Auto merge of #115677 - matthewjasper:let-expr-recovery, r=b-naber

Improve invalid let expression handling

- Move all of the checks for valid let expression positions to parsing.
- Add a field to ExprKind::Let in AST/HIR to mark whether it's in a valid location.
- Suppress some later errors and MIR construction for invalid let expressions.
- Fix a (drop) scope issue that was also responsible for #104172.

Fixes #104172
Fixes #104868

clippy_lints/src/suspicious_operation_groupings.rs

@@ -586,7 +586,7 @@ fn ident_difference_expr_with_base_location(
         | (ForLoop(_, _, _, _), ForLoop(_, _, _, _))
         | (While(_, _, _), While(_, _, _))
         | (If(_, _, _), If(_, _, _))
-        | (Let(_, _, _), Let(_, _, _))
+        | (Let(_, _, _, _), Let(_, _, _, _))
         | (Type(_, _), Type(_, _))
         | (Cast(_, _), Cast(_, _))
         | (Lit(_), Lit(_))

clippy_lints/src/unnested_or_patterns.rs

@@ -68,7 +68,7 @@ impl EarlyLintPass for UnnestedOrPatterns {
 
     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
         if self.msrv.meets(msrvs::OR_PATTERNS) {
-            if let ast::ExprKind::Let(pat, _, _) = &e.kind {
+            if let ast::ExprKind::Let(pat, _, _, _) = &e.kind {
                 lint_unnested_or_patterns(cx, pat);
             }
         }

clippy_utils/src/ast_utils.rs

@@ -166,7 +166,7 @@ pub fn eq_expr(l: &Expr, r: &Expr) -> bool {
         (Unary(lo, l), Unary(ro, r)) => mem::discriminant(lo) == mem::discriminant(ro) && eq_expr(l, r),
         (Lit(l), Lit(r)) => l == r,
         (Cast(l, lt), Cast(r, rt)) | (Type(l, lt), Type(r, rt)) => eq_expr(l, r) && eq_ty(lt, rt),
-        (Let(lp, le, _), Let(rp, re, _)) => eq_pat(lp, rp) && eq_expr(le, re),
+        (Let(lp, le, _, _), Let(rp, re, _, _)) => eq_pat(lp, rp) && eq_expr(le, re),
         (If(lc, lt, le), If(rc, rt, re)) => eq_expr(lc, rc) && eq_block(lt, rt) && eq_expr_opt(le, re),
         (While(lc, lt, ll), While(rc, rt, rl)) => eq_label(ll, rl) && eq_expr(lc, rc) && eq_block(lt, rt),
         (ForLoop(lp, li, lt, ll), ForLoop(rp, ri, rt, rl)) => {