Rollup merge of #45927 - sinkuu:mir-borrowck-closure, r=estebank

MIR-borrowck: fix diagnostics for closures

Emit notes for captured variables in the same manner as AST borrowck.

```
error[E0499]: cannot borrow `x` as mutable more than once at a time (Ast)
  --> $DIR/borrowck-closures-two-mut.rs:24:24
   |
23 |     let c1 = to_fn_mut(|| x = 4);
   |                        -- - previous borrow occurs due to use of `x` in closure
   |                        |
   |                        first mutable borrow occurs here
24 |     let c2 = to_fn_mut(|| x = 5); //~ ERROR cannot borrow `x` as mutable more than once
   |                        ^^ - borrow occurs due to use of `x` in closure
   |                        |
   |                        second mutable borrow occurs here
25 | }
   | - first borrow ends here

error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
  --> $DIR/borrowck-closures-two-mut.rs:24:24
   |
23 |     let c1 = to_fn_mut(|| x = 4);
   |                        -- - previous borrow occurs due to use of `x` in closure
   |                        |
   |                        first mutable borrow occurs here
24 |     let c2 = to_fn_mut(|| x = 5); //~ ERROR cannot borrow `x` as mutable more than once
   |                        ^^ - borrow occurs due to use of `x` in closure
   |                        |
   |                        second mutable borrow occurs here
25 | }
   | - first borrow ends here
```

Fixes #45362.

5 files changed, 303 insertions, 9 deletions

diff --git a/src/librustc/hir/mod.rs b/src/librustc/hir/mod.rs
index c9b1d70e7b6..39e222230e5 100644
--- a/src/librustc/hir/mod.rs
+++ b/src/librustc/hir/mod.rs
@@ -1949,7 +1949,7 @@ impl ForeignItem_ {
 }
 
 /// A free variable referred to in a function.
-#[derive(Copy, Clone, RustcEncodable, RustcDecodable)]
+#[derive(Debug, Copy, Clone, RustcEncodable, RustcDecodable)]
 pub struct Freevar {
     /// The variable being accessed free.
     pub def: Def,
diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs
index 0fa40f56196..cd017650633 100644
--- a/src/librustc/mir/mod.rs
+++ b/src/librustc/mir/mod.rs
@@ -267,10 +267,10 @@ impl<'tcx> Mir<'tcx> {
         let block = &self[location.block];
         let stmts = &block.statements;
         let idx = location.statement_index;
-        if location.statement_index < stmts.len() {
+        if idx < stmts.len() {
             &stmts[idx].source_info
         } else {
-            assert!(location.statement_index == stmts.len());
+            assert!(idx == stmts.len());
             &block.terminator().source_info
         }
     }
diff --git a/src/librustc_mir/borrow_check.rs b/src/librustc_mir/borrow_check.rs
index d2524b306cf..b723b86776b 100644
--- a/src/librustc_mir/borrow_check.rs
+++ b/src/librustc_mir/borrow_check.rs
@@ -1169,8 +1169,72 @@ impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx>
         err.emit();
     }
 
+    /// Finds the span of arguments of a closure (within `maybe_closure_span`) and its usage of
+    /// the local assigned at `location`.
+    /// This is done by searching in statements succeeding `location`
+    /// and originating from `maybe_closure_span`.
+    fn find_closure_span(
+        &self,
+        maybe_closure_span: Span,
+        location: Location,
+    ) -> Option<(Span, Span)> {
+        use rustc::hir::ExprClosure;
+        use rustc::mir::AggregateKind;
+
+        let local = if let StatementKind::Assign(Lvalue::Local(local), _) =
+            self.mir[location.block].statements[location.statement_index].kind
+        {
+            local
+        } else {
+            return None;
+        };
+
+        for stmt in &self.mir[location.block].statements[location.statement_index + 1..] {
+            if maybe_closure_span != stmt.source_info.span {
+                break;
+            }
+
+            if let StatementKind::Assign(_, Rvalue::Aggregate(ref kind, ref lvs)) = stmt.kind {
+                if let AggregateKind::Closure(def_id, _) = **kind {
+                    debug!("find_closure_span: found closure {:?}", lvs);
+
+                    return if let Some(node_id) = self.tcx.hir.as_local_node_id(def_id) {
+                        let args_span = if let ExprClosure(_, _, _, span, _) =
+                            self.tcx.hir.expect_expr(node_id).node
+                        {
+                            span
+                        } else {
+                            return None;
+                        };
+
+                        self.tcx
+                            .with_freevars(node_id, |freevars| {
+                                for (v, lv) in freevars.iter().zip(lvs) {
+                                    if let Operand::Consume(Lvalue::Local(l)) = *lv {
+                                        if local == l {
+                                            debug!(
+                                                "find_closure_span: found captured local {:?}",
+                                                l
+                                            );
+                                            return Some(v.span);
+                                        }
+                                    }
+                                }
+                                None
+                            })
+                            .map(|var_span| (args_span, var_span))
+                    } else {
+                        None
+                    };
+                }
+            }
+        }
+
+        None
+    }
+
     fn report_conflicting_borrow(&mut self,
-                                 _context: Context,
+                                 context: Context,
                                  common_prefix: &Lvalue,
                                  (lvalue, span): (&Lvalue, Span),
                                  gen_borrow_kind: BorrowKind,
@@ -1183,38 +1247,60 @@ impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx>
 
         let issued_span = self.retrieve_borrow_span(issued_borrow);
 
