Detect missing if let or let-else

During `let` binding parse error and encountering a block, detect if there is a likely missing `if` or `else`:

```
error: expected one of `.`, `;`, `?`, `else`, or an operator, found `{`
  --> $DIR/missing-if-let-or-let-else.rs:14:25
   |
LL |     let Some(x) = foo() {
   |                         ^ expected one of `.`, `;`, `?`, `else`, or an operator
   |
help: you might have meant to use `if let`
   |
LL |     if let Some(x) = foo() {
   |     ++
help: alternatively, you might have meant to use `let else`
   |
LL |     let Some(x) = foo() else {
   |                         ++++
```

Author: estebank

compiler/rustc_parse/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(debug_closure_helpers)]
+#![feature(default_field_values)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![recursion_limit = "256"]

compiler/rustc_parse/src/parser/stmt.rs

@@ -6,6 +6,7 @@ use ast::Label;
 use rustc_ast as ast;
 use rustc_ast::token::{self, Delimiter, InvisibleOrigin, MetaVarKind, TokenKind};
 use rustc_ast::util::classify::{self, TrailingBrace};
+use rustc_ast::visit::{Visitor, walk_expr};
 use rustc_ast::{
     AttrStyle, AttrVec, Block, BlockCheckMode, DUMMY_NODE_ID, Expr, ExprKind, HasAttrs, Local,
     LocalKind, MacCall, MacCallStmt, MacStmtStyle, Recovered, Stmt, StmtKind,
@@ -783,6 +784,71 @@ impl<'a> Parser<'a> {
         Ok(self.mk_block(stmts, s, lo.to(self.prev_token.span)))
     }
 
+    fn recover_missing_let_else(&mut self, err: &mut Diag<'_>, pat: &ast::Pat, stmt_span: Span) {
+        if self.token.kind != token::OpenBrace {
+            return;
+        }
+        match pat.kind {
+            ast::PatKind::Ident(..) | ast::PatKind::Missing | ast::PatKind::Wild => {
+                // Not if let or let else
+                return;
+            }
+            _ => {}
+        }
+        let snapshot = self.create_snapshot_for_diagnostic();
+        let block_span = self.token.span;
+        let (if_let, let_else) = match self.parse_block() {
+            Ok(block) => {
+                let mut idents = vec![];
+                pat.walk(&mut |pat: &ast::Pat| {
+                    if let ast::PatKind::Ident(_, ident, _) = pat.kind {
+                        idents.push(ident);
+                    }
+                    true
+                });
+                // Collect all bindings in pattern and see if they appear in the block. Likely meant
+                // to write `if let`. See if the block has a return. Likely meant to write
+                // `let else`.
+                let mut visitor = IdentFinder { idents, .. };
+                visitor.visit_block(&block);
+
+                (visitor.references_ident, visitor.has_return)
+            }
+            Err(e) => {
+                e.cancel();
+                self.restore_snapshot(snapshot);
+                (false, false)
+            }
+        };
+
+        let mut alternatively = "";
+        if if_let || !let_else {
+            alternatively = "alternatively, ";
+            err.span_suggestion_verbose(
+                stmt_span.shrink_to_lo(),
+                "you might have meant to use `if let`",
+                "if ".to_string(),
+                if if_let {
+                    Applicability::MachineApplicable
+                } else {
+                    Applicability::MaybeIncorrect
+                },
+            );
+        }
+        if let_else || !if_let {
+            err.span_suggestion_verbose(
+                block_span.shrink_to_lo(),
+                format!("{alternatively}you might have meant to use `let else`"),
+                "else ".to_string(),
+                if let_else {
+                    Applicability::MachineApplicable
+                } else {
+                    Applicability::MaybeIncorrect
+                },
+            );
+        }
+    }
+
     fn recover_missing_dot(&mut self, err: &mut Diag<'_>) {
         let Some((ident, _)) = self.token.ident() else {
             return;
@@ -977,6 +1043,7 @@ impl<'a> Parser<'a> {
                         self.check_mistyped_turbofish_with_multiple_type_params(e, expr).map_err(
                             |mut e| {
                                 self.recover_missing_dot(&mut e);
+                                self.recover_missing_let_else(&mut e, &local.pat, stmt.span);
                                 e
                             },
                         )?;
@@ -1065,3 +1132,30 @@ impl<'a> Parser<'a> {
         self.mk_block(thin_vec![self.mk_stmt_err(span, guar)], BlockCheckMode::Default, span)
     }
 }
+
+struct IdentFinder {
+    idents: Vec<Ident>,
+    /// If a block references one of the bindings introduced by the let pattern, we likely meant to
+    /// use `if let`.
+    /// This is pre-expansion, so if we encounter `let Some(x) = foo() { println!("{x}") }` we won't
+    /// find it.
+    references_ident: bool = false,
+    /// If a block has a `return`, then we know with high certainty that the
+    has_return: bool = false,
+}
+
+impl<'a> Visitor<'a> for IdentFinder {
+    fn visit_ident(&mut self, ident: &Ident) {
+        for i in &self.idents {
+            if ident.name == i.name {
+                self.references_ident = true;
+            }
+        }
+    }
+    fn visit_expr(&mut self, node: &'a Expr) {
+        if let ExprKind::Ret(..) = node.kind {
+            self.has_return = true;
+        }
+        walk_expr(self, node);
+    }
+}

