Rollup merge of #23859 - pnkfelix:fsk-lesser-box, r=nikomatsakis

Disallow writing through mutable pointers stored in non-mut Box.

Fix #14270 

The fix works by making `cmt::freely_aliasable` result more fine-grained.

Instead of encoding the aliasability (i.e. whether the cmt is uniquely writable or not) as an option, now pass back an enum indicating either: 1. freely-aliasable (thus not uniquely-writable), 2. non-aliasable (thus uniquely writable), or 3. unique but immutable (and thus not uniquely writable, according to proposal from issue #14270.)

This is all of course a giant hack that will hopefully go away with an eventually removal of special treatment of `Box<T>` (aka `ty_unique`) from the compiler.

src/test/compile-fail/borrowck-issue-14498.rs

@@ -9,26 +9,48 @@
 // except according to those terms.
 
 // This tests that we can't modify Box<&mut T> contents while they
-// are borrowed.
+// are borrowed (#14498).
+//
+// Also includes tests of the errors reported when the Box in question
+// is immutable (#14270).
 
 #![feature(box_syntax)]
 
 struct A { a: isize }
 struct B<'a> { a: Box<&'a mut isize> }
 
+fn indirect_write_to_imm_box() {
+    let mut x: isize = 1;
+    let y: Box<_> = box &mut x;
+    let p = &y;
+    ***p = 2; //~ ERROR cannot assign to data in an immutable container
+    drop(p);
+}
+
 fn borrow_in_var_from_var() {
+    let mut x: isize = 1;
+    let mut y: Box<_> = box &mut x;
+    let p = &y;
+    let q = &***p;
+    **y = 2; //~ ERROR cannot assign to `**y` because it is borrowed
+    drop(p);
+    drop(q);
+}
+
+fn borrow_in_var_from_var_via_imm_box() {
     let mut x: isize = 1;
     let y: Box<_> = box &mut x;
     let p = &y;
     let q = &***p;
     **y = 2; //~ ERROR cannot assign to `**y` because it is borrowed
+    //~^         ERROR cannot assign to data in an immutable container
     drop(p);
     drop(q);
 }
 
 fn borrow_in_var_from_field() {
     let mut x = A { a: 1 };
-    let y: Box<_> = box &mut x.a;
+    let mut y: Box<_> = box &mut x.a;
     let p = &y;
     let q = &***p;
     **y = 2; //~ ERROR cannot assign to `**y` because it is borrowed
@@ -36,19 +58,41 @@ fn borrow_in_var_from_field() {
     drop(q);
 }
 
+fn borrow_in_var_from_field_via_imm_box() {
+    let mut x = A { a: 1 };
+    let y: Box<_> = box &mut x.a;
+    let p = &y;
+    let q = &***p;
+    **y = 2; //~ ERROR cannot assign to `**y` because it is borrowed
+    //~^         ERROR cannot assign to data in an immutable container
+    drop(p);
+    drop(q);
+}
+
 fn borrow_in_field_from_var() {
+    let mut x: isize = 1;
+    let mut y = B { a: box &mut x };
+    let p = &y.a;
+    let q = &***p;
+    **y.a = 2; //~ ERROR cannot assign to `**y.a` because it is borrowed
+    drop(p);
+    drop(q);
+}
+
+fn borrow_in_field_from_var_via_imm_box() {
     let mut x: isize = 1;
     let y = B { a: box &mut x };
     let p = &y.a;
     let q = &***p;
     **y.a = 2; //~ ERROR cannot assign to `**y.a` because it is borrowed
+    //~^           ERROR cannot assign to data in an immutable container
     drop(p);
     drop(q);
 }
 
 fn borrow_in_field_from_field() {
     let mut x = A { a: 1 };
-    let y = B { a: box &mut x.a };
+    let mut y = B { a: box &mut x.a };
     let p = &y.a;
     let q = &***p;
     **y.a = 2; //~ ERROR cannot assign to `**y.a` because it is borrowed
@@ -56,9 +100,25 @@ fn borrow_in_field_from_field() {
     drop(q);
 }
 
-fn main() {
-    borrow_in_var_from_var();
-    borrow_in_var_from_field();
-    borrow_in_field_from_var();
-    borrow_in_field_from_field();
+fn borrow_in_field_from_field_via_imm_box() {
+    let mut x = A { a: 1 };
+    let y = B { a: box &mut x.a };
+    let p = &y.a;
+    let q = &***p;
+    **y.a = 2; //~ ERROR cannot assign to `**y.a` because it is borrowed
+    //~^           ERROR cannot assign to data in an immutable container
+    drop(p);
+    drop(q);
+}
+
+fn main() {
+    indirect_write_to_imm_box();
+    borrow_in_var_from_var();
+    borrow_in_var_from_var_via_imm_box();
+    borrow_in_var_from_field();
+    borrow_in_var_from_field_via_imm_box();
+    borrow_in_field_from_var();
+    borrow_in_field_from_var_via_imm_box();
+    borrow_in_field_from_field();
+    borrow_in_field_from_field_via_imm_box();
 }