Suggest using slice when encountering let x = ""[..];

src/test/ui/suggestions/str-array-assignment.rs

@@ -0,0 +1,17 @@
+// 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.
+
+fn main() {
+  let s = "abc";
+  let t = if true { s[..2] } else { s };
+  let u: &str = if true { s[..2] } else { s };
+  let v = s[..2];
+  let w: &str = s[..2];
+}

src/test/ui/suggestions/str-array-assignment.stderr

@@ -0,0 +1,44 @@
+error[E0308]: if and else have incompatible types
+  --> $DIR/str-array-assignment.rs:13:11
+   |
+13 |   let t = if true { s[..2] } else { s };
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected str, found &str
+   |
+   = note: expected type `str`
+              found type `&str`
+
+error[E0308]: mismatched types
+  --> $DIR/str-array-assignment.rs:14:27
+   |
+11 | fn main() {
+   |           - expected `()` because of default return type
+...
+14 |   let u: &str = if true { s[..2] } else { s };
+   |                           ^^^^^^ expected &str, found str
+   |
+   = note: expected type `&str`
+              found type `str`
+
+error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
+  --> $DIR/str-array-assignment.rs:15:7
+   |
+15 |   let v = s[..2];
+   |       ^   ------ help: consider a slice instead: `&s[..2]`
+   |       |
+   |       `str` does not have a constant size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `str`
+   = note: all local variables must have a statically known size
+
+error[E0308]: mismatched types
+  --> $DIR/str-array-assignment.rs:16:17
+   |
+16 |   let w: &str = s[..2];
+   |                 ^^^^^^ expected &str, found str
+   |
+   = note: expected type `&str`
+              found type `str`
+   = help: try with `&s[..2]`
+
+error: aborting due to 4 previous errors
+