tests/ui/uninhabited/missing-if-let-or-let-else.rs

@@ -0,0 +1,24 @@
+fn a() {
+    let Some(x) = foo() { //~ ERROR expected one of
+        //~^ HELP you might have meant to use `if let`
+        let y = x;
+    }
+}
+fn b() {
+    let Some(x) = foo() { //~ ERROR expected one of
+        //~^ HELP you might have meant to use `let else`
+        return;
+    }
+}
+fn c() {
+    let Some(x) = foo() { //~ ERROR expected one of
+        //~^ HELP you might have meant to use `if let`
+        //~| HELP alternatively, you might have meant to use `let else`
+        // The parser check happens pre-macro-expansion, so we don't know for sure.
+        println!("{x}");
+    }
+}
+fn foo() -> Option<i32> {
+    Some(42)
+}
+fn main() {}

tests/ui/uninhabited/missing-if-let-or-let-else.stderr

@@ -0,0 +1,39 @@
+error: expected one of `.`, `;`, `?`, `else`, or an operator, found `{`
+  --> $DIR/missing-if-let-or-let-else.rs:2:25
+   |
+LL |     let Some(x) = foo() {
+   |                         ^ expected one of `.`, `;`, `?`, `else`, or an operator
+   |
+help: you might have meant to use `if let`
+   |
+LL |     if let Some(x) = foo() {
+   |     ++
+
+error: expected one of `.`, `;`, `?`, `else`, or an operator, found `{`
+  --> $DIR/missing-if-let-or-let-else.rs:8:25
+   |
+LL |     let Some(x) = foo() {
+   |                         ^ expected one of `.`, `;`, `?`, `else`, or an operator
+   |
+help: you might have meant to use `let else`
+   |
+LL |     let Some(x) = foo() else {
+   |                         ++++
+
+error: expected one of `.`, `;`, `?`, `else`, or an operator, found `{`
+  --> $DIR/missing-if-let-or-let-else.rs:14:25
+   |
+LL |     let Some(x) = foo() {
+   |                         ^ expected one of `.`, `;`, `?`, `else`, or an operator
+   |
+help: you might have meant to use `if let`
+   |
+LL |     if let Some(x) = foo() {
+   |     ++
+help: alternatively, you might have meant to use `let else`
+   |
+LL |     let Some(x) = foo() else {
+   |                         ++++
+
+error: aborting due to 3 previous errors
+