Change the use of moved value error to be more accurate

Previously it output `partially moved` to eagerly. This updates it to be more
accurate and output `collaterally moved` for use of values that were invalidated
by moves out of different fields in the same struct.

Closes #15630.

src/librustc/middle/borrowck/mod.rs

@@ -461,24 +461,58 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
             MovedInCapture => "capture",
         };
 
-        match the_move.kind {
+        let (ol, moved_lp_msg) = match the_move.kind {
             move_data::Declared => {
                 self.tcx.sess.span_err(
                     use_span,
                     format!("{} of possibly uninitialized variable: `{}`",
                             verb,
                             self.loan_path_to_string(lp)).as_slice());
+                (self.loan_path_to_string(moved_lp),
+                 String::new())
             }
             _ => {
-                let partially = if lp == moved_lp {""} else {"partially "};
+                // If moved_lp is something like `x.a`, and lp is something like `x.b`, we would
+                // normally generate a rather confusing message:
+                //
+                //     error: use of moved value: `x.b`
+                //     note: `x.a` moved here...
+                //
+                // What we want to do instead is get the 'common ancestor' of the two moves and
+                // use that for most of the message instead, giving is something like this:
+                //
+                //     error: use of moved value: `x`
+                //     note: `x` moved here (through moving `x.a`)...
+
+                let common = moved_lp.common(lp);
+                let has_common = common.is_some();
+                let has_fork = moved_lp.has_fork(lp);
+                let (nl, ol, moved_lp_msg) =
+                    if has_fork && has_common {
+                        let nl = self.loan_path_to_string(&common.unwrap());
+                        let ol = nl.clone();
+                        let moved_lp_msg = format!(" (through moving `{}`)",
+                                                   self.loan_path_to_string(moved_lp));
+                        (nl, ol, moved_lp_msg)
+                    } else {
+                        (self.loan_path_to_string(lp),
+                         self.loan_path_to_string(moved_lp),
+                         String::new())
+                    };
+
+                let partial = moved_lp.depth() > lp.depth();
+                let msg = if !has_fork && partial { "partially " }
+                          else if has_fork && !has_common { "collaterally "}
+                          else { "" };
                 self.tcx.sess.span_err(
                     use_span,
                     format!("{} of {}moved value: `{}`",
                             verb,
-                            partially,
-                            self.loan_path_to_string(lp)).as_slice());
+                            msg,
+                            nl).as_slice());
+                (ol, moved_lp_msg)
             }
-        }
+        };
 
         match the_move.kind {
             move_data::Declared => {}
@@ -501,19 +535,21 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                         "moved by default (use `copy` to override)");
                 self.tcx.sess.span_note(
                     expr_span,
-                    format!("`{}` moved here because it has type `{}`, which is {}",
-                            self.loan_path_to_string(moved_lp),
+                    format!("`{}` moved here{} because it has type `{}`, which is {}",
+                            ol,
+                            moved_lp_msg,
                             expr_ty.user_string(self.tcx),
                             suggestion).as_slice());
             }
 
             move_data::MovePat => {
                 let pat_ty = ty::node_id_to_type(self.tcx, the_move.id);
                 self.tcx.sess.span_note(self.tcx.map.span(the_move.id),
-                    format!("`{}` moved here because it has type `{}`, \
+                    format!("`{}` moved here{} because it has type `{}`, \
                              which is moved by default (use `ref` to \
                              override)",
-                            self.loan_path_to_string(moved_lp),
+                            ol,
+                            moved_lp_msg,
                             pat_ty.user_string(self.tcx)).as_slice());
             }
 