+        let new_closure_span = self.find_closure_span(span, context.loc);
+        let span = new_closure_span.map(|(args, _)| args).unwrap_or(span);
+        let old_closure_span = self.find_closure_span(issued_span, issued_borrow.location);
+        let issued_span = old_closure_span.map(|(args, _)| args).unwrap_or(issued_span);
+
+        let desc_lvalue = self.describe_lvalue(lvalue);
+
         // FIXME: supply non-"" `opt_via` when appropriate
         let mut err = match (gen_borrow_kind, "immutable", "mutable",
                              issued_borrow.kind, "immutable", "mutable") {
             (BorrowKind::Shared, lft, _, BorrowKind::Mut, _, rgt) |
             (BorrowKind::Mut, _, lft, BorrowKind::Shared, rgt, _) =>
                 self.tcx.cannot_reborrow_already_borrowed(
-                    span, &self.describe_lvalue(lvalue), "", lft, issued_span,
+                    span, &desc_lvalue, "", lft, issued_span,
                     "it", rgt, "", end_issued_loan_span, Origin::Mir),
 
             (BorrowKind::Mut, _, _, BorrowKind::Mut, _, _) =>
                 self.tcx.cannot_mutably_borrow_multiply(
-                    span, &self.describe_lvalue(lvalue), "", issued_span,
+                    span, &desc_lvalue, "", issued_span,
                     "", end_issued_loan_span, Origin::Mir),
 
             (BorrowKind::Unique, _, _, BorrowKind::Unique, _, _) =>
                 self.tcx.cannot_uniquely_borrow_by_two_closures(
-                    span, &self.describe_lvalue(lvalue), issued_span,
+                    span, &desc_lvalue, issued_span,
                     end_issued_loan_span, Origin::Mir),
 
             (BorrowKind::Unique, _, _, _, _, _) =>
                 self.tcx.cannot_uniquely_borrow_by_one_closure(
-                    span, &self.describe_lvalue(lvalue), "",
+                    span, &desc_lvalue, "",
                     issued_span, "it", "", end_issued_loan_span, Origin::Mir),
 
             (_, _, _, BorrowKind::Unique, _, _) =>
                 self.tcx.cannot_reborrow_already_uniquely_borrowed(
-                    span, &self.describe_lvalue(lvalue), "it", "",
+                    span, &desc_lvalue, "it", "",
                     issued_span, "", end_issued_loan_span, Origin::Mir),
 
             (BorrowKind::Shared, _, _, BorrowKind::Shared, _, _) =>
                 unreachable!(),
         };
+
+        if let Some((_, var_span)) = old_closure_span {
+            err.span_label(
+                var_span,
+                format!("previous borrow occurs due to use of `{}` in closure", desc_lvalue),
+            );
+        }
+
+        if let Some((_, var_span)) = new_closure_span {
+            err.span_label(
+                var_span,
+                format!("borrow occurs due to use of `{}` in closure", desc_lvalue),
+            );
+        }
+
         err.emit();
     }
 
