Rollup merge of #80629 - sexxi-goose:migrations_1, r=nikomatsakis

Add lint for 2229 migrations

Implements the first for RFC 2229 where we make the decision to migrate a root variable based on if the type of the variable needs Drop and if the root variable would be moved into the closure when the feature isn't enabled.

r? `@nikomatsakis`

compiler/rustc_lint_defs/src/builtin.rs

@@ -2968,6 +2968,7 @@ declare_lint_pass! {
         UNSUPPORTED_NAKED_FUNCTIONS,
         MISSING_ABI,
         SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
+        DISJOINT_CAPTURE_DROP_REORDER,
     ]
 }
 
@@ -2994,6 +2995,51 @@ declare_lint! {
     "detects doc comments that aren't used by rustdoc"
 }
 
+declare_lint! {
+    /// The `disjoint_capture_drop_reorder` lint detects variables that aren't completely
+    /// captured when the feature `capture_disjoint_fields` is enabled and it affects the Drop
+    /// order of at least one path starting at this variable.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// # #![deny(disjoint_capture_drop_reorder)]
+    /// # #![allow(unused)]
+    /// struct FancyInteger(i32);
+    ///
+    /// impl Drop for FancyInteger {
+    ///     fn drop(&mut self) {
+    ///         println!("Just dropped {}", self.0);
+    ///     }
+    /// }
+    ///
+    /// struct Point { x: FancyInteger, y: FancyInteger }
+    ///
+    /// fn main() {
+    ///   let p = Point { x: FancyInteger(10), y: FancyInteger(20) };
+    ///
+    ///   let c = || {
+    ///      let x = p.x;
+    ///   };
+    ///
+    ///   c();
+    ///
+    ///   // ... More code ...
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// In the above example `p.y` will be dropped at the end of `f` instead of with `c` if
+    /// the feature `capture_disjoint_fields` is enabled.
+    pub DISJOINT_CAPTURE_DROP_REORDER,
+    Allow,
+    "Drop reorder because of `capture_disjoint_fields`"
+
+}
+
 declare_lint_pass!(UnusedDocComment => [UNUSED_DOC_COMMENTS]);
 
 declare_lint! {

compiler/rustc_typeck/src/check/upvar.rs

@@ -30,6 +30,7 @@
 //! then mean that all later passes would have to check for these figments
 //! and report an error, and it just seems like more mess in the end.)
 
+use super::writeback::Resolver;
 use super::FnCtxt;
 
 use crate::expr_use_visitor as euv;
@@ -40,7 +41,9 @@ use rustc_hir::def_id::LocalDefId;
 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_infer::infer::UpvarRegion;
 use rustc_middle::hir::place::{Place, PlaceBase, PlaceWithHirId, ProjectionKind};
-use rustc_middle::ty::{self, Ty, TyCtxt, UpvarSubsts};
+use rustc_middle::ty::fold::TypeFoldable;
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeckResults, UpvarSubsts};
+use rustc_session::lint;
 use rustc_span::sym;
 use rustc_span::{MultiSpan, Span, Symbol};
 
@@ -55,6 +58,11 @@ enum PlaceAncestryRelation {
     Divergent,
 }
 
+/// Intermediate format to store a captured `Place` and associated `ty::CaptureInfo`
+/// during capture analysis. Information in this map feeds into the minimum capture
+/// analysis pass.
+type InferredCaptureInformation<'tcx> = FxIndexMap<Place<'tcx>, ty::CaptureInfo<'tcx>>;
+
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn closure_analyze(&self, body: &'tcx hir::Body<'tcx>) {
         InferBorrowKindVisitor { fcx: self }.visit_body(body);
@@ -92,7 +100,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         closure_hir_id: hir::HirId,
         span: Span,
-        body: &hir::Body<'_>,
+        body: &'tcx hir::Body<'tcx>,
         capture_clause: hir::CaptureBy,
     ) {
         debug!("analyze_closure(id={:?}, body.id={:?})", closure_hir_id, body.id());
@@ -124,28 +132,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let local_def_id = closure_def_id.expect_local();
 
