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`)

compiler/rustc_lint/src/unused.rs

@@ -1154,7 +1154,7 @@ impl EarlyLintPass for UnusedParens {
 
     fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
         if let StmtKind::Local(ref local) = s.kind {
-            self.check_unused_parens_pat(cx, &local.pat, true, false, (false, false));
+            self.check_unused_parens_pat(cx, &local.pat, true, false, (true, false));
         }
 
         <Self as UnusedDelimLint>::check_stmt(self, cx, s)

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);
 }

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);
 }

tests/ui/lint/lint-unnecessary-parens.stderr

@@ -267,5 +267,77 @@ LL -     _a += (1);
 LL +     _a += 1;
    |
 
-error: aborting due to 22 previous errors
+error: unnecessary parentheses around pattern
+  --> $DIR/lint-unnecessary-parens.rs:87:8
+   |
+LL |     let(mut _a) = 3;
+   |        ^      ^
+   |
+help: remove these parentheses
+   |
+LL -     let(mut _a) = 3;
+LL +     let mut _a = 3;
+   |
+
+error: unnecessary parentheses around pattern
+  --> $DIR/lint-unnecessary-parens.rs:88:9
+   |
+LL |     let (mut _a) = 3;
+   |         ^      ^
+   |
+help: remove these parentheses
+   |
+LL -     let (mut _a) = 3;
+LL +     let mut _a = 3;
+   |
+
+error: unnecessary parentheses around pattern
+  --> $DIR/lint-unnecessary-parens.rs:89:8
+   |
+LL |     let( mut _a) = 3;
+   |        ^^      ^
+   |
+help: remove these parentheses
+   |
+LL -     let( mut _a) = 3;
+LL +     let mut _a = 3;
+   |
+
+error: unnecessary parentheses around pattern
+  --> $DIR/lint-unnecessary-parens.rs:91:8
+   |
+LL |     let(_a) = 3;
+   |        ^  ^
+   |
+help: remove these parentheses
+   |
+LL -     let(_a) = 3;
+LL +     let _a = 3;
+   |
+
+error: unnecessary parentheses around pattern
+  --> $DIR/lint-unnecessary-parens.rs:92:9
+   |
+LL |     let (_a) = 3;
+   |         ^  ^
+   |
+help: remove these parentheses
+   |
+LL -     let (_a) = 3;
+LL +     let _a = 3;
+   |
+
+error: unnecessary parentheses around pattern
+  --> $DIR/lint-unnecessary-parens.rs:93:8
+   |
+LL |     let( _a) = 3;
+   |        ^^  ^
+   |
+help: remove these parentheses
+   |
+LL -     let( _a) = 3;
+LL +     let _a = 3;
+   |
+
+error: aborting due to 28 previous errors