Auto merge of #47124 - estebank:loan-paths, r=nikomatsakis

Reword reason for move note

On move errors, when encountering an enum variant, be more ambiguous and do not refer to the type on the cause note, to avoid referring to `(maybe as std::prelude::v1::Some).0`, and instead refer to `the value`.

Sidesteps part of the problem with #41962:

```
error[E0382]: use of partially moved value: `maybe`
 --> file.rs:5:30
  |
5 |         if let Some(thing) = maybe {
  |                     -----    ^^^^^ value used here after move
  |                     |
  |                     value moved here
  = note: move occurs because the value has type `std::vec::Vec<bool>`, which does not implement the `Copy` trait

error[E0382]: use of moved value: `(maybe as std::prelude::v1::Some).0`
 --> file.rs:5:21
  |
5 |         if let Some(thing) = maybe {
  |                     ^^^^^ value moved here in previous iteration of loop
  = note: move occurs because the value has type `std::vec::Vec<bool>`, which does not implement the `Copy` trait

error: aborting due to 2 previous errors
```

Previous discussion: #44360

r? @arielb1

Author: bors

src/librustc_borrowck/borrowck/mod.rs

@@ -346,6 +346,16 @@ impl<'tcx> LoanPath<'tcx> {
     }
 
     fn to_type(&self) -> Ty<'tcx> { self.ty }
+
+    fn has_downcast(&self) -> bool {
+        match self.kind {
+            LpDowncast(_, _) => true,
+            LpExtend(ref lp, _, LpInterior(_, _)) => {
+                lp.has_downcast()
+            }
+            _ => false,
+        }
+    }
 }
 
 // FIXME (pnkfelix): See discussion here
@@ -721,16 +731,20 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                          move_note));
             err
         } else {
-            err.span_label(use_span, format!("value {} here after move", verb_participle))
-               .span_label(move_span, format!("value moved{} here", move_note));
+            err.span_label(use_span, format!("value {} here after move", verb_participle));
+            err.span_label(move_span, format!("value moved{} here", move_note));
             err
         };
 
         if need_note {
-            err.note(&format!("move occurs because `{}` has type `{}`, \
-                               which does not implement the `Copy` trait",
-                              self.loan_path_to_string(moved_lp),
-                              moved_lp.ty));
+            err.note(&format!(
+                "move occurs because {} has type `{}`, which does not implement the `Copy` trait",
+                if moved_lp.has_downcast() {
+                    "the value".to_string()
+                } else {
+                    format!("`{}`", self.loan_path_to_string(moved_lp))
+                },
+                moved_lp.ty));
         }
 
         // Note: we used to suggest adding a `ref binding` or calling
@@ -1414,7 +1428,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
             LpDowncast(ref lp_base, variant_def_id) => {
                 out.push('(');
                 self.append_autoderefd_loan_path_to_string(&lp_base, out);
-                out.push(':');
+                out.push_str(DOWNCAST_PRINTED_OPERATOR);
                 out.push_str(&self.tcx.item_path_str(variant_def_id));
                 out.push(')');
             }

src/librustc_mir/borrow_check/error_reporting.rs

@@ -62,13 +62,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                 Origin::Mir,
             );
 
-            err.span_label(
-                span,
-                format!(
-                    "value {} here after move",
-                    desired_action.as_verb_in_past_tense()
-                ),
-            );
+            let mut is_loop_move = false;
             for moi in mois {
                 let move_msg = ""; //FIXME: add " (into closure)"
                 let move_span = self.mir.source_info(self.move_data.moves[*moi].source).span;
@@ -77,10 +71,20 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                         span,
                         format!("value moved{} here in previous iteration of loop", move_msg),
                     );
+                    is_loop_move = true;
                 } else {
                     err.span_label(move_span, format!("value moved{} here", move_msg));
                 };
             }
+            if !is_loop_move {
+                err.span_label(
+                    span,
+                    format!(
+                        "value {} here after move",
+                        desired_action.as_verb_in_past_tense()
+                    ),
+                );
+            }
 