-        let mut capture_information: FxIndexMap<Place<'tcx>, ty::CaptureInfo<'tcx>> =
-            Default::default();
-        if !self.tcx.features().capture_disjoint_fields {
-            if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
-                for (&var_hir_id, _) in upvars.iter() {
-                    let place = self.place_for_root_variable(local_def_id, var_hir_id);
-
-                    debug!("seed place {:?}", place);
-
-                    let upvar_id = ty::UpvarId::new(var_hir_id, local_def_id);
-                    let capture_kind = self.init_capture_kind(capture_clause, upvar_id, span);
-                    let info = ty::CaptureInfo {
-                        capture_kind_expr_id: None,
-                        path_expr_id: None,
-                        capture_kind,
-                    };
-
-                    capture_information.insert(place, info);
-                }
-            }
-        }
-
         let body_owner_def_id = self.tcx.hir().body_owner_def_id(body.id());
         assert_eq!(body_owner_def_id.to_def_id(), closure_def_id);
         let mut delegate = InferBorrowKind {
@@ -155,7 +141,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             capture_clause,
             current_closure_kind: ty::ClosureKind::LATTICE_BOTTOM,
             current_origin: None,
-            capture_information,
+            capture_information: Default::default(),
         };
         euv::ExprUseVisitor::new(
             &mut delegate,
@@ -172,6 +158,40 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         );
         self.log_capture_analysis_first_pass(closure_def_id, &delegate.capture_information, span);
 
+        self.compute_min_captures(closure_def_id, delegate.capture_information);
+
+        let closure_hir_id = self.tcx.hir().local_def_id_to_hir_id(local_def_id);
+        if should_do_migration_analysis(self.tcx, closure_hir_id) {
+            self.perform_2229_migration_anaysis(closure_def_id, capture_clause, span, body);
+        }
+
+        // We now fake capture information for all variables that are mentioned within the closure
+        // We do this after handling migrations so that min_captures computes before
+        if !self.tcx.features().capture_disjoint_fields {
+            let mut capture_information: InferredCaptureInformation<'tcx> = Default::default();
+
+            if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
+                for var_hir_id in upvars.keys() {
+                    let place = self.place_for_root_variable(local_def_id, *var_hir_id);
+
+                    debug!("seed place {:?}", place);
+
+                    let upvar_id = ty::UpvarId::new(*var_hir_id, local_def_id);
+                    let capture_kind = self.init_capture_kind(capture_clause, upvar_id, span);
+                    let fake_info = ty::CaptureInfo {
+                        capture_kind_expr_id: None,
+                        path_expr_id: None,
+                        capture_kind,
+                    };
+
+                    capture_information.insert(place, fake_info);
+                }
+            }
+
+            // This will update the min captures based on this new fake information.
+            self.compute_min_captures(closure_def_id, capture_information);
+        }
+
         if let Some(closure_substs) = infer_kind {
             // Unify the (as yet unbound) type variable in the closure
             // substs with the kind we inferred.
@@ -197,7 +217,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
-        self.compute_min_captures(closure_def_id, delegate);
         self.log_closure_min_capture_info(closure_def_id, span);
 
         self.min_captures_to_closure_captures_bridge(closure_def_id);
@@ -344,6 +363,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Places (and corresponding capture kind) that we need to keep track of to support all
     /// the required captured paths.
     ///
+    ///
+    /// Note: If this function is called multiple times for the same closure, it will update
+    ///       the existing min_capture map that is stored in TypeckResults.
+    ///
     /// Eg:
     /// ```rust,no_run
     /// struct Point { x: i32, y: i32 }
@@ -408,11 +431,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn compute_min_captures(
         &self,
         closure_def_id: DefId,
-        inferred_info: InferBorrowKind<'_, 'tcx>,
+        capture_information: InferredCaptureInformation<'tcx>,
     ) {
-        let mut root_var_min_capture_list: ty::RootVariableMinCaptureList<'_> = Default::default();
+        if capture_information.is_empty() {
+            return;
+        }
 
