Auto merge of #42649 - estebank:if-cond, r=nikomatsakis

Report error for assignment in `if` condition

For code like `if x = 3 {}`, output:

```
error[E0308]: mismatched types
  --> $DIR/issue-17283.rs:25:8
   |
25 |     if x = x {
   |        ^^^^^
   |        |
   |        help: did you mean to compare equality? `x == x`
   |        expected bool, found ()
   |
   = note: expected type `bool`
              found type `()`
```

Fix #40926.

src/test/ui/type-check/assignment-in-if.rs

@@ -0,0 +1,52 @@
+// 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.
+
+// Test that the parser does not attempt to parse struct literals
+// within assignments in if expressions.
+
+#![allow(unused_parens)]
+
+struct Foo {
+    foo: usize
+}
+
+fn main() {
+    let x = 1;
+    let y: Foo;
+
+    // `x { ... }` should not be interpreted as a struct literal here
+    if x = x {
+        //~^ ERROR mismatched types
+        //~| HELP did you mean to compare equality?
+        println!("{}", x);
+    }
+    // Explicit parentheses on the left should match behavior of above
+    if (x = x) {
+        //~^ ERROR mismatched types
+        //~| HELP did you mean to compare equality?
+        println!("{}", x);
+    }
+    // The struct literal interpretation is fine with explicit parentheses on the right
+    if y = (Foo { foo: x }) {
+        //~^ ERROR mismatched types
+        //~| HELP did you mean to compare equality?
+        println!("{}", x);
+    }
+    // "invalid left-hand side expression" error is suppresed
+    if 3 = x {
+        //~^ ERROR mismatched types
+        //~| HELP did you mean to compare equality?
+        println!("{}", x);
+    }
+    if (if true { x = 4 } else { x = 5 }) {
+        //~^ ERROR mismatched types
+        println!("{}", x);
+    }
+}

src/test/ui/type-check/assignment-in-if.stderr

@@ -0,0 +1,59 @@
+error[E0308]: mismatched types
+  --> $DIR/assignment-in-if.rs:25:8
+   |
+25 |     if x = x {
+   |        ^^^^^
+   |        |
+   |        help: did you mean to compare equality? `x == x`
+   |        expected bool, found ()
+   |
+   = note: expected type `bool`
+              found type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/assignment-in-if.rs:31:8
+   |
+31 |     if (x = x) {
+   |        ^^^^^^^
+   |        |
+   |        help: did you mean to compare equality? `x == x`
+   |        expected bool, found ()
+   |
+   = note: expected type `bool`
+              found type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/assignment-in-if.rs:37:8
+   |
+37 |     if y = (Foo { foo: x }) {
+   |        ^^^^^^^^^^^^^^^^^^^^
+   |        |
+   |        help: did you mean to compare equality? `y == (Foo { foo: x })`
+   |        expected bool, found ()
+   |
+   = note: expected type `bool`
+              found type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/assignment-in-if.rs:43:8
+   |
+43 |     if 3 = x {
+   |        ^^^^^
+   |        |
+   |        help: did you mean to compare equality? `3 == x`
+   |        expected bool, found ()
+   |
+   = note: expected type `bool`
+              found type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/assignment-in-if.rs:48:8
+   |
+48 |     if (if true { x = 4 } else { x = 5 }) {
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found ()
+   |
+   = note: expected type `bool`
+              found type `()`
+
+error: aborting due to previous error(s)
+