Rollup merge of rust-lang#55696 - davidtwco:issue-55675, r=pnkfelix

NLL Diagnostic Review 3: Missing errors for borrows of union fields

Fixes rust-lang#55675.

This PR modifies a test to make it more robust (it also fixes indentation on a doc comment, but that's not the point of the PR). See the linked issue for details.

r? @pnkfelix

src/librustc/mir/mod.rs

@@ -506,25 +506,25 @@ pub enum BorrowKind {
     /// implicit closure bindings. It is needed when the closure is
     /// borrowing or mutating a mutable referent, e.g.:
     ///
-    ///    let x: &mut isize = ...;
-    ///    let y = || *x += 5;
+    ///     let x: &mut isize = ...;
+    ///     let y = || *x += 5;
     ///
     /// If we were to try to translate this closure into a more explicit
     /// form, we'd encounter an error with the code as written:
     ///
-    ///    struct Env { x: & &mut isize }
-    ///    let x: &mut isize = ...;
-    ///    let y = (&mut Env { &x }, fn_ptr);  // Closure is pair of env and fn
-    ///    fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    ///     struct Env { x: & &mut isize }
+    ///     let x: &mut isize = ...;
+    ///     let y = (&mut Env { &x }, fn_ptr);  // Closure is pair of env and fn
+    ///     fn fn_ptr(env: &mut Env) { **env.x += 5; }
     ///
     /// This is then illegal because you cannot mutate an `&mut` found
     /// in an aliasable location. To solve, you'd have to translate with
     /// an `&mut` borrow:
     ///
-    ///    struct Env { x: & &mut isize }
-    ///    let x: &mut isize = ...;
-    ///    let y = (&mut Env { &mut x }, fn_ptr); // changed from &x to &mut x
-    ///    fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    ///     struct Env { x: & &mut isize }
+    ///     let x: &mut isize = ...;
+    ///     let y = (&mut Env { &mut x }, fn_ptr); // changed from &x to &mut x
+    ///     fn fn_ptr(env: &mut Env) { **env.x += 5; }
     ///
     /// Now the assignment to `**env.x` is legal, but creating a
     /// mutable pointer to `x` is not because `x` is not mutable. We

src/test/ui/union/union-borrow-move-parent-sibling.nll.stderr

@@ -1,33 +1,64 @@
+error[E0502]: cannot borrow `u.y` as immutable because it is also borrowed as mutable
+  --> $DIR/union-borrow-move-parent-sibling.rs:25:13
+   |
+LL |     let a = &mut u.x.0;
+   |             ---------- mutable borrow occurs here
+LL |     let b = &u.y; //~ ERROR cannot borrow `u.y`
+   |             ^^^^ immutable borrow occurs here
+LL |     use_borrow(a);
+   |                - mutable borrow later used here
+
 error[E0382]: use of moved value: `u`
-  --> $DIR/union-borrow-move-parent-sibling.rs:29:13
+  --> $DIR/union-borrow-move-parent-sibling.rs:32:13
    |
 LL |     let a = u.x.0;
    |             ----- value moved here
-LL |     let a = u.y; //~ ERROR use of moved value: `u.y`
+LL |     let b = u.y; //~ ERROR use of moved value: `u.y`
    |             ^^^ value used here after move
    |
    = note: move occurs because `u` has type `U`, which does not implement the `Copy` trait
 
+error[E0502]: cannot borrow `u.y` as immutable because it is also borrowed as mutable
+  --> $DIR/union-borrow-move-parent-sibling.rs:38:13
+   |
+LL |     let a = &mut (u.x.0).0;
+   |             -------------- mutable borrow occurs here
+LL |     let b = &u.y; //~ ERROR cannot borrow `u.y`
+   |             ^^^^ immutable borrow occurs here
+LL |     use_borrow(a);
+   |                - mutable borrow later used here
+
 error[E0382]: use of moved value: `u`
-  --> $DIR/union-borrow-move-parent-sibling.rs:41:13
+  --> $DIR/union-borrow-move-parent-sibling.rs:45:13
    |
 LL |     let a = (u.x.0).0;
    |             --------- value moved here
-LL |     let a = u.y; //~ ERROR use of moved value: `u.y`
+LL |     let b = u.y; //~ ERROR use of moved value: `u.y`
    |             ^^^ value used here after move
    |
    = note: move occurs because `u` has type `U`, which does not implement the `Copy` trait
 