-        for (place, capture_info) in inferred_info.capture_information.into_iter() {
+        let mut typeck_results = self.typeck_results.borrow_mut();
+
+        let mut root_var_min_capture_list =
+            typeck_results.closure_min_captures.remove(&closure_def_id).unwrap_or_default();
+
+        for (place, capture_info) in capture_information.into_iter() {
             let var_hir_id = match place.base {
                 PlaceBase::Upvar(upvar_id) => upvar_id.var_path.hir_id,
                 base => bug!("Expected upvar, found={:?}", base),
@@ -422,7 +452,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             let min_cap_list = match root_var_min_capture_list.get_mut(&var_hir_id) {
                 None => {
-                    let mutability = self.determine_capture_mutability(&place);
+                    let mutability = self.determine_capture_mutability(&typeck_results, &place);
                     let min_cap_list =
                         vec![ty::CapturedPlace { place, info: capture_info, mutability }];
                     root_var_min_capture_list.insert(var_hir_id, min_cap_list);
@@ -487,7 +517,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             // Only need to insert when we don't have an ancestor in the existing min capture list
             if !ancestor_found {
-                let mutability = self.determine_capture_mutability(&place);
+                let mutability = self.determine_capture_mutability(&typeck_results, &place);
                 let captured_place =
                     ty::CapturedPlace { place, info: updated_capture_info, mutability };
                 min_cap_list.push(captured_place);
@@ -495,15 +525,123 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
 
         debug!("For closure={:?}, min_captures={:#?}", closure_def_id, root_var_min_capture_list);
+        typeck_results.closure_min_captures.insert(closure_def_id, root_var_min_capture_list);
+    }
 
-        if !root_var_min_capture_list.is_empty() {
+    /// Perform the migration analysis for RFC 2229, and emit lint
-            self.typeck_results
+    /// `disjoint_capture_drop_reorder` if needed.
-                .borrow_mut()
+    fn perform_2229_migration_anaysis(
-                .closure_min_captures
+        &self,
-                .insert(closure_def_id, root_var_min_capture_list);
+        closure_def_id: DefId,
+        capture_clause: hir::CaptureBy,
+        span: Span,
+        body: &'tcx hir::Body<'tcx>,
+    ) {
+        let need_migrations = self.compute_2229_migrations_first_pass(
+            closure_def_id,
+            span,
+            capture_clause,
+            body,
+            self.typeck_results.borrow().closure_min_captures.get(&closure_def_id),
+        );
+
+        if !need_migrations.is_empty() {
+            let need_migrations_hir_id = need_migrations.iter().map(|m| m.0).collect::<Vec<_>>();
+
+            let migrations_text = migration_suggestion_for_2229(self.tcx, &need_migrations_hir_id);
+
+            let local_def_id = closure_def_id.expect_local();
+            let closure_hir_id = self.tcx.hir().local_def_id_to_hir_id(local_def_id);
+            self.tcx.struct_span_lint_hir(
+                lint::builtin::DISJOINT_CAPTURE_DROP_REORDER,
+                closure_hir_id,
+                span,
+                |lint| {
+                    let mut diagnostics_builder = lint.build(
+                        "drop order affected for closure because of `capture_disjoint_fields`",
+                    );
+                    diagnostics_builder.note(&migrations_text);
+                    diagnostics_builder.emit();
+                },
+            );
         }
     }
 
