Account for `if (let pat = expr) {}`

Partially address #82827.

1 files changed, 47 insertions, 2 deletions

diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 82b41e13ccc..63fc4e0ceda 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -97,6 +97,23 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     ExprKind::Let(ref pat, ref scrutinee) => {
                         self.lower_expr_if_let(e.span, pat, scrutinee, then, else_opt.as_deref())
                     }
+                    ExprKind::Paren(ref paren) => match paren.peel_parens().kind {
+                        ExprKind::Let(ref pat, ref scrutinee) => {
+                            // A user has written `if (let Some(x) = foo) {`, we want to avoid
+                            // confusing them with mentions of nightly features.
+                            // If this logic is changed, you will also likely need to touch
+                            // `unused::UnusedParens::check_expr`.
+                            self.if_let_expr_with_parens(cond, &paren.peel_parens());
+                            self.lower_expr_if_let(
+                                e.span,
+                                pat,
+                                scrutinee,
+                                then,
+                                else_opt.as_deref(),
+                            )
+                        }
+                        _ => self.lower_expr_if(cond, then, else_opt.as_deref()),
+                    },
                     _ => self.lower_expr_if(cond, then, else_opt.as_deref()),
                 },
                 ExprKind::While(ref cond, ref body, opt_label) => self
@@ -346,6 +363,32 @@ impl<'hir> LoweringContext<'_, 'hir> {
         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
     }
 
+    fn if_let_expr_with_parens(&mut self, cond: &Expr, paren: &Expr) {
+        let start = cond.span.until(paren.span);
+        let end = paren.span.shrink_to_hi().until(cond.span.shrink_to_hi());
+        let mut err = self.sess.struct_span_err(
+            vec![start, end],
+            "invalid parentheses around `let` expression in `if let`",
+        );
+        if self.sess.opts.unstable_features.is_nightly_build() {
+            err.note(
+                "only supported directly without parentheses in conditions of `if`- and \
+                 `while`-expressions, as well as in `let` chains within parentheses",
+            );
+        } else {
+            err.note("variable declaration using `let` is a statement, not a condition");
+        }
+        err.multipart_suggestion(
+            "`if let` needs to be written without parentheses",
+            vec![(start, String::new()), (end, String::new())],
+            rustc_errors::Applicability::MachineApplicable,
+        );
+        err.emit();
+        // Ideally, we'd remove the feature gating of a `let` expression since we are already
+        // complaining about it here, but `feature_gate::check_crate` has already run by now:
+        // self.sess.parse_sess.gated_spans.ungate_last(sym::let_chains, paren.span);
+    }
+
     /// Emit an error and lower `ast::ExprKind::Let(pat, scrutinee)` into:
     /// ```rust
     /// match scrutinee { pats => true, _ => false }
@@ -356,8 +399,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         if self.sess.opts.unstable_features.is_nightly_build() {
             self.sess
                 .struct_span_err(span, "`let` expressions are not supported here")
-                .note("only supported directly in conditions of `if`- and `while`-expressions")
-                .note("as well as when nested within `&&` and parenthesis in those conditions")
+                .note(
+                    "only supported directly without parentheses in conditions of `if`- and \
+                     `while`-expressions, as well as in `let` chains within parentheses",
+                )
                 .emit();
         } else {
             self.sess