Fix suggestions for redundant_pattern_matching

Fixes the problem displayed in rust-lang#4344 (comment).

We now append `{}` to the suggestion so that the conditional has the
correct syntax again.

(If we were to _remove_ the `if` instead, it would trigger the
`unused_must_use` warning for `#[must_use]` types.

clippy_lints/src/redundant_pattern_matching.rs

@@ -90,8 +90,8 @@ fn find_sugg_for_if_let<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr,
                 db.span_suggestion(
                     span,
                     "try this",
-                    format!("if {}.{}", snippet(cx, op.span, "_"), good_method),
-                    Applicability::MachineApplicable, // snippet
+                    format!("{}.{}", snippet(cx, op.span, "_"), good_method),
+                    Applicability::MaybeIncorrect, // snippet
                 );
             },
         );
@@ -154,7 +154,7 @@ fn find_sugg_for_match<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr, o
                         span,
                         "try this",
                         format!("{}.{}", snippet(cx, op.span, "_"), good_method),
-                        Applicability::MachineApplicable, // snippet
+                        Applicability::MaybeIncorrect, // snippet
                     );
                 },
             );

tests/ui/redundant_pattern_matching.stderr

@@ -2,27 +2,27 @@ error: redundant pattern matching, consider using `is_ok()`
   --> $DIR/redundant_pattern_matching.rs:5:12
    |
 LL |     if let Ok(_) = Ok::<i32, i32>(42) {}
-   |     -------^^^^^------------------------ help: try this: `if Ok::<i32, i32>(42).is_ok()`
+   |     -------^^^^^------------------------ help: try this: `Ok::<i32, i32>(42).is_ok()`
    |
    = note: `-D clippy::redundant-pattern-matching` implied by `-D warnings`
 
 error: redundant pattern matching, consider using `is_err()`
   --> $DIR/redundant_pattern_matching.rs:7:12
    |
 LL |     if let Err(_) = Err::<i32, i32>(42) {}
-   |     -------^^^^^^------------------------- help: try this: `if Err::<i32, i32>(42).is_err()`
+   |     -------^^^^^^------------------------- help: try this: `Err::<i32, i32>(42).is_err()`
 
 error: redundant pattern matching, consider using `is_none()`
   --> $DIR/redundant_pattern_matching.rs:9:12
    |
 LL |     if let None = None::<()> {}
-   |     -------^^^^---------------- help: try this: `if None::<()>.is_none()`
+   |     -------^^^^---------------- help: try this: `None::<()>.is_none()`
 
 error: redundant pattern matching, consider using `is_some()`
   --> $DIR/redundant_pattern_matching.rs:11:12
    |
 LL |     if let Some(_) = Some(42) {}
-   |     -------^^^^^^^-------------- help: try this: `if Some(42).is_some()`
+   |     -------^^^^^^^-------------- help: try this: `Some(42).is_some()`
 
 error: redundant pattern matching, consider using `is_ok()`
   --> $DIR/redundant_pattern_matching.rs:25:5