Auto merge of #52681 - pnkfelix:z-borrowck-migrate, r=nikomatsakis

Add `-Z borrowck=migrate`

This adds `-Z borrowck=migrate`, which represents the way we want to migrate to NLL under Rust versions to come. It also hooks this new mode into `--edition 2018`, which means we're officially turning NLL on in the 2018 edition.

The basic idea of `-Z borrowck=migrate` that there are cases where NLL is fixing old soundness bugs in the borrow-checker, but in order to avoid just breaking code by immediately rejecting the programs that hit those soundness bugs, we instead use the following strategy:

If your code is accepted by NLL, then we accept it.
If your code is rejected by both NLL and the old AST-borrowck, then we reject it.
If your code is rejected by NLL but accepted by the old AST-borrowck, then we emit the new NLL errors as **warnings**.

These warnings will be turned into hard errors in the future, and they say so in these diagnostics.

Fix #46908

src/test/ui/borrowck/borrowck-feature-nll-overrides-migrate.edition.stderr

@@ -0,0 +1,9 @@
+error[E0507]: cannot move out of borrowed content
+  --> $DIR/borrowck-feature-nll-overrides-migrate.rs:32:17
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot move out of borrowed content
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0507`.

src/test/ui/borrowck/borrowck-feature-nll-overrides-migrate.rs

@@ -0,0 +1,40 @@
+// Copyright 2017 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.
+
+// This is a test that the `#![feature(nll)]` opt-in overrides the
+// migration mode. The intention here is to emulate the goal behavior
+// that `--edition 2018` effects on borrowck (modeled here by `-Z
+// borrowck=migrate`) are themselves overridden by the
+// `#![feature(nll)]` opt-in.
+//
+// Therefore, for developer convenience, under `#[feature(nll)]` the
+// NLL checks will be emitted as errors *even* in the presence of `-Z
+// borrowck=migrate`.
+
+// revisions: zflag edition
+// [zflag]compile-flags: -Z borrowck=migrate
+// [edition]compile-flags: --edition 2018
+
+#![feature(nll)]
+
+fn main() {
+    match Some(&4) {
+        None => {},
+        ref mut foo
+            if {
+                (|| { let bar = foo; bar.take() })();
+                //[zflag]~^ ERROR cannot move out of borrowed content [E0507]
+                //[edition]~^^ ERROR cannot move out of borrowed content [E0507]
+                false
+            } => {},
+        Some(ref _s) => println!("Note this arm is bogus; the `Some` became `None` in the guard."),
+        _ => println!("Here is some supposedly unreachable code."),
+    }
+}

src/test/ui/borrowck/borrowck-feature-nll-overrides-migrate.zflag.stderr

@@ -0,0 +1,9 @@
+error[E0507]: cannot move out of borrowed content
+  --> $DIR/borrowck-feature-nll-overrides-migrate.rs:32:17
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot move out of borrowed content
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0507`.

src/test/ui/borrowck/borrowck-migrate-to-nll.edition.stderr

@@ -0,0 +1,24 @@
+warning[E0507]: cannot move out of borrowed content
+  --> $DIR/borrowck-migrate-to-nll.rs:35:17
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot move out of borrowed content
+   |
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
+
+warning[E0507]: cannot move out of `foo`, as it is immutable for the pattern guard
+  --> $DIR/borrowck-migrate-to-nll.rs:35:17
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                 |
+   |                 cannot move out of `foo`, as it is immutable for the pattern guard
+   |                 cannot move
+   |
+   = note: variables bound in patterns are immutable until the end of the pattern guard
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
+

src/test/ui/borrowck/borrowck-migrate-to-nll.rs

@@ -0,0 +1,41 @@
+// Copyright 2017 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.
+
+// This is a test of the borrowck migrate mode. It leverages #27282, a
+// bug that is fixed by NLL: this code is (unsoundly) accepted by
+// AST-borrowck, but is correctly rejected by the NLL borrowck.
+//
+// Therefore, for backwards-compatiblity, under borrowck=migrate the
+// NLL checks will be emitted as *warnings*.
+
+// NLL mode makes this compile-fail; we cannot currently encode a
+// test that is run-pass or compile-fail based on compare-mode. So
+// just ignore it instead:
+
+// ignore-compare-mode-nll
+
+// revisions: zflag edition
+//[zflag]compile-flags: -Z borrowck=migrate
+//[edition]compile-flags: --edition 2018
+//[zflag] run-pass
+//[edition] run-pass
+
+fn main() {
+    match Some(&4) {
+        None => {},
+        ref mut foo
+            if {
+                (|| { let bar = foo; bar.take() })();
+                false
+            } => {},
+        Some(ref _s) => println!("Note this arm is bogus; the `Some` became `None` in the guard."),
+        _ => println!("Here is some supposedly unreachable code."),
+    }
+}

src/test/ui/borrowck/borrowck-migrate-to-nll.zflag.stderr

@@ -0,0 +1,24 @@
+warning[E0507]: cannot move out of borrowed content
+  --> $DIR/borrowck-migrate-to-nll.rs:35:17
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot move out of borrowed content
+   |
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
+
+warning[E0507]: cannot move out of `foo`, as it is immutable for the pattern guard
+  --> $DIR/borrowck-migrate-to-nll.rs:35:17
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                 |
+   |                 cannot move out of `foo`, as it is immutable for the pattern guard
+   |                 cannot move
+   |
+   = note: variables bound in patterns are immutable until the end of the pattern guard
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
+

src/test/ui/generator/generator-with-nll.rs

@@ -8,17 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z borrowck=compare
-
 #![feature(generators)]
 #![feature(nll)]
 
 fn main() {
     || {
         // The reference in `_a` is a Legal with NLL since it ends before the yield
-        let _a = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
-        let b = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
-        //~^ borrow may still be in use when generator yields (Mir)
+        let _a = &mut true;
+        let b = &mut true;
+        //~^ borrow may still be in use when generator yields
         yield ();
         println!("{}", b);
     };

src/test/ui/generator/generator-with-nll.stderr

@@ -1,30 +1,12 @@
-error[E0626]: borrow may still be in use when generator yields (Ast)
-  --> $DIR/generator-with-nll.rs:19:23
+error[E0626]: borrow may still be in use when generator yields
+  --> $DIR/generator-with-nll.rs:18:17
    |
-LL |         let _a = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
-   |                       ^^^^
-...
-LL |         yield ();
-   |         -------- possible yield occurs here
-
-error[E0626]: borrow may still be in use when generator yields (Ast)
-  --> $DIR/generator-with-nll.rs:20:22
-   |
-LL |         let b = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
-   |                      ^^^^
-LL |         //~^ borrow may still be in use when generator yields (Mir)
-LL |         yield ();
-   |         -------- possible yield occurs here
-
-error[E0626]: borrow may still be in use when generator yields (Mir)
-  --> $DIR/generator-with-nll.rs:20:17
-   |
-LL |         let b = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
+LL |         let b = &mut true;
    |                 ^^^^^^^^^
-LL |         //~^ borrow may still be in use when generator yields (Mir)
+LL |         //~^ borrow may still be in use when generator yields
 LL |         yield ();
    |         -------- possible yield occurs here
 
-error: aborting due to 3 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0626`.