diff --git a/src/test/ui/borrowck/borrowck-closures-two-mut.rs b/src/test/ui/borrowck/borrowck-closures-two-mut.rs
new file mode 100644
index 00000000000..182b3d75442
--- /dev/null
+++ b/src/test/ui/borrowck/borrowck-closures-two-mut.rs
@@ -0,0 +1,62 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Tests that two closures cannot simultaneously have mutable
+// access to the variable, whether that mutable access be used
+// for direct assignment or for taking mutable ref. Issue #6801.
+
+// compile-flags: -Z emit-end-regions -Z borrowck-mir
+
+#![feature(box_syntax)]
+
+fn to_fn_mut<F: FnMut()>(f: F) -> F { f }
+
+fn a() {
+    let mut x = 3;
+    let c1 = to_fn_mut(|| x = 4);
+    let c2 = to_fn_mut(|| x = 5); //~ ERROR cannot borrow `x` as mutable more than once
+}
+
+fn set(x: &mut isize) {
+    *x = 4;
+}
+
+fn b() {
+    let mut x = 3;
+    let c1 = to_fn_mut(|| set(&mut x));
+    let c2 = to_fn_mut(|| set(&mut x)); //~ ERROR cannot borrow `x` as mutable more than once
+}
+
+fn c() {
+    let mut x = 3;
+    let c1 = to_fn_mut(|| x = 5);
+    let c2 = to_fn_mut(|| set(&mut x)); //~ ERROR cannot borrow `x` as mutable more than once
+}
+
+fn d() {
+    let mut x = 3;
+    let c1 = to_fn_mut(|| x = 5);
+    let c2 = to_fn_mut(|| { let _y = to_fn_mut(|| set(&mut x)); }); // (nested closure)
+    //~^ ERROR cannot borrow `x` as mutable more than once
+}
+
+fn g() {
+    struct Foo {
+        f: Box<isize>
+    }
+
+    let mut x: Box<_> = box Foo { f: box 3 };
+    let c1 = to_fn_mut(|| set(&mut *x.f));
+    let c2 = to_fn_mut(|| set(&mut *x.f));
+    //~^ ERROR cannot borrow `x` as mutable more than once
+}
+
+fn main() {
+}
diff --git a/src/test/ui/borrowck/borrowck-closures-two-mut.stderr b/src/test/ui/borrowck/borrowck-closures-two-mut.stderr
new file mode 100644
index 00000000000..fc8a7f2ab60
--- /dev/null
+++ b/src/test/ui/borrowck/borrowck-closures-two-mut.stderr
@@ -0,0 +1,146 @@
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Ast)
+  --> $DIR/borrowck-closures-two-mut.rs:24:24
+   |
+23 |     let c1 = to_fn_mut(|| x = 4);
+   |                        -- - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+24 |     let c2 = to_fn_mut(|| x = 5); //~ ERROR cannot borrow `x` as mutable more than once
+   |                        ^^ - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+25 | }
+   | - first borrow ends here
+
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Ast)
+  --> $DIR/borrowck-closures-two-mut.rs:34:24
+   |
+33 |     let c1 = to_fn_mut(|| set(&mut x));
+   |                        --          - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+34 |     let c2 = to_fn_mut(|| set(&mut x)); //~ ERROR cannot borrow `x` as mutable more than once
+   |                        ^^          - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+35 | }
+   | - first borrow ends here
+
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Ast)
+  --> $DIR/borrowck-closures-two-mut.rs:40:24
+   |
+39 |     let c1 = to_fn_mut(|| x = 5);
+   |                        -- - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+40 |     let c2 = to_fn_mut(|| set(&mut x)); //~ ERROR cannot borrow `x` as mutable more than once
+   |                        ^^          - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+41 | }
+   | - first borrow ends here
+
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Ast)
+  --> $DIR/borrowck-closures-two-mut.rs:46:24
+   |
+45 |     let c1 = to_fn_mut(|| x = 5);
+   |                        -- - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+46 |     let c2 = to_fn_mut(|| { let _y = to_fn_mut(|| set(&mut x)); }); // (nested closure)
+   |                        ^^                                  - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+47 |     //~^ ERROR cannot borrow `x` as mutable more than once
+48 | }
+   | - first borrow ends here
+
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Ast)
+  --> $DIR/borrowck-closures-two-mut.rs:57:24
+   |
+56 |     let c1 = to_fn_mut(|| set(&mut *x.f));
+   |                        --           - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+57 |     let c2 = to_fn_mut(|| set(&mut *x.f));
+   |                        ^^           - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+58 |     //~^ ERROR cannot borrow `x` as mutable more than once
+59 | }
+   | - first borrow ends here
+
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
+  --> $DIR/borrowck-closures-two-mut.rs:24:24
+   |
+23 |     let c1 = to_fn_mut(|| x = 4);
+   |                        -- - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+24 |     let c2 = to_fn_mut(|| x = 5); //~ ERROR cannot borrow `x` as mutable more than once
+   |                        ^^ - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+25 | }
+   | - first borrow ends here
+
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
+  --> $DIR/borrowck-closures-two-mut.rs:34:24
+   |
+33 |     let c1 = to_fn_mut(|| set(&mut x));
+   |                        --          - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+34 |     let c2 = to_fn_mut(|| set(&mut x)); //~ ERROR cannot borrow `x` as mutable more than once
+   |                        ^^          - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+35 | }
+   | - first borrow ends here
+
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
+  --> $DIR/borrowck-closures-two-mut.rs:40:24
+   |
+39 |     let c1 = to_fn_mut(|| x = 5);
+   |                        -- - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+40 |     let c2 = to_fn_mut(|| set(&mut x)); //~ ERROR cannot borrow `x` as mutable more than once
+   |                        ^^          - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+41 | }
+   | - first borrow ends here
+
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
+  --> $DIR/borrowck-closures-two-mut.rs:46:24
+   |
+45 |     let c1 = to_fn_mut(|| x = 5);
+   |                        -- - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+46 |     let c2 = to_fn_mut(|| { let _y = to_fn_mut(|| set(&mut x)); }); // (nested closure)
+   |                        ^^                                  - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+47 |     //~^ ERROR cannot borrow `x` as mutable more than once
+48 | }
+   | - first borrow ends here
+
+error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
+  --> $DIR/borrowck-closures-two-mut.rs:57:24
+   |
+56 |     let c1 = to_fn_mut(|| set(&mut *x.f));
+   |                        --           - previous borrow occurs due to use of `x` in closure
+   |                        |
+   |                        first mutable borrow occurs here
+57 |     let c2 = to_fn_mut(|| set(&mut *x.f));
+   |                        ^^           - borrow occurs due to use of `x` in closure
+   |                        |
+   |                        second mutable borrow occurs here
+58 |     //~^ ERROR cannot borrow `x` as mutable more than once
+59 | }
+   | - first borrow ends here
+
+error: aborting due to 10 previous errors
+