+    /// Figures out the list of root variables (and their types) that aren't completely
+    /// captured by the closure when `capture_disjoint_fields` is enabled and drop order of
+    /// some path starting at that root variable **might** be affected.
+    ///
+    /// The output list would include a root variable if:
+    /// - It would have been moved into the closure when `capture_disjoint_fields` wasn't
+    ///   enabled, **and**
+    /// - It wasn't completely captured by the closure, **and**
+    /// - The type of the root variable needs Drop.
+    fn compute_2229_migrations_first_pass(
+        &self,
+        closure_def_id: DefId,
+        closure_span: Span,
+        closure_clause: hir::CaptureBy,
+        body: &'tcx hir::Body<'tcx>,
+        min_captures: Option<&ty::RootVariableMinCaptureList<'tcx>>,
+    ) -> Vec<(hir::HirId, Ty<'tcx>)> {
+        fn resolve_ty<T: TypeFoldable<'tcx>>(
+            fcx: &FnCtxt<'_, 'tcx>,
+            span: Span,
+            body: &'tcx hir::Body<'tcx>,
+            ty: T,
+        ) -> T {
+            let mut resolver = Resolver::new(fcx, &span, body);
+            ty.fold_with(&mut resolver)
+        }
+
+        let upvars = if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
+            upvars
+        } else {
+            return vec![];
+        };
+
+        let mut need_migrations = Vec::new();
+
+        for (&var_hir_id, _) in upvars.iter() {
+            let ty = resolve_ty(self, closure_span, body, self.node_ty(var_hir_id));
+
+            if !ty.needs_drop(self.tcx, self.tcx.param_env(closure_def_id.expect_local())) {
+                continue;
+            }
+
+            let root_var_min_capture_list = if let Some(root_var_min_capture_list) =
+                min_captures.and_then(|m| m.get(&var_hir_id))
+            {
+                root_var_min_capture_list
+            } else {
+                // The upvar is mentioned within the closure but no path starting from it is
+                // used.
+
+                match closure_clause {
+                    // Only migrate if closure is a move closure
+                    hir::CaptureBy::Value => need_migrations.push((var_hir_id, ty)),
+
+                    hir::CaptureBy::Ref => {}
+                }
+
+                continue;
+            };
+
+            let is_moved = root_var_min_capture_list
+                .iter()
+                .any(|capture| matches!(capture.info.capture_kind, ty::UpvarCapture::ByValue(_)));
+
+            let is_not_completely_captured =
+                root_var_min_capture_list.iter().any(|capture| capture.place.projections.len() > 0);
+
+            if is_moved && is_not_completely_captured {
+                need_migrations.push((var_hir_id, ty));
+            }
+        }
+
+        need_migrations
+    }
+
     fn init_capture_kind(
         &self,
         capture_clause: hir::CaptureBy,
@@ -613,18 +751,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// A captured place is mutable if
     /// 1. Projections don't include a Deref of an immut-borrow, **and**
     /// 2. PlaceBase is mut or projections include a Deref of a mut-borrow.
-    fn determine_capture_mutability(&self, place: &Place<'tcx>) -> hir::Mutability {
+    fn determine_capture_mutability(
+        &self,
+        typeck_results: &'a TypeckResults<'tcx>,
+        place: &Place<'tcx>,
+    ) -> hir::Mutability {
         let var_hir_id = match place.base {
             PlaceBase::Upvar(upvar_id) => upvar_id.var_path.hir_id,
             _ => unreachable!(),
         };
 
-        let bm = *self
+        let bm = *typeck_results.pat_binding_modes().get(var_hir_id).expect("missing binding mode");
-            .typeck_results
-            .borrow()
-            .pat_binding_modes()
-            .get(var_hir_id)
-            .expect("missing binding mode");
 
         let mut is_mutbl = match bm {
             ty::BindByValue(mutability) => mutability,
@@ -698,9 +835,11 @@ struct InferBorrowKind<'a, 'tcx> {
     ///
     /// For closure `fix_s`, (at a high level) the map contains
     ///
+    /// ```
     /// Place { V1, [ProjectionKind::Field(Index=0, Variant=0)] } : CaptureKind { E1, ImmutableBorrow }
     /// Place { V1, [ProjectionKind::Field(Index=1, Variant=0)] } : CaptureKind { E2, MutableBorrow }
-    capture_information: FxIndexMap<Place<'tcx>, ty::CaptureInfo<'tcx>>,
+    /// ```
+    capture_information: InferredCaptureInformation<'tcx>,
 }
 
 impl<'a, 'tcx> InferBorrowKind<'a, 'tcx> {
@@ -1119,6 +1258,21 @@ fn var_name(tcx: TyCtxt<'_>, var_hir_id: hir::HirId) -> Symbol {
     tcx.hir().name(var_hir_id)
 }
 
