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.rs b/tests/ui/lint/lint-unnecessary-parens.rs
index ce537a4dc1d..4cbd6562cd3 100644
--- a/tests/ui/lint/lint-unnecessary-parens.rs
+++ b/tests/ui/lint/lint-unnecessary-parens.rs
@@ -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);
 }