+error[E0502]: cannot borrow `u.x` as immutable because it is also borrowed as mutable
+  --> $DIR/union-borrow-move-parent-sibling.rs:51:13
+   |
+LL |     let a = &mut *u.y;
+   |             --------- mutable borrow occurs here
+LL |     let b = &u.x; //~ ERROR cannot borrow `u` (via `u.x`)
+   |             ^^^^ immutable borrow occurs here
+LL |     use_borrow(a);
+   |                - mutable borrow later used here
+
 error[E0382]: use of moved value: `u`
-  --> $DIR/union-borrow-move-parent-sibling.rs:53:13
+  --> $DIR/union-borrow-move-parent-sibling.rs:58:13
    |
 LL |     let a = *u.y;
    |             ---- value moved here
-LL |     let a = u.x; //~ ERROR use of moved value: `u.x`
+LL |     let b = u.x; //~ ERROR use of moved value: `u.x`
    |             ^^^ value used here after move
    |
    = note: move occurs because `u` has type `U`, which does not implement the `Copy` trait
 
-error: aborting due to 3 previous errors
+error: aborting due to 6 previous errors
 
-For more information about this error, try `rustc --explain E0382`.
+Some errors occurred: E0382, E0502.
+For more information about an error, try `rustc --explain E0382`.

src/test/ui/union/union-borrow-move-parent-sibling.rs

@@ -17,40 +17,45 @@ union U {
     y: Box<Vec<u8>>,
 }
 
+fn use_borrow<T>(_: &T) {}
+
 unsafe fn parent_sibling_borrow() {
     let mut u = U { x: ((Vec::new(), Vec::new()), Vec::new()) };
     let a = &mut u.x.0;
-    let a = &u.y; //~ ERROR cannot borrow `u.y`
+    let b = &u.y; //~ ERROR cannot borrow `u.y`
+    use_borrow(a);
 }
 
 unsafe fn parent_sibling_move() {
     let u = U { x: ((Vec::new(), Vec::new()), Vec::new()) };
     let a = u.x.0;
-    let a = u.y; //~ ERROR use of moved value: `u.y`
+    let b = u.y; //~ ERROR use of moved value: `u.y`
 }
 
 unsafe fn grandparent_sibling_borrow() {
     let mut u = U { x: ((Vec::new(), Vec::new()), Vec::new()) };
     let a = &mut (u.x.0).0;
-    let a = &u.y; //~ ERROR cannot borrow `u.y`
+    let b = &u.y; //~ ERROR cannot borrow `u.y`
+    use_borrow(a);
 }
 
 unsafe fn grandparent_sibling_move() {
     let u = U { x: ((Vec::new(), Vec::new()), Vec::new()) };
     let a = (u.x.0).0;
-    let a = u.y; //~ ERROR use of moved value: `u.y`
+    let b = u.y; //~ ERROR use of moved value: `u.y`
 }
 
 unsafe fn deref_sibling_borrow() {
     let mut u = U { y: Box::default() };
     let a = &mut *u.y;
-    let a = &u.x; //~ ERROR cannot borrow `u` (via `u.x`)
+    let b = &u.x; //~ ERROR cannot borrow `u` (via `u.x`)
+    use_borrow(a);
 }
 
 unsafe fn deref_sibling_move() {
     let u = U { x: ((Vec::new(), Vec::new()), Vec::new()) };
     let a = *u.y;
-    let a = u.x; //~ ERROR use of moved value: `u.x`
+    let b = u.x; //~ ERROR use of moved value: `u.x`
 }
 