+fn should_do_migration_analysis(tcx: TyCtxt<'_>, closure_id: hir::HirId) -> bool {
+    let (level, _) =
+        tcx.lint_level_at_node(lint::builtin::DISJOINT_CAPTURE_DROP_REORDER, closure_id);
+
+    !matches!(level, lint::Level::Allow)
+}
+
+fn migration_suggestion_for_2229(tcx: TyCtxt<'_>, need_migrations: &Vec<hir::HirId>) -> String {
+    let need_migrations_strings =
+        need_migrations.iter().map(|v| format!("{}", var_name(tcx, *v))).collect::<Vec<_>>();
+    let migrations_list_concat = need_migrations_strings.join(", ");
+
+    format!("drop(&({}));", migrations_list_concat)
+}
+
 /// Helper function to determine if we need to escalate CaptureKind from
 /// CaptureInfo A to B and returns the escalated CaptureInfo.
 /// (Note: CaptureInfo contains CaptureKind and an expression that led to capture it in that way)

compiler/rustc_typeck/src/check/writeback.rs

@@ -650,7 +650,7 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
     }
 }
 
-trait Locatable {
+crate trait Locatable {
     fn to_span(&self, tcx: TyCtxt<'_>) -> Span;
 }
 
@@ -668,7 +668,7 @@ impl Locatable for hir::HirId {
 
 /// The Resolver. This is the type folding engine that detects
 /// unresolved types and so forth.
-struct Resolver<'cx, 'tcx> {
+crate struct Resolver<'cx, 'tcx> {
     tcx: TyCtxt<'tcx>,
     infcx: &'cx InferCtxt<'cx, 'tcx>,
     span: &'cx dyn Locatable,
@@ -679,7 +679,7 @@ struct Resolver<'cx, 'tcx> {
 }
 
 impl<'cx, 'tcx> Resolver<'cx, 'tcx> {
-    fn new(
+    crate fn new(
         fcx: &'cx FnCtxt<'cx, 'tcx>,
         span: &'cx dyn Locatable,
         body: &'tcx hir::Body<'tcx>,

src/test/ui/closures/2229_closure_analysis/migrations/insignificant_drop.rs

@@ -0,0 +1,130 @@
+#![deny(disjoint_capture_drop_reorder)]
+//~^ NOTE: the lint level is defined here
+
+// Test cases for types that implement a insignificant drop (stlib defined)
+
+// `t` needs Drop because one of its elements needs drop,
+// therefore precise capture might affect drop ordering
+fn test1_all_need_migration() {
+    let t = (String::new(), String::new());
+    let t1 = (String::new(), String::new());
+    let t2 = (String::new(), String::new());
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t, t1, t2));
+        let _t = t.0;
+        let _t1 = t1.0;
+        let _t2 = t2.0;
+    };
+
+    c();
+}
+
+// String implements drop and therefore should be migrated.
+// But in this test cases, `t2` is completely captured and when it is dropped won't be affected
+fn test2_only_precise_paths_need_migration() {
+    let t = (String::new(), String::new());
+    let t1 = (String::new(), String::new());
+    let t2 = (String::new(), String::new());
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t, t1));
+        let _t = t.0;
+        let _t1 = t1.0;
+        let _t2 = t2;
+    };
+
+    c();
+}
+
+// If a variable would've not been captured by value then it would've not been
+// dropped with the closure and therefore doesn't need migration.
+fn test3_only_by_value_need_migration() {
+    let t = (String::new(), String::new());
+    let t1 = (String::new(), String::new());
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t));
+        let _t = t.0;
+        println!("{}", t1.1);
+    };
+
+    c();
+}
+
+// Copy types get copied into the closure instead of move. Therefore we don't need to
+// migrate then as their drop order isn't tied to the closure.
+fn test4_only_non_copy_types_need_migration() {
+    let t = (String::new(), String::new());
+
+    // `t1` is Copy because all of its elements are Copy
+    let t1 = (0i32, 0i32);
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t));
+        let _t = t.0;
+        let _t1 = t1.0;
+    };
+
+    c();
+}
+
+fn test5_only_drop_types_need_migration() {
+    struct S(i32, i32);
+
+    let t = (String::new(), String::new());
+
+    // `s` doesn't implement Drop or any elements within it, and doesn't need migration
+    let s = S(0i32, 0i32);
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t));
+        let _t = t.0;
+        let _s = s.0;
+    };
+
+    c();
+}
+
+// Since we are using a move closure here, both `t` and `t1` get moved
+// even though they are being used by ref inside the closure.
+fn test6_move_closures_non_copy_types_might_need_migration() {
+    let t = (String::new(), String::new());
+    let t1 = (String::new(), String::new());
+    let c = move || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t1, t));
+        println!("{} {}", t1.1, t.1);
+    };
+
+    c();
+}
+
+// Test migration analysis in case of Drop + Non Drop aggregates.
+// Note we need migration here only because the non-copy (because Drop type) is captured,
+// otherwise we won't need to, since we can get away with just by ref capture in that case.
+fn test7_drop_non_drop_aggregate_need_migration() {
+    let t = (String::new(), String::new(), 0i32);
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t));
+        let _t = t.0;
+    };
+
+    c();
+}
+
+fn main() {
+    test1_all_need_migration();
+    test2_only_precise_paths_need_migration();
+    test3_only_by_value_need_migration();
+    test4_only_non_copy_types_need_migration();
+    test5_only_drop_types_need_migration();
+    test6_move_closures_non_copy_types_might_need_migration();
+    test7_drop_non_drop_aggregate_need_migration();
+}