@@ -536,9 +572,10 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                          capture that instead to override)");
                 self.tcx.sess.span_note(
                     expr_span,
-                    format!("`{}` moved into closure environment here because it \
+                    format!("`{}` moved into closure environment here{} because it \
                             has type `{}`, which is {}",
-                            self.loan_path_to_string(moved_lp),
+                            ol,
+                            moved_lp_msg,
                             expr_ty.user_string(self.tcx),
                             suggestion).as_slice());
             }

src/test/compile-fail/borrowck-box-insensitivity.rs

@@ -31,19 +31,22 @@ struct D {
 fn copy_after_move() {
     let a = box A { x: box 0, y: 1 };
     let _x = a.x;
-    let _y = a.y; //~ ERROR use of partially moved
+    let _y = a.y; //~ ERROR use of moved
+    //~^^ NOTE `a` moved here (through moving `a.x`)
 }
 
 fn move_after_move() {
     let a = box B { x: box 0, y: box 1 };
     let _x = a.x;
-    let _y = a.y; //~ ERROR use of partially moved
+    let _y = a.y; //~ ERROR use of moved
+    //~^^ NOTE `a` moved here (through moving `a.x`)
 }
 
 fn borrow_after_move() {
     let a = box A { x: box 0, y: 1 };
     let _x = a.x;
-    let _y = &a.y; //~ ERROR use of partially moved
+    let _y = &a.y; //~ ERROR use of moved
+    //~^^ NOTE `a` moved here (through moving `a.x`)
 }
 
 fn move_after_borrow() {
@@ -79,19 +82,19 @@ fn mut_borrow_after_borrow() {
 fn copy_after_move_nested() {
     let a = box C { x: box A { x: box 0, y: 1 }, y: 2 };
     let _x = a.x.x;
-    let _y = a.y; //~ ERROR use of partially moved
+    let _y = a.y; //~ ERROR use of collaterally moved
 }
 
 fn move_after_move_nested() {
     let a = box D { x: box A { x: box 0, y: 1 }, y: box 2 };
     let _x = a.x.x;
-    let _y = a.y; //~ ERROR use of partially moved
+    let _y = a.y; //~ ERROR use of collaterally moved
 }
 
 fn borrow_after_move_nested() {
     let a = box C { x: box A { x: box 0, y: 1 }, y: 2 };
     let _x = a.x.x;
-    let _y = &a.y; //~ ERROR use of partially moved
+    let _y = &a.y; //~ ERROR use of collaterally moved
 }
 
 fn move_after_borrow_nested() {

src/test/compile-fail/borrowck-field-sensitivity.rs

@@ -13,13 +13,13 @@ struct A { a: int, b: Box<int> }
 fn deref_after_move() {
     let x = A { a: 1, b: box 2 };
     drop(x.b);
-    drop(*x.b); //~ ERROR use of partially moved value: `*x.b`
+    drop(*x.b); //~ ERROR use of moved value: `*x.b`
 }
 
 fn deref_after_fu_move() {
     let x = A { a: 1, b: box 2 };
     let y = A { a: 3, .. x };
-    drop(*x.b); //~ ERROR use of partially moved value: `*x.b`
+    drop(*x.b); //~ ERROR use of moved value: `*x.b`
 }
 
 fn borrow_after_move() {

src/test/compile-fail/liveness-use-after-move.rs

@@ -13,6 +13,6 @@ extern crate debug;
 fn main() {
     let x = box 5i;
     let y = x;
-    println!("{:?}", *x); //~ ERROR use of partially moved value: `*x`
+    println!("{:?}", *x); //~ ERROR use of moved value: `*x`
     y.clone();
 }

src/test/compile-fail/use-after-move-self-based-on-type.rs

@@ -19,7 +19,7 @@ impl Drop for S {
 impl S {
     pub fn foo(self) -> int {
         self.bar();
-        return self.x;  //~ ERROR use of partially moved value: `self.x`
+        return self.x;  //~ ERROR use of moved value: `self.x`
     }
 
     pub fn bar(self) {}

src/test/compile-fail/use-after-move-self.rs

@@ -16,7 +16,7 @@ struct S {
 impl S {
     pub fn foo(self) -> int {
         self.bar();
-        return *self.x;  //~ ERROR use of partially moved value: `*self.x`
+        return *self.x;  //~ ERROR use of moved value: `*self.x`
     }
 
     pub fn bar(self) {}