 

src/test/ui/union/union-borrow-move-parent-sibling.stderr

@@ -1,59 +1,62 @@
 error[E0502]: cannot borrow `u.y` as immutable because `u.x.0` is also borrowed as mutable
-  --> $DIR/union-borrow-move-parent-sibling.rs:23:14
+  --> $DIR/union-borrow-move-parent-sibling.rs:25:14
    |
 LL |     let a = &mut u.x.0;
    |                  ----- mutable borrow occurs here
-LL |     let a = &u.y; //~ ERROR cannot borrow `u.y`
+LL |     let b = &u.y; //~ ERROR cannot borrow `u.y`
    |              ^^^ immutable borrow occurs here
+LL |     use_borrow(a);
 LL | }
    | - mutable borrow ends here
 
 error[E0382]: use of moved value: `u.y`
-  --> $DIR/union-borrow-move-parent-sibling.rs:29:9
+  --> $DIR/union-borrow-move-parent-sibling.rs:32:9
    |
 LL |     let a = u.x.0;
    |         - value moved here
-LL |     let a = u.y; //~ ERROR use of moved value: `u.y`
+LL |     let b = u.y; //~ ERROR use of moved value: `u.y`
    |         ^ value used here after move
    |
    = note: move occurs because `u.y` has type `[type error]`, which does not implement the `Copy` trait
 
 error[E0502]: cannot borrow `u.y` as immutable because `u.x.0.0` is also borrowed as mutable
-  --> $DIR/union-borrow-move-parent-sibling.rs:35:14
+  --> $DIR/union-borrow-move-parent-sibling.rs:38:14
    |
 LL |     let a = &mut (u.x.0).0;
    |                  --------- mutable borrow occurs here
-LL |     let a = &u.y; //~ ERROR cannot borrow `u.y`
+LL |     let b = &u.y; //~ ERROR cannot borrow `u.y`
    |              ^^^ immutable borrow occurs here
+LL |     use_borrow(a);
 LL | }
    | - mutable borrow ends here
 
 error[E0382]: use of moved value: `u.y`
-  --> $DIR/union-borrow-move-parent-sibling.rs:41:9
+  --> $DIR/union-borrow-move-parent-sibling.rs:45:9
    |
 LL |     let a = (u.x.0).0;
    |         - value moved here
-LL |     let a = u.y; //~ ERROR use of moved value: `u.y`
+LL |     let b = u.y; //~ ERROR use of moved value: `u.y`
    |         ^ value used here after move
    |
    = note: move occurs because `u.y` has type `[type error]`, which does not implement the `Copy` trait
 
 error[E0502]: cannot borrow `u` (via `u.x`) as immutable because `u` is also borrowed as mutable (via `*u.y`)
-  --> $DIR/union-borrow-move-parent-sibling.rs:47:14
+  --> $DIR/union-borrow-move-parent-sibling.rs:51:14
    |
 LL |     let a = &mut *u.y;
    |                  ---- mutable borrow occurs here (via `*u.y`)
-LL |     let a = &u.x; //~ ERROR cannot borrow `u` (via `u.x`)
+LL |     let b = &u.x; //~ ERROR cannot borrow `u` (via `u.x`)
    |              ^^^ immutable borrow occurs here (via `u.x`)
+LL |     use_borrow(a);
 LL | }
    | - mutable borrow ends here
 
 error[E0382]: use of moved value: `u.x`
-  --> $DIR/union-borrow-move-parent-sibling.rs:53:9
+  --> $DIR/union-borrow-move-parent-sibling.rs:58:9
    |
 LL |     let a = *u.y;
    |         - value moved here
-LL |     let a = u.x; //~ ERROR use of moved value: `u.x`
+LL |     let b = u.x; //~ ERROR use of moved value: `u.x`
    |         ^ value used here after move
    |
    = note: move occurs because `u.x` has type `[type error]`, which does not implement the `Copy` trait