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

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

Tracking issue #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 #103108.
Related crater runs: https://github.com/rust-lang/rust/pull/129466.

2 files changed, 122 insertions, 17 deletions

diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
index 5dec019126a..4a31aee5c96 100644
--- a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
+++ b/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,
diff --git a/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs b/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs
index 419e72df00b..d9b5fd8b5c1 100644
--- a/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs
+++ b/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,
@@ -99,7 +99,12 @@ 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",
@@ -107,9 +112,26 @@ impl<'tcx> BorrowExplanation<'tcx> {
                     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,
+                    )
                 }
             }