Auto merge of rust-lang#107251 - dingxiangfei2009:let-chain-rescope, r=jieyouxu

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

Tracking issue rust-lang#124085

This PR shortens the temporary lifetime to cover only the pattern matching and consequent branch of a `if let`.

At the expression location, means that the lifetime is shortened from previously the deepest enclosing block or statement in Edition 2021. This warrants an Edition change.

Coming with the Edition change, this patch also implements an edition lint to warn about the change and a safe rewrite suggestion to preserve the 2021 semantics in most cases.

Related to rust-lang#103108.
Related crater runs: rust-lang#129466.

Author: bors

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 {
@@ -2859,7 +2872,7 @@ 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

@@ -495,6 +495,8 @@ 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 ->