Rescope temp lifetime in if-let into IfElse with migration lint

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1999,19 +1999,32 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
     ) {
         let used_in_call = matches!(
             explanation,
-            BorrowExplanation::UsedLater(LaterUseKind::Call | LaterUseKind::Other, _call_span, _)
+            BorrowExplanation::UsedLater(
+                _,
+                LaterUseKind::Call | LaterUseKind::Other,
+                _call_span,
+                _
+            )
         );
         if !used_in_call {
             debug!("not later used in call");
             return;
         }
+        if matches!(
+            self.body.local_decls[issued_borrow.borrowed_place.local].local_info(),
+            LocalInfo::IfThenRescopeTemp { .. }
+        ) {
+            // A better suggestion will be issued by the `if_let_rescope` lint
+            return;
+        }
 
-        let use_span =
-            if let BorrowExplanation::UsedLater(LaterUseKind::Other, use_span, _) = explanation {
-                Some(use_span)
-            } else {
-                None
-            };
+        let use_span = if let BorrowExplanation::UsedLater(_, LaterUseKind::Other, use_span, _) =
+            explanation
+        {
+            Some(use_span)
+        } else {
+            None
+        };
 
         let outer_call_loc =
             if let TwoPhaseActivation::ActivatedAt(loc) = issued_borrow.activation_location {
@@ -2862,7 +2875,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             // and `move` will not help here.
             (
                 Some(name),
-                BorrowExplanation::UsedLater(LaterUseKind::ClosureCapture, var_or_use_span, _),
+                BorrowExplanation::UsedLater(_, LaterUseKind::ClosureCapture, var_or_use_span, _),
             ) if borrow_spans.for_coroutine() || borrow_spans.for_closure() => self
                 .report_escaping_closure_capture(
                     borrow_spans,

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -30,7 +30,7 @@ use crate::{MirBorrowckCtxt, WriteKind};
 
 #[derive(Debug)]
 pub(crate) enum BorrowExplanation<'tcx> {
-    UsedLater(LaterUseKind, Span, Option<Span>),
+    UsedLater(Local, LaterUseKind, Span, Option<Span>),
     UsedLaterInLoop(LaterUseKind, Span, Option<Span>),
     UsedLaterWhenDropped {
         drop_loc: Location,
@@ -99,17 +99,39 @@ impl<'tcx> BorrowExplanation<'tcx> {
             }
         }
         match *self {
-            BorrowExplanation::UsedLater(later_use_kind, var_or_use_span, path_span) => {
+            BorrowExplanation::UsedLater(
+                dropped_local,
+                later_use_kind,
+                var_or_use_span,
+                path_span,
+            ) => {
                 let message = match later_use_kind {
                     LaterUseKind::TraitCapture => "captured here by trait object",
                     LaterUseKind::ClosureCapture => "captured here by closure",
                     LaterUseKind::Call => "used by call",
                     LaterUseKind::FakeLetRead => "stored here",
                     LaterUseKind::Other => "used here",
                 };
-                // We can use `var_or_use_span` if either `path_span` is not present, or both spans are the same
-                if path_span.map(|path_span| path_span == var_or_use_span).unwrap_or(true) {
-                    if borrow_span.map(|sp| !sp.overlaps(var_or_use_span)).unwrap_or(true) {
+                let local_decl = &body.local_decls[dropped_local];
+
+                if let &LocalInfo::IfThenRescopeTemp { if_then } = local_decl.local_info()
+                    && let Some((_, hir::Node::Expr(expr))) = tcx.hir().parent_iter(if_then).next()
+                    && let hir::ExprKind::If(cond, conseq, alt) = expr.kind
+                    && let hir::ExprKind::Let(&hir::LetExpr {
+                        span: _,
+                        pat,
+                        init,
+                        // FIXME(#101728): enable rewrite when type ascription is stabilized again
+                        ty: None,
+                        recovered: _,
+                    }) = cond.kind
+                    && pat.span.can_be_used_for_suggestions()
+                    && let Ok(pat) = tcx.sess.source_map().span_to_snippet(pat.span)
+                {
+                    suggest_rewrite_if_let(tcx, expr, &pat, init, conseq, alt, err);
+                } else if path_span.map_or(true, |path_span| path_span == var_or_use_span) {
+                    // We can use `var_or_use_span` if either `path_span` is not present, or both spans are the same
+                    if borrow_span.map_or(true, |sp| !sp.overlaps(var_or_use_span)) {
                         err.span_label(
                             var_or_use_span,
                             format!("{borrow_desc}borrow later {message}"),
@@ -255,6 +277,22 @@ impl<'tcx> BorrowExplanation<'tcx> {
                                     Applicability::MaybeIncorrect,
                                 );
                             };
+                        } else if let &LocalInfo::IfThenRescopeTemp { if_then } =
+                            local_decl.local_info()
+                            && let hir::Node::Expr(expr) = tcx.hir_node(if_then)
+                            && let hir::ExprKind::If(cond, conseq, alt) = expr.kind
+                            && let hir::ExprKind::Let(&hir::LetExpr {
+                                span: _,
+                                pat,
+                                init,
+                                // FIXME(#101728): enable rewrite when type ascription is stabilized again
+                                ty: None,
+                                recovered: _,
+                            }) = cond.kind
+                            && pat.span.can_be_used_for_suggestions()
+                            && let Ok(pat) = tcx.sess.source_map().span_to_snippet(pat.span)
+                        {
+                            suggest_rewrite_if_let(tcx, expr, &pat, init, conseq, alt, err);
                         }
                     }
                 }
@@ -390,6 +428,53 @@ impl<'tcx> BorrowExplanation<'tcx> {
     }
 }
 
