mir-borrowck: Replace all constant index and sublices output with [..] to match the AST borrowck output

Author: zilbuz

src/librustc_mir/borrow_check.rs

@@ -1090,20 +1090,13 @@ impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx>
                         autoderef = true;
                         ("",   format!(""), Some(index))
                     },
-                    ProjectionElem::ConstantIndex { offset, from_end: false, .. } => {
+                    ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {
                         autoderef = true;
-                        ("",   format!("[{}]", offset), None)
+                        // Since it isn't possible to borrow an element on a particular index and
+                        // then use another while the borrow is held, don't output indices details
+                        // to avoid confusing the end-user
+                        ("",   format!("[..]"), None)
                     },
-                    ProjectionElem::ConstantIndex { offset, from_end: true, .. } => {
-                        autoderef = true;
-                        ("",   format!("[-{}]", offset), None)
-                    },
-                    ProjectionElem::Subslice { from, to: 0 } =>
-                        ("",   format!("[{}:]", from), None),
-                    ProjectionElem::Subslice { from: 0, to } =>
-                        ("",   format!("[:-{}]", to), None),
-                    ProjectionElem::Subslice { from, to } =>
-                        ("",   format!("[{}:-{}]", from, to), None),
                 };
                 buf.push_str(prefix);
                 self.append_lvalue_to_string(&proj.base, buf, Some(autoderef));

src/test/compile-fail/borrowck/borrowck-describe-lvalue.rs

@@ -8,9 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-tidy-linelength
 // revisions: ast mir
 //[mir]compile-flags: -Z emit-end-regions -Z borrowck-mir
 
+#![feature(slice_patterns)]
 #![feature(advanced_slice_patterns)]
 
 pub struct Foo {
@@ -173,29 +175,62 @@ fn main() {
             &[x, _, .., _, _] => println!("{}", x),
                 //[ast]~^ ERROR cannot use `v[..]` because it was mutably borrowed
                 //[mir]~^^ ERROR cannot use `v[..]` because it was mutably borrowed (Ast)
-                //[mir]~| ERROR cannot use `v[0]` because it was mutably borrowed (Mir)
+                //[mir]~| ERROR cannot use `v[..]` because it was mutably borrowed (Mir)
                             _ => panic!("other case"),
         }
         match v {
             &[_, x, .., _, _] => println!("{}", x),
                 //[ast]~^ ERROR cannot use `v[..]` because it was mutably borrowed
                 //[mir]~^^ ERROR cannot use `v[..]` because it was mutably borrowed (Ast)
-                //[mir]~| ERROR cannot use `v[1]` because it was mutably borrowed (Mir)
+                //[mir]~| ERROR cannot use `v[..]` because it was mutably borrowed (Mir)
                             _ => panic!("other case"),
         }
         match v {
             &[_, _, .., x, _] => println!("{}", x),
                 //[ast]~^ ERROR cannot use `v[..]` because it was mutably borrowed
                 //[mir]~^^ ERROR cannot use `v[..]` because it was mutably borrowed (Ast)
-                //[mir]~| ERROR cannot use `v[-2]` because it was mutably borrowed (Mir)
+                //[mir]~| ERROR cannot use `v[..]` because it was mutably borrowed (Mir)
                             _ => panic!("other case"),
         }
         match v {
             &[_, _, .., _, x] => println!("{}", x),
                 //[ast]~^ ERROR cannot use `v[..]` because it was mutably borrowed
                 //[mir]~^^ ERROR cannot use `v[..]` because it was mutably borrowed (Ast)
-                //[mir]~| ERROR cannot use `v[-1]` because it was mutably borrowed (Mir)
+                //[mir]~| ERROR cannot use `v[..]` because it was mutably borrowed (Mir)
                             _ => panic!("other case"),
         }
     }
+    // Subslices
+    {
+        let mut v = &[1, 2, 3, 4, 5];
+        let _v = &mut v;
+        match v {
+            &[x..] => println!("{:?}", x),
+                //[ast]~^ ERROR cannot use `v[..]` because it was mutably borrowed
+                //[mir]~^^ ERROR cannot use `v[..]` because it was mutably borrowed (Ast)
+                //[mir]~| ERROR cannot use `v[..]` because it was mutably borrowed (Mir)
+            _ => panic!("other case"),
+        }
+        match v {
+            &[_, x..] => println!("{:?}", x),
+                //[ast]~^ ERROR cannot use `v[..]` because it was mutably borrowed
+                //[mir]~^^ ERROR cannot use `v[..]` because it was mutably borrowed (Ast)
+                //[mir]~| ERROR cannot use `v[..]` because it was mutably borrowed (Mir)
+            _ => panic!("other case"),
+        }
+        match v {
+            &[x.., _] => println!("{:?}", x),
+                //[ast]~^ ERROR cannot use `v[..]` because it was mutably borrowed
+                //[mir]~^^ ERROR cannot use `v[..]` because it was mutably borrowed (Ast)
+                //[mir]~| ERROR cannot use `v[..]` because it was mutably borrowed (Mir)
+            _ => panic!("other case"),
+        }
+        match v {
+            &[_, x.., _] => println!("{:?}", x),
+                //[ast]~^ ERROR cannot use `v[..]` because it was mutably borrowed
+                //[mir]~^^ ERROR cannot use `v[..]` because it was mutably borrowed (Ast)
+                //[mir]~| ERROR cannot use `v[..]` because it was mutably borrowed (Mir)
+            _ => panic!("other case"),
+        }
+    }
 }