src/test/ui/closures/2229_closure_analysis/migrations/insignificant_drop.stderr

@@ -0,0 +1,105 @@
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/insignificant_drop.rs:13:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |         let _t1 = t1.0;
+LL | |         let _t2 = t2.0;
+LL | |     };
+   | |_____^
+   |
+note: the lint level is defined here
+  --> $DIR/insignificant_drop.rs:1:9
+   |
+LL | #![deny(disjoint_capture_drop_reorder)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: drop(&(t, t1, t2));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/insignificant_drop.rs:31:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |         let _t1 = t1.0;
+LL | |         let _t2 = t2;
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t, t1));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/insignificant_drop.rs:47:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |         println!("{}", t1.1);
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/insignificant_drop.rs:65:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |         let _t1 = t1.0;
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/insignificant_drop.rs:83:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |         let _s = s.0;
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/insignificant_drop.rs:98:13
+   |
+LL |       let c = move || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         println!("{} {}", t1.1, t.1);
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t1, t));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/insignificant_drop.rs:113:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t));
+
+error: aborting due to 7 previous errors
+

src/test/ui/closures/2229_closure_analysis/migrations/no_migrations.rs

@@ -0,0 +1,84 @@
+// run-pass
+
+// Set of test cases that don't need migrations
+
+#![deny(disjoint_capture_drop_reorder)]
+
+
+// Copy types as copied by the closure instead of being moved into the closure
+// Therefore their drop order isn't tied to the closure and won't be requiring any
+// migrations.
+fn test1_only_copy_types() {
+    let t = (0i32, 0i32);
+
+    let c = || {
+        let _t = t.0;
+    };
+
+    c();
+}
+
+// Same as test1 but using a move closure
+fn test2_only_copy_types_move_closure() {
+    let t = (0i32, 0i32);
+
+    let c = move || {
+        println!("{}", t.0);
+    };
+
+    c();
+}
+
+// Don't need to migrate if captured by ref
+fn test3_only_copy_types_move_closure() {
+    let t = (String::new(), String::new());
+
+    let c = || {
+        println!("{}", t.0);
+    };
+
+    c();
+}
+
+// Test migration analysis in case of Insignificant Drop + Non Drop aggregates.
+// Note in this test the closure captures a non Drop type and therefore the variable
+// is only captured by ref.
+fn test4_insignificant_drop_non_drop_aggregate() {
+    let t = (String::new(), 0i32);
+
+    let c = || {
+        let _t = t.1;
+    };
+
+    c();
+}
+
+
+struct Foo(i32);
+impl Drop for Foo {
+    fn drop(&mut self) {
+        println!("{:?} dropped", self.0);
+    }
+}
+
+// Test migration analysis in case of Significant Drop + Non Drop aggregates.
+// Note in this test the closure captures a non Drop type and therefore the variable
+// is only captured by ref.
+fn test5_significant_drop_non_drop_aggregate() {
+    let t = (Foo(0), 0i32);
+
+    let c = || {
+        let _t = t.1;
+    };
+
+    c();
+}
+
+fn main() {
+    test1_only_copy_types();
+    test2_only_copy_types_move_closure();
+    test3_only_copy_types_move_closure();
+    test4_insignificant_drop_non_drop_aggregate();
+    test5_significant_drop_non_drop_aggregate();
+
+}

