice.rs:25:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+LL |     x.0 = 22; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.1`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_twice.rs:26:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+...
+LL |     x.1 = 44; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0594`.