             if let Some(ty) = self.retrieve_type_for_place(place) {
                 let needs_note = match ty.sty {

src/test/compile-fail/issue-24357.rs

@@ -12,9 +12,9 @@ struct NoCopy;
 fn main() {
    let x = NoCopy;
    let f = move || { let y = x; };
-   //~^ value moved (into closure) here
+   //~^ NOTE value moved (into closure) here
    let z = x;
    //~^ ERROR use of moved value: `x`
-   //~| value used here after move
-   //~| move occurs because `x` has type `NoCopy`
+   //~| NOTE value used here after move
+   //~| NOTE move occurs because `x` has type `NoCopy`
 }

src/test/compile-fail/moves-based-on-type-distribute-copy-over-paren.rs

@@ -17,17 +17,19 @@ fn touch<A>(_a: &A) {}
 fn f00() {
     let x = "hi".to_string();
     let _y = Foo { f:x };
-    //~^ value moved here
+    //~^ NOTE value moved here
     touch(&x); //~ ERROR use of moved value: `x`
-    //~^ value used here after move
-    //~| move occurs because `x` has type `std::string::String`
+    //~^ NOTE value used here after move
+    //~| NOTE move occurs because `x` has type `std::string::String`
 }
 
 fn f05() {
     let x = "hi".to_string();
     let _y = Foo { f:(((x))) };
-    //~^ value moved here
+    //~^ NOTE value moved here
     touch(&x); //~ ERROR use of moved value: `x`
+    //~^ NOTE value used here after move
+    //~| NOTE move occurs because `x` has type `std::string::String`
 }
 
 fn f10() {

src/test/ui/borrowck/issue-41962.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+// compile-flags: -Z borrowck=compare
+
+pub fn main(){
+    let maybe = Some(vec![true, true]);
+
+    loop {
+        if let Some(thing) = maybe {
+        //~^ ERROR use of partially moved value: `maybe` (Ast) [E0382]
+        //~| ERROR use of moved value: `(maybe as std::prelude::v1::Some).0` (Ast) [E0382]
+        //~| ERROR use of moved value: `maybe` (Mir) [E0382]
+        //~| ERROR use of moved value: `maybe.0` (Mir) [E0382]
+        }
+    }
+}

src/test/ui/borrowck/issue-41962.stderr

@@ -0,0 +1,39 @@
+error[E0382]: use of partially moved value: `maybe` (Ast)
+  --> $DIR/issue-41962.rs:17:30
+   |
+17 |         if let Some(thing) = maybe {
+   |                     -----    ^^^^^ value used here after move
+   |                     |
+   |                     value moved here
+   |
+   = note: move occurs because the value has type `std::vec::Vec<bool>`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `(maybe as std::prelude::v1::Some).0` (Ast)
+  --> $DIR/issue-41962.rs:17:21
+   |
+17 |         if let Some(thing) = maybe {
+   |                     ^^^^^ value moved here in previous iteration of loop
+   |
+   = note: move occurs because the value has type `std::vec::Vec<bool>`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `maybe` (Mir)
+  --> $DIR/issue-41962.rs:17:16
+   |
+17 |         if let Some(thing) = maybe {
+   |                ^^^^^-----^
+   |                |    |
+   |                |    value moved here
+   |                value used here after move
+   |
+   = note: move occurs because `maybe` has type `std::option::Option<std::vec::Vec<bool>>`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `maybe.0` (Mir)
+  --> $DIR/issue-41962.rs:17:21
+   |
+17 |         if let Some(thing) = maybe {
+   |                     ^^^^^ value moved here in previous iteration of loop
+   |
+   = note: move occurs because `maybe.0` has type `std::vec::Vec<bool>`, which does not implement the `Copy` trait
+
+error: aborting due to 4 previous errors
+