Make migrate mode work at item level granularity

src/librustc/middle/expr_use_visitor.rs

@@ -83,6 +83,9 @@ pub trait Delegate<'tcx> {
               assignment_span: Span,
               assignee_cmt: &mc::cmt_<'tcx>,
               mode: MutateMode);
+
+    // A nested closure or generator - only one layer deep.
+    fn nested_body(&mut self, _body_id: hir::BodyId) {}
 }
 
 #[derive(Copy, Clone, PartialEq, Debug)]
@@ -532,8 +535,9 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
                 self.consume_expr(&base);
             }
 
-            hir::ExprKind::Closure(.., fn_decl_span, _) => {
-                self.walk_captures(expr, fn_decl_span)
+            hir::ExprKind::Closure(_, _, body_id, fn_decl_span, _) => {
+                self.delegate.nested_body(body_id);
+                self.walk_captures(expr, fn_decl_span);
             }
 
             hir::ExprKind::Box(ref base) => {

src/librustc_borrowck/borrowck/check_loans.rs

@@ -178,6 +178,22 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for CheckLoanCtxt<'a, 'tcx> {
     }
 
     fn decl_without_init(&mut self, _id: ast::NodeId, _span: Span) { }
+
+    fn nested_body(&mut self, body_id: hir::BodyId) {
+        // rust-lang/rust#58776: MIR and AST borrow check disagree on where
+        // certain closure errors are reported. As such migrate borrowck has to
+        // operate at the level of items, rather than bodies. Check if the
+        // contained closure had any errors and set `signalled_any_error` if it
+        // has.
+        let bccx = self.bccx;
+        if bccx.tcx.migrate_borrowck() {
+            if let SignalledError::NoErrorsSeen = bccx.signalled_any_error.get() {
+                let closure_def_id = bccx.tcx.hir().body_owner_def_id(body_id);
+
+                bccx.signalled_any_error.set(bccx.tcx.borrowck(closure_def_id).signalled_any_error);
+            }
+        }
+    }
 }
 
 pub fn check_loans<'a, 'b, 'c, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,

src/librustc_mir/borrow_check/mod.rs

@@ -329,30 +329,12 @@ fn do_mir_borrowck<'a, 'gcx, 'tcx>(
             // When borrowck=migrate, check if AST-borrowck would
             // error on the given code.
 
-            // rust-lang/rust#55492: loop over parents to ensure that
-            // errors that AST-borrowck only detects in some parent of
-            // a closure still allows NLL to signal an error.
-            let mut curr_def_id = def_id;
-            let signalled_any_error = loop {
-                match tcx.borrowck(curr_def_id).signalled_any_error {
-                    SignalledError::NoErrorsSeen => {
-                        // keep traversing (and borrow-checking) parents
-                    }
-                    SignalledError::SawSomeError => {
-                        // stop search here
-                        break SignalledError::SawSomeError;
-                    }
-                }
-
-                if tcx.is_closure(curr_def_id) {
-                    curr_def_id = tcx.parent_def_id(curr_def_id)
-                        .expect("a closure must have a parent_def_id");
-                } else {
-                    break SignalledError::NoErrorsSeen;
-                }
-            };
+            // rust-lang/rust#55492, rust-lang/rust#58776 check the base def id
+            // for errors. AST borrowck is responsible for aggregating
+            // `signalled_any_error` from all of the nested closures here.
+            let base_def_id = tcx.closure_base_def_id(def_id);
 
-            match signalled_any_error {
+            match tcx.borrowck(base_def_id).signalled_any_error {
                 SignalledError::NoErrorsSeen => {
                     // if AST-borrowck signalled no errors, then
                     // downgrade all the buffered MIR-borrowck errors

src/test/ui/borrowck/issue-58776-borrowck-scans-children.ast.stderr

@@ -0,0 +1,15 @@
+error[E0597]: `**greeting` does not live long enough
+  --> $DIR/issue-58776-borrowck-scans-children.rs:10:24
+   |
+LL |     let res = (|| (|| &greeting)())();
+   |                    --  ^^^^^^^^     - borrowed value only lives until here
+   |                    |   |
+   |                    |   borrowed value does not live long enough
+   |                    capture occurs here
+...
+LL | }
+   | - borrowed value needs to live until here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/borrowck/issue-58776-borrowck-scans-children.migrate.stderr

@@ -0,0 +1,32 @@
+error[E0506]: cannot assign to `greeting` because it is borrowed
+  --> $DIR/issue-58776-borrowck-scans-children.rs:13:5
+   |
+LL |     let res = (|| (|| &greeting)())();
+   |                --      -------- borrow occurs due to use in closure
+   |                |
+   |                borrow of `greeting` occurs here
+...
+LL |     greeting = "DEALLOCATED".to_string();
+   |     ^^^^^^^^ assignment to borrowed `greeting` occurs here
+...
+LL |     println!("thread result: {:?}", res);
+   |                                     --- borrow later used here
+
+error[E0505]: cannot move out of `greeting` because it is borrowed
+  --> $DIR/issue-58776-borrowck-scans-children.rs:16:10
+   |
+LL |     let res = (|| (|| &greeting)())();
+   |                --      -------- borrow occurs due to use in closure
+   |                |
+   |                borrow of `greeting` occurs here
+...
+LL |     drop(greeting);
+   |          ^^^^^^^^ move out of `greeting` occurs here
+...
+LL |     println!("thread result: {:?}", res);
+   |                                     --- borrow later used here
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0505, E0506.
+For more information about an error, try `rustc --explain E0505`.

src/test/ui/borrowck/issue-58776-borrowck-scans-children.nll.stderr

@@ -0,0 +1,32 @@
+error[E0506]: cannot assign to `greeting` because it is borrowed
+  --> $DIR/issue-58776-borrowck-scans-children.rs:13:5
+   |
+LL |     let res = (|| (|| &greeting)())();
+   |                --      -------- borrow occurs due to use in closure
+   |                |
+   |                borrow of `greeting` occurs here
+...
+LL |     greeting = "DEALLOCATED".to_string();
+   |     ^^^^^^^^ assignment to borrowed `greeting` occurs here
+...
+LL |     println!("thread result: {:?}", res);
+   |                                     --- borrow later used here
+
+error[E0505]: cannot move out of `greeting` because it is borrowed
+  --> $DIR/issue-58776-borrowck-scans-children.rs:16:10
+   |
+LL |     let res = (|| (|| &greeting)())();
+   |                --      -------- borrow occurs due to use in closure
+   |                |
+   |                borrow of `greeting` occurs here
+...
+LL |     drop(greeting);
+   |          ^^^^^^^^ move out of `greeting` occurs here
+...
+LL |     println!("thread result: {:?}", res);
+   |                                     --- borrow later used here
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0505, E0506.
+For more information about an error, try `rustc --explain E0505`.

src/test/ui/borrowck/issue-58776-borrowck-scans-children.rs

@@ -0,0 +1,21 @@
+// ignore-compare-mode-nll
+
+// revisions: ast migrate nll
+
+//[migrate]compile-flags: -Z borrowck=migrate
+#![cfg_attr(nll, feature(nll))]
+
+fn main() {
+    let mut greeting = "Hello world!".to_string();
+    let res = (|| (|| &greeting)())();
+    //[ast]~^ ERROR does not live long enough
+
+    greeting = "DEALLOCATED".to_string();
+    //[migrate]~^ ERROR cannot assign
+    //[nll]~^^ ERROR cannot assign
+    drop(greeting);
+    //[migrate]~^ ERROR cannot move
+    //[nll]~^^ ERROR cannot move
+
+    println!("thread result: {:?}", res);
+}