Rollup merge of #98574 - dingxiangfei2009:let-else-thir, r=oli-obk

Lower let-else in MIR

This MR will switch to lower let-else statements in MIR building instead.

To lower let-else in MIR, we build a mini-switch two branches. One branch leads to the matching case, and the other leads to the `else` block. This arrangement will allow temporary lifetime analysis running as-is so that the temporaries are properly extended according to the same rule applied to regular `let` statements.

cc #87335

Fix #98672

clippy_lints/src/loops/while_let_loop.rs

@@ -11,7 +11,7 @@ use rustc_lint::LateContext;
 pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, loop_block: &'tcx Block<'_>) {
     let (init, has_trailing_exprs) = match (loop_block.stmts, loop_block.expr) {
         ([stmt, stmts @ ..], expr) => {
-            if let StmtKind::Local(&Local { init: Some(e), .. }) | StmtKind::Semi(e) | StmtKind::Expr(e) = stmt.kind {
+            if let StmtKind::Local(&Local { init: Some(e), els: None, .. }) | StmtKind::Semi(e) | StmtKind::Expr(e) = stmt.kind {
                 (e, !stmts.is_empty() || expr.is_some())
             } else {
                 return;

clippy_lints/src/matches/mod.rs

@@ -1041,7 +1041,8 @@ impl<'tcx> LateLintPass<'tcx> for Matches {
     }
 
     fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx Local<'_>) {
-        self.infallible_destructuring_match_linted |= infallible_destructuring_match::check(cx, local);
+        self.infallible_destructuring_match_linted |=
+            local.els.is_none() && infallible_destructuring_match::check(cx, local);
     }
 
     fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {

clippy_lints/src/no_effect.rs

@@ -92,6 +92,7 @@ fn check_no_effect(cx: &LateContext<'_>, stmt: &Stmt<'_>) -> bool {
         if_chain! {
             if !is_lint_allowed(cx, NO_EFFECT_UNDERSCORE_BINDING, local.hir_id);
             if let Some(init) = local.init;
+            if local.els.is_none();
             if !local.pat.span.from_expansion();
             if has_no_effect(cx, init);
             if let PatKind::Binding(_, _, ident, _) = local.pat.kind;

clippy_lints/src/returns.rs

@@ -10,7 +10,6 @@ use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
-use rustc_span::hygiene::DesugaringKind;
 use rustc_span::source_map::Span;
 use rustc_span::sym;
 
@@ -203,9 +202,7 @@ fn check_final_expr<'tcx>(
                 check_block_return(cx, ifblock);
             }
             if let Some(else_clause) = else_clause_opt {
-                if expr.span.desugaring_kind() != Some(DesugaringKind::LetElse) {
-                    check_final_expr(cx, else_clause, None, RetReplacement::Empty);
-                }
+                check_final_expr(cx, else_clause, None, RetReplacement::Empty);
             }
         },
         // a match expr, check all arms

clippy_utils/src/hir_utils.rs

@@ -102,7 +102,7 @@ pub struct HirEqInterExpr<'a, 'b, 'tcx> {
 impl HirEqInterExpr<'_, '_, '_> {
     pub fn eq_stmt(&mut self, left: &Stmt<'_>, right: &Stmt<'_>) -> bool {
         match (&left.kind, &right.kind) {
-            (&StmtKind::Local(l), &StmtKind::Local(r)) => {
+            (&StmtKind::Local(l, ), &StmtKind::Local(r, )) => {
                 // This additional check ensures that the type of the locals are equivalent even if the init
                 // expression or type have some inferred parts.
                 if let Some((typeck_lhs, typeck_rhs)) = self.inner.maybe_typeck_results {
@@ -117,6 +117,7 @@ impl HirEqInterExpr<'_, '_, '_> {
                 // these only get added if the init and type is equal.
                 both(&l.init, &r.init, |l, r| self.eq_expr(l, r))
                     && both(&l.ty, &r.ty, |l, r| self.eq_ty(l, r))
+                    && both(&l.els, &r.els, |l, r| self.eq_block(l, r))
                     && self.eq_pat(l.pat, r.pat)
             },
             (&StmtKind::Expr(l), &StmtKind::Expr(r)) | (&StmtKind::Semi(l), &StmtKind::Semi(r)) => self.eq_expr(l, r),
@@ -921,11 +922,14 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
         std::mem::discriminant(&b.kind).hash(&mut self.s);
 
         match &b.kind {
-            StmtKind::Local(local) => {
+            StmtKind::Local(local, ) => {
                 self.hash_pat(local.pat);
                 if let Some(init) = local.init {
                     self.hash_expr(init);
                 }
+                if let Some(els) = local.els {
+                    self.hash_block(els);
+                }
             },
             StmtKind::Item(..) => {},
             StmtKind::Expr(expr) | StmtKind::Semi(expr) => {