Fix into() cast paren check precedence

As discussed in #47699 the logic for determining if an expression needs
parenthesis when suggesting an `.into()` cast is incorrect. Two broken
examples from nightly are:

```
error[E0308]: mismatched types
 --> main.rs:4:10
  |
4 |     test(foo as i8);
  |          ^^^^^^^^^ expected i32, found i8
help: you can cast an `i8` to `i32`, which will sign-extend the source value
  |
4 |     test(foo as i8.into());
  |
```

```
error[E0308]: mismatched types
 --> main.rs:4:10
  |
4 |     test(*foo);
  |          ^^^^ expected i32, found i8
help: you can cast an `i8` to `i32`, which will sign-extend the source value
  |
4 |     test(*foo.into());
  |
```

As suggested by @petrochenkov switch the precedence check to
PREC_POSTFIX. This catches both `as` and unary operators. Fixes #47699.

src/librustc_typeck/check/demand.rs

@@ -15,7 +15,7 @@
 use rustc::traits::ObligationCause;
 
 use syntax::ast;
-use syntax::util::parser::AssocOp;
+use syntax::util::parser::PREC_POSTFIX;
 use syntax_pos::{self, Span};
 use rustc::hir;
 use rustc::hir::print;
@@ -336,7 +336,7 @@ fn check_for_cast(&self,
         // For now, don't suggest casting with `as`.
         let can_cast = false;
 
-        let needs_paren = expr.precedence().order() < (AssocOp::As.precedence() as i8);
+        let needs_paren = expr.precedence().order() < (PREC_POSTFIX as i8);
 
         if let Ok(src) = self.tcx.sess.codemap().span_to_snippet(expr.span) {
             let msg = format!("you can cast an `{}` to `{}`", checked_ty, expected_ty);

src/test/ui/suggestions/numeric-cast.rs

@@ -312,4 +312,9 @@ fn main() {
     foo::<f32>(x_f64);
     //~^ ERROR mismatched types
     foo::<f32>(x_f32);
+
+    foo::<u32>(x_u8 as u16);
+    //~^ ERROR mismatched types
+    foo::<i32>(-x_i8);
+    //~^ ERROR mismatched types
 }

src/test/ui/suggestions/numeric-cast.stderr

@@ -882,5 +882,25 @@ error[E0308]: mismatched types
 312 |     foo::<f32>(x_f64);
     |                ^^^^^ expected f32, found f64
 
-error: aborting due to 132 previous errors
+error[E0308]: mismatched types
+   --> $DIR/numeric-cast.rs:316:16
+    |
+316 |     foo::<u32>(x_u8 as u16);
+    |                ^^^^^^^^^^^ expected u32, found u16
+help: you can cast an `u16` to `u32`, which will zero-extend the source value
+    |
+316 |     foo::<u32>((x_u8 as u16).into());
+    |                ^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+   --> $DIR/numeric-cast.rs:318:16
+    |
+318 |     foo::<i32>(-x_i8);
+    |                ^^^^^ expected i32, found i8
+help: you can cast an `i8` to `i32`, which will sign-extend the source value
+    |
+318 |     foo::<i32>((-x_i8).into());
+    |                ^^^^^^^^^^^^^^
+
+error: aborting due to 134 previous errors