+fn suggest_rewrite_if_let(
+    tcx: TyCtxt<'_>,
+    expr: &hir::Expr<'_>,
+    pat: &str,
+    init: &hir::Expr<'_>,
+    conseq: &hir::Expr<'_>,
+    alt: Option<&hir::Expr<'_>>,
+    err: &mut Diag<'_>,
+) {
+    let source_map = tcx.sess.source_map();
+    err.span_note(
+        source_map.end_point(conseq.span),
+        "lifetimes for temporaries generated in `if let`s have been shortened in Edition 2024 so that they are dropped here instead",
+    );
+    if expr.span.can_be_used_for_suggestions() && conseq.span.can_be_used_for_suggestions() {
+        let needs_block = if let Some(hir::Node::Expr(expr)) =
+            alt.and_then(|alt| tcx.hir().parent_iter(alt.hir_id).next()).map(|(_, node)| node)
+        {
+            matches!(expr.kind, hir::ExprKind::If(..))
+        } else {
+            false
+        };
+        let mut sugg = vec![
+            (
+                expr.span.shrink_to_lo().between(init.span),
+                if needs_block { "{ match ".into() } else { "match ".into() },
+            ),
+            (conseq.span.shrink_to_lo(), format!(" {{ {pat} => ")),
+        ];
+        let expr_end = expr.span.shrink_to_hi();
+        let mut expr_end_code;
+        if let Some(alt) = alt {
+            sugg.push((conseq.span.between(alt.span), " _ => ".into()));
+            expr_end_code = "}".to_string();
+        } else {
+            expr_end_code = " _ => {} }".into();
+        }
+        expr_end_code.push('}');
+        sugg.push((expr_end, expr_end_code));
+        err.multipart_suggestion(
+            "consider rewriting the `if` into `match` which preserves the extended lifetime",
+            sugg,
+            Applicability::MaybeIncorrect,
+        );
+    }
+}
+
 impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
     fn free_region_constraint_info(
         &self,
@@ -465,14 +550,21 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
                     .or_else(|| self.borrow_spans(span, location));
 
