From 5693a34db2f0e929995fe7aa03d307bafa9360aa Mon Sep 17 00:00:00 2001
From: sjwang05 <63834813+sjwang05@users.noreply.github.com>
Date: Thu, 9 Nov 2023 00:31:42 -0800
Subject: [PATCH] Suggest fix for ; within let-chains

---
 compiler/rustc_parse/src/parser/expr.rs  | 24 ++++++++++--
 tests/ui/parser/semi-in-let-chain.rs     | 27 +++++++++++++
 tests/ui/parser/semi-in-let-chain.stderr | 50 ++++++++++++++++++++++++
 3 files changed, 97 insertions(+), 4 deletions(-)
 create mode 100644 tests/ui/parser/semi-in-let-chain.rs
 create mode 100644 tests/ui/parser/semi-in-let-chain.stderr

diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 19690a6964be9..235b28b6e26e2 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -2441,10 +2441,26 @@ impl<'a> Parser<'a> {
                     self.error_on_extra_if(&cond)?;
                     // Parse block, which will always fail, but we can add a nice note to the error
                     self.parse_block().map_err(|mut err| {
-                        err.span_note(
-                            cond_span,
-                            "the `if` expression is missing a block after this condition",
-                        );
+                        if self.prev_token == token::Semi
+                            && self.token == token::AndAnd
+                            && let maybe_let = self.look_ahead(1, |t| t.clone())
+                            && maybe_let.is_keyword(kw::Let)
+                        {
+                            err.span_suggestion(
+                                self.prev_token.span,
+                                "consider removing this semicolon to parse the `let` as part of the same chain",
+                                "",
+                                Applicability::MachineApplicable,
+                            ).span_note(
+                                self.token.span.to(maybe_let.span),
+                                "you likely meant to continue parsing the let-chain starting here",
+                            );
+                        } else {
+                            err.span_note(
+                                cond_span,
+                                "the `if` expression is missing a block after this condition",
+                            );
+                        }
                         err
                     })?
                 }
diff --git a/tests/ui/parser/semi-in-let-chain.rs b/tests/ui/parser/semi-in-let-chain.rs
new file mode 100644
index 0000000000000..9c21af0372d25
--- /dev/null
+++ b/tests/ui/parser/semi-in-let-chain.rs
@@ -0,0 +1,27 @@
+// Issue #117720
+
+#![feature(let_chains)]
+
+fn main() {
+    if let () = ()
+        && let () = (); //~ERROR
+        && let () = ()
+    {
+    }
+}
+
+fn foo() {
+    if let () = ()
+        && () == (); //~ERROR
+        && 1 < 0
+    {
+    }
+}
+
+fn bar() {
+    if let () = ()
+        && () == (); //~ERROR
+        && let () = ()
+    {
+    }
+}
diff --git a/tests/ui/parser/semi-in-let-chain.stderr b/tests/ui/parser/semi-in-let-chain.stderr
new file mode 100644
index 0000000000000..c1a8f92965eb8
--- /dev/null
+++ b/tests/ui/parser/semi-in-let-chain.stderr
@@ -0,0 +1,50 @@
+error: expected `{`, found `;`
+  --> $DIR/semi-in-let-chain.rs:7:23
+   |
+LL |         && let () = ();
+   |                       ^ expected `{`
+   |
+note: you likely meant to continue parsing the let-chain starting here
+  --> $DIR/semi-in-let-chain.rs:8:9
+   |
+LL |         && let () = ()
+   |         ^^^^^^
+help: consider removing this semicolon to parse the `let` as part of the same chain
+   |
+LL -         && let () = ();
+LL +         && let () = ()
+   |
+
+error: expected `{`, found `;`
+  --> $DIR/semi-in-let-chain.rs:15:20
+   |
+LL |         && () == ();
+   |                    ^ expected `{`
+   |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/semi-in-let-chain.rs:14:8
+   |
+LL |       if let () = ()
+   |  ________^
+LL | |         && () == ();
+   | |___________________^
+
+error: expected `{`, found `;`
+  --> $DIR/semi-in-let-chain.rs:23:20
+   |
+LL |         && () == ();
+   |                    ^ expected `{`
+   |
+note: you likely meant to continue parsing the let-chain starting here
+  --> $DIR/semi-in-let-chain.rs:24:9
+   |
+LL |         && let () = ()
+   |         ^^^^^^
+help: consider removing this semicolon to parse the `let` as part of the same chain
+   |
+LL -         && () == ();
+LL +         && () == ()
+   |
+
+error: aborting due to 3 previous errors
+