src/test/ui/closures/2229_closure_analysis/migrations/significant_drop.rs

@@ -0,0 +1,137 @@
+#![deny(disjoint_capture_drop_reorder)]
+//~^ NOTE: the lint level is defined here
+
+// Test cases for types that implement a significant drop (user defined)
+
+#[derive(Debug)]
+struct Foo(i32);
+impl Drop for Foo {
+    fn drop(&mut self) {
+        println!("{:?} dropped", self.0);
+    }
+}
+
+#[derive(Debug)]
+struct ConstainsDropField(Foo, Foo);
+
+// `t` needs Drop because one of its elements needs drop,
+// therefore precise capture might affect drop ordering
+fn test1_all_need_migration() {
+    let t = (Foo(0), Foo(0));
+    let t1 = (Foo(0), Foo(0));
+    let t2 = (Foo(0), Foo(0));
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t, t1, t2));
+        let _t = t.0;
+        let _t1 = t1.0;
+        let _t2 = t2.0;
+    };
+
+    c();
+}
+
+// String implements drop and therefore should be migrated.
+// But in this test cases, `t2` is completely captured and when it is dropped won't be affected
+fn test2_only_precise_paths_need_migration() {
+    let t = (Foo(0), Foo(0));
+    let t1 = (Foo(0), Foo(0));
+    let t2 = (Foo(0), Foo(0));
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t, t1));
+        let _t = t.0;
+        let _t1 = t1.0;
+        let _t2 = t2;
+    };
+
+    c();
+}
+
+// If a variable would've not been captured by value then it would've not been
+// dropped with the closure and therefore doesn't need migration.
+fn test3_only_by_value_need_migration() {
+    let t = (Foo(0), Foo(0));
+    let t1 = (Foo(0), Foo(0));
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t));
+        let _t = t.0;
+        println!("{:?}", t1.1);
+    };
+
+    c();
+}
+
+// The root variable might not implement drop themselves but some path starting
+// at the root variable might implement Drop.
+//
+// If this path isn't captured we need to migrate for the root variable.
+fn test4_type_contains_drop_need_migration() {
+    let t = ConstainsDropField(Foo(0), Foo(0));
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t));
+        let _t = t.0;
+    };
+
+    c();
+}
+
+// Test migration analysis in case of Drop + Non Drop aggregates.
+// Note we need migration here only because the non-copy (because Drop type) is captured,
+// otherwise we won't need to, since we can get away with just by ref capture in that case.
+fn test5_drop_non_drop_aggregate_need_migration() {
+    let t = (Foo(0), Foo(0), 0i32);
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t));
+        let _t = t.0;
+    };
+
+    c();
+}
+
+// Test migration analysis in case of Significant and Insignificant Drop aggregates.
+fn test6_significant_insignificant_drop_aggregate_need_migration() {
+    struct S(i32, i32);
+
+    let t = (Foo(0), String::new());
+
+    let c = || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t));
+        let _t = t.1;
+    };
+
+    c();
+}
+
+// Since we are using a move closure here, both `t` and `t1` get moved
+// even though they are being used by ref inside the closure.
+fn test7_move_closures_non_copy_types_might_need_migration() {
+    let t = (Foo(0), Foo(0));
+    let t1 = (Foo(0), Foo(0), Foo(0));
+
+    let c = move || {
+    //~^ERROR: drop order affected for closure because of `capture_disjoint_fields`
+    //~| NOTE: drop(&(t1, t));
+        println!("{:?} {:?}", t1.1, t.1);
+    };
+
+    c();
+}
+
+fn main() {
+    test1_all_need_migration();
+    test2_only_precise_paths_need_migration();
+    test3_only_by_value_need_migration();
+    test4_type_contains_drop_need_migration();
+    test5_drop_non_drop_aggregate_need_migration();
+    test6_significant_insignificant_drop_aggregate_need_migration();
+    test7_move_closures_non_copy_types_might_need_migration();
+}