                 if use_in_later_iteration_of_loop {
-                    let later_use = self.later_use_kind(borrow, spans, use_location);
-                    BorrowExplanation::UsedLaterInLoop(later_use.0, later_use.1, later_use.2)
+                    let (later_use_kind, var_or_use_span, path_span) =
+                        self.later_use_kind(borrow, spans, use_location);
+                    BorrowExplanation::UsedLaterInLoop(later_use_kind, var_or_use_span, path_span)
                 } else {
                     // Check if the location represents a `FakeRead`, and adapt the error
                     // message to the `FakeReadCause` it is from: in particular,
                     // the ones inserted in optimized `let var = <expr>` patterns.
-                    let later_use = self.later_use_kind(borrow, spans, location);
-                    BorrowExplanation::UsedLater(later_use.0, later_use.1, later_use.2)
+                    let (later_use_kind, var_or_use_span, path_span) =
+                        self.later_use_kind(borrow, spans, location);
+                    BorrowExplanation::UsedLater(
+                        borrow.borrowed_place.local,
+                        later_use_kind,
+                        var_or_use_span,
+                        path_span,
+                    )
                 }
             }
 

compiler/rustc_feature/src/unstable.rs

@@ -497,6 +497,8 @@ declare_features! (
     (unstable, half_open_range_patterns_in_slices, "1.66.0", Some(67264)),
     /// Allows `if let` guard in match arms.
     (unstable, if_let_guard, "1.47.0", Some(51114)),
+    /// Rescoping temporaries in `if let` to align with Rust 2024.
+    (unstable, if_let_rescope, "CURRENT_RUSTC_VERSION", Some(124085)),
     /// Allows `impl Trait` to be used inside associated types (RFC 2515).
     (unstable, impl_trait_in_assoc_type, "1.70.0", Some(63063)),
     /// Allows `impl Trait` as output type in `Fn` traits in return position of functions.

compiler/rustc_hir_analysis/src/check/region.rs

@@ -472,7 +472,12 @@ fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx h
 
         hir::ExprKind::If(cond, then, Some(otherwise)) => {
             let expr_cx = visitor.cx;
-            visitor.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen });
+            let data = if expr.span.at_least_rust_2024() && visitor.tcx.features().if_let_rescope {
+                ScopeData::IfThenRescope
+            } else {
+                ScopeData::IfThen
+            };
+            visitor.enter_scope(Scope { id: then.hir_id.local_id, data });
             visitor.cx.var_parent = visitor.cx.parent;
             visitor.visit_expr(cond);
             visitor.visit_expr(then);
@@ -482,7 +487,12 @@ fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx h
 
         hir::ExprKind::If(cond, then, None) => {
             let expr_cx = visitor.cx;
-            visitor.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen });
+            let data = if expr.span.at_least_rust_2024() && visitor.tcx.features().if_let_rescope {
+                ScopeData::IfThenRescope
+            } else {
+                ScopeData::IfThen
+            };
+            visitor.enter_scope(Scope { id: then.hir_id.local_id, data });
             visitor.cx.var_parent = visitor.cx.parent;
             visitor.visit_expr(cond);
             visitor.visit_expr(then);

compiler/rustc_lint/messages.ftl

@@ -334,6 +334,11 @@ lint_identifier_uncommon_codepoints = identifier contains {$codepoints_len ->
         *[other] {" "}{$identifier_type}
     } Unicode general security profile
 
+lint_if_let_rescope = `if let` assigns a shorter lifetime since Edition 2024
+    .label = this value has a significant drop implementation which may observe a major change in drop order and requires your discretion
+    .help = the value is now dropped here in Edition 2024
+    .suggestion = a `match` with a single arm can preserve the drop order up to Edition 2021
+
 lint_ignored_unless_crate_specified = {$level}({$name}) is ignored unless specified at crate level
 
 lint_ill_formed_attribute_input = {$num_suggestions ->