Fix missing leading space in suggestion

For a local pattern with no space between `let` and `(` e.g.:

  let(_a) = 3;

we were previously suggesting this illegal code:

  let_a =3;

After this change the suggestion will instead be:

  let _a =3;

(Note the space after `let`)

1 files changed, 8 insertions, 0 deletions

diff --git a/tests/ui/lint/lint-unnecessary-parens.fixed b/tests/ui/lint/lint-unnecessary-parens.fixed
index bafac05d8da..b17914da6e6 100644
--- a/tests/ui/lint/lint-unnecessary-parens.fixed
+++ b/tests/ui/lint/lint-unnecessary-parens.fixed
@@ -84,6 +84,14 @@ fn main() {
     _a = 0; //~ ERROR unnecessary parentheses around assigned value
     _a += 1; //~ ERROR unnecessary parentheses around assigned value
 
+    let mut _a = 3; //~ ERROR unnecessary parentheses around pattern
+    let mut _a = 3; //~ ERROR unnecessary parentheses around pattern
+    let mut _a = 3; //~ ERROR unnecessary parentheses around pattern
+
+    let _a = 3; //~ ERROR unnecessary parentheses around pattern
+    let _a = 3; //~ ERROR unnecessary parentheses around pattern
+    let _a = 3; //~ ERROR unnecessary parentheses around pattern
+
     let _a = baz!(3, 4);
     let _b = baz!(3);
 }