Do not trigger unused_braces for while let

Author: JohnTitor

src/librustc_lint/unused.rs

@@ -520,7 +520,10 @@ trait UnusedDelimLint {
                 (cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
             }
 
-            While(ref cond, ref block, ..) => {
+            // Do not lint `unused_braces` in `while let` expressions.
+            While(ref cond, ref block, ..)
+                if !matches!(cond.kind, Let(_, _)) || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
+            {
                 let left = e.span.lo() + rustc_span::BytePos(5);
                 let right = block.span.lo();
                 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))

src/test/ui/lint/issue-74883-unused-paren-baren-yield.rs

@@ -15,15 +15,8 @@ fn main() {
         while let Some(_) = ((yield)) {} //~ ERROR: unnecessary parentheses
         {{yield}}; //~ ERROR: unnecessary braces
         {( yield )}; //~ ERROR: unnecessary parentheses
-
-        // FIXME: Reduce duplicate warnings.
-        // Perhaps we should tweak checks in `BlockRetValue`?
-        while let Some(_) = {(yield)} {}
-        //~^ ERROR: unnecessary braces
-        //~| ERROR: unnecessary parentheses
-        while let Some(_) = {{yield}} {}
-        //~^ ERROR: unnecessary braces
-        //~| ERROR: unnecessary braces
+        while let Some(_) = {(yield)} {} //~ ERROR: unnecessary parentheses
+        while let Some(_) = {{yield}} {} //~ ERROR: unnecessary braces
 
         // FIXME: It'd be great if we could also warn them.
         ((yield));

src/test/ui/lint/issue-74883-unused-paren-baren-yield.stderr

@@ -34,29 +34,17 @@ error: unnecessary parentheses around block return value
 LL |         {( yield )};
    |          ^^^^^^^^^ help: remove these parentheses
 
-error: unnecessary braces around `let` scrutinee expression
-  --> $DIR/issue-74883-unused-paren-baren-yield.rs:21:29
-   |
-LL |         while let Some(_) = {(yield)} {}
-   |                             ^^^^^^^^^ help: remove these braces
-
 error: unnecessary parentheses around block return value
-  --> $DIR/issue-74883-unused-paren-baren-yield.rs:21:30
+  --> $DIR/issue-74883-unused-paren-baren-yield.rs:18:30
    |
 LL |         while let Some(_) = {(yield)} {}
    |                              ^^^^^^^ help: remove these parentheses
 
-error: unnecessary braces around `let` scrutinee expression
-  --> $DIR/issue-74883-unused-paren-baren-yield.rs:24:29
-   |
-LL |         while let Some(_) = {{yield}} {}
-   |                             ^^^^^^^^^ help: remove these braces
-
 error: unnecessary braces around block return value
-  --> $DIR/issue-74883-unused-paren-baren-yield.rs:24:30
+  --> $DIR/issue-74883-unused-paren-baren-yield.rs:19:30
    |
 LL |         while let Some(_) = {{yield}} {}
    |                              ^^^^^^^ help: remove these braces
 
-error: aborting due to 8 previous errors
+error: aborting due to 6 previous errors
 

src/test/ui/lint/unused-braces-while-let-with-mutable-value.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#![deny(unused_braces)]
+
+fn main() {
+    let mut a = Some(3);
+    // Shouldn't warn below `a`.
+    while let Some(ref mut v) = {a} {
+        a.as_mut().map(|a| std::mem::swap(a, v));
+        break;
+    }
+}