Add compile-fail style comments to tests

Author: estebank

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

@@ -8,10 +8,25 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn main() {
+fn main() { //~ NOTE expected `()` because of default return type
   let s = "abc";
   let t = if true { s[..2] } else { s };
+  //~^ ERROR if and else have incompatible types
+  //~| NOTE expected str, found &str
+  //~| NOTE expected type
   let u: &str = if true { s[..2] } else { s };
+  //~^ ERROR mismatched types
+  //~| NOTE expected &str, found str
+  //~| NOTE expected type
   let v = s[..2];
+  //~^ ERROR the trait bound `str: std::marker::Sized` is not satisfied
+  //~| NOTE consider a slice instead
+  //~| NOTE `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
   let w: &str = s[..2];
+  //~^ ERROR mismatched types
+  //~| NOTE expected &str, found str
+  //~| NOTE expected type
+  //~| HELP try with `&s[..2]`
 }

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

@@ -8,21 +8,21 @@ error[E0308]: if and else have incompatible types
               found type `&str`
 
 error[E0308]: mismatched types
-  --> $DIR/str-array-assignment.rs:14:27
+  --> $DIR/str-array-assignment.rs:17:27
    |
-11 | fn main() {
+11 | fn main() { //~ NOTE expected `()` because of default return type
    |           - expected `()` because of default return type
 ...
-14 |   let u: &str = if true { s[..2] } else { s };
+17 |   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
+  --> $DIR/str-array-assignment.rs:21:7
    |
-15 |   let v = s[..2];
+21 |   let v = s[..2];
    |       ^   ------ help: consider a slice instead: `&s[..2]`
    |       |
    |       `str` does not have a constant size known at compile-time
@@ -31,9 +31,9 @@ error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
    = note: all local variables must have a statically known size
 
 error[E0308]: mismatched types
-  --> $DIR/str-array-assignment.rs:16:17
+  --> $DIR/str-array-assignment.rs:27:17
    |
-16 |   let w: &str = s[..2];
+27 |   let w: &str = s[..2];
    |                 ^^^^^^ expected &str, found str
    |
    = note: expected type `&str`