src/test/ui/closures/2229_closure_analysis/migrations/significant_drop.stderr

@@ -0,0 +1,103 @@
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/significant_drop.rs:24:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |         let _t1 = t1.0;
+LL | |         let _t2 = t2.0;
+LL | |     };
+   | |_____^
+   |
+note: the lint level is defined here
+  --> $DIR/significant_drop.rs:1:9
+   |
+LL | #![deny(disjoint_capture_drop_reorder)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: drop(&(t, t1, t2));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/significant_drop.rs:42:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |         let _t1 = t1.0;
+LL | |         let _t2 = t2;
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t, t1));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/significant_drop.rs:58:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |         println!("{:?}", t1.1);
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/significant_drop.rs:75:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/significant_drop.rs:90:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.0;
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/significant_drop.rs:105:13
+   |
+LL |       let c = || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         let _t = t.1;
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t));
+
+error: drop order affected for closure because of `capture_disjoint_fields`
+  --> $DIR/significant_drop.rs:120:13
+   |
+LL |       let c = move || {
+   |  _____________^
+LL | |
+LL | |
+LL | |         println!("{:?} {:?}", t1.1, t.1);
+LL | |     };
+   | |_____^
+   |
+   = note: drop(&(t1, t));
+
+error: aborting due to 7 previous errors
+

src/test/ui/nll/closure-requirements/escape-upvar-nested.stderr

@@ -7,10 +7,10 @@ LL |             let mut closure1 = || p = &y;
    = note: defining type: test::{closure#0}::{closure#0} with closure substs [
                i16,
                extern "rust-call" fn(()),
-               (&'_#1r i32, &'_#2r mut &'_#3r i32),
+               (&'_#1r mut &'_#2r i32, &'_#3r i32),
            ]
    = note: number of external vids: 4
-   = note: where '_#1r: '_#3r
+   = note: where '_#3r: '_#2r
 
 note: external requirements
   --> $DIR/escape-upvar-nested.rs:20:27
@@ -25,10 +25,10 @@ LL | |         };
    = note: defining type: test::{closure#0} with closure substs [
                i16,
                extern "rust-call" fn(()),
-               (&'_#1r i32, &'_#2r mut &'_#3r i32),
+               (&'_#1r mut &'_#2r i32, &'_#3r i32),
            ]
    = note: number of external vids: 4
-   = note: where '_#1r: '_#3r
+   = note: where '_#3r: '_#2r
 
 note: no external requirements
   --> $DIR/escape-upvar-nested.rs:13:1

src/test/ui/nll/closure-requirements/escape-upvar-ref.stderr

@@ -7,10 +7,10 @@ LL |         let mut closure = || p = &y;
    = note: defining type: test::{closure#0} with closure substs [
                i16,
                extern "rust-call" fn(()),
-               (&'_#1r i32, &'_#2r mut &'_#3r i32),
+               (&'_#1r mut &'_#2r i32, &'_#3r i32),
            ]
    = note: number of external vids: 4
-   = note: where '_#1r: '_#3r
+   = note: where '_#3r: '_#2r
 
 note: no external requirements
   --> $DIR/escape-upvar-ref.rs:17:1