Auto merge of rust-lang#54034 - pnkfelix:issue-15287-bind-by-move-pattern-guards, r=nikomatsakis

Add feature to enable bind by move pattern guards

Implement rust-lang#15287 as described on rust-lang#15287 (comment)

Author: bors

src/librustc/ty/context.rs

@@ -1448,10 +1448,37 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.queries.on_disk_cache.serialize(self.global_tcx(), encoder)
     }
 
+    /// This checks whether one is allowed to have pattern bindings
+    /// that bind-by-move on a match arm that has a guard, e.g.:
+    ///
+    /// ```rust
+    /// match foo { A(inner) if { /* something */ } => ..., ... }
+    /// ```
+    ///
+    /// It is separate from check_for_mutation_in_guard_via_ast_walk,
+    /// because that method has a narrower effect that can be toggled
+    /// off via a separate `-Z` flag, at least for the short term.
+    pub fn allow_bind_by_move_patterns_with_guards(self) -> bool {
+        self.features().bind_by_move_pattern_guards && self.use_mir_borrowck()
+    }
+
     /// If true, we should use a naive AST walk to determine if match
     /// guard could perform bad mutations (or mutable-borrows).
     pub fn check_for_mutation_in_guard_via_ast_walk(self) -> bool {
-        !self.sess.opts.debugging_opts.disable_ast_check_for_mutation_in_guard
+        // If someone passes the `-Z` flag, they're asking for the footgun.
+        if self.sess.opts.debugging_opts.disable_ast_check_for_mutation_in_guard {
+            return false;
+        }
+
+        // If someone requests the feature, then be a little more
+        // careful and ensure that MIR-borrowck is enabled (which can
+        // happen via edition selection, via `feature(nll)`, or via an
+        // appropriate `-Z` flag) before disabling the mutation check.
+        if self.allow_bind_by_move_patterns_with_guards() {
+            return false;
+        }
+
+        return true;
     }
 
     /// If true, we should use the AST-based borrowck (we may *also* use

src/librustc_mir/hair/pattern/check_match.rs

@@ -537,11 +537,15 @@ fn check_legality_of_move_bindings(cx: &MatchVisitor,
                              "cannot bind by-move with sub-bindings")
                 .span_label(p.span, "binds an already bound by-move value by moving it")
                 .emit();
-        } else if has_guard {
-            struct_span_err!(cx.tcx.sess, p.span, E0008,
-                      "cannot bind by-move into a pattern guard")
-                .span_label(p.span, "moves value into pattern guard")
-                .emit();
+        } else if has_guard && !cx.tcx.allow_bind_by_move_patterns_with_guards() {
+            let mut err = struct_span_err!(cx.tcx.sess, p.span, E0008,
+                                       "cannot bind by-move into a pattern guard");
+            err.span_label(p.span, "moves value into pattern guard");
+            if cx.tcx.sess.opts.unstable_features.is_nightly_build() && cx.tcx.use_mir_borrowck() {
+                err.help("add #![feature(bind_by_move_pattern_guards)] to the \
+                          crate attributes to enable");
+            }
+            err.emit();
         } else if let Some(by_ref_span) = by_ref_span {
             struct_span_err!(
                 cx.tcx.sess,
@@ -613,10 +617,16 @@ impl<'a, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'tcx> {
               _: LoanCause) {
         match kind {
             ty::MutBorrow => {
-                struct_span_err!(self.cx.tcx.sess, span, E0301,
-                          "cannot mutably borrow in a pattern guard")
-                    .span_label(span, "borrowed mutably in pattern guard")
-                    .emit();
+                let mut err = struct_span_err!(self.cx.tcx.sess, span, E0301,
+                          "cannot mutably borrow in a pattern guard");
+                err.span_label(span, "borrowed mutably in pattern guard");
+                if self.cx.tcx.sess.opts.unstable_features.is_nightly_build() &&
+                    self.cx.tcx.use_mir_borrowck()
+                {
+                    err.help("add #![feature(bind_by_move_pattern_guards)] to the \
+                              crate attributes to enable");
+                }
+                err.emit();
             }
             ty::ImmBorrow | ty::UniqueImmBorrow => {}
         }

src/libsyntax/feature_gate.rs

@@ -515,6 +515,12 @@ declare_features! (
 
     // Self struct constructor  (RFC 2302)
     (active, self_struct_ctor, "1.31.0", Some(51994), None),
+
+    // allow mixing of bind-by-move in patterns and references to
+    // those identifiers in guards, *if* we are using MIR-borrowck
+    // (aka NLL). Essentially this means you need to be on
+    // edition:2018 or later.
+    (active, bind_by_move_pattern_guards, "1.30.0", Some(15287), None),
 );
 
 declare_features! (

src/test/ui/bind-by-move/bind-by-move-no-guards.nll.stderr

@@ -0,0 +1,11 @@
+error[E0008]: cannot bind by-move into a pattern guard
+  --> $DIR/bind-by-move-no-guards.rs:8:14
+   |
+LL |         Some(z) if z.recv().unwrap() => { panic!() },
+   |              ^ moves value into pattern guard
+   |
+   = help: add #![feature(bind_by_move_pattern_guards)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0008`.

src/test/ui/bind-by-move/bind-by-move-no-guards.rs

@@ -1,13 +1,3 @@
-// Copyright 2012 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.
-
 use std::sync::mpsc::channel;
 
 fn main() {

src/test/ui/bind-by-move/bind-by-move-no-guards.stderr

@@ -1,5 +1,5 @@
 error[E0008]: cannot bind by-move into a pattern guard
-  --> $DIR/bind-by-move-no-guards.rs:18:14
+  --> $DIR/bind-by-move-no-guards.rs:8:14
    |
 LL |         Some(z) if z.recv().unwrap() => { panic!() },
    |              ^ moves value into pattern guard

src/test/ui/borrowck/borrowck-mutate-in-guard.nll.stderr

@@ -0,0 +1,24 @@
+error[E0302]: cannot assign in a pattern guard
+  --> $DIR/borrowck-mutate-in-guard.rs:20:25
+   |
+LL |         Enum::A(_) if { x = Enum::B(false); false } => 1,
+   |                         ^^^^^^^^^^^^^^^^^^ assignment in pattern guard
+
+error[E0301]: cannot mutably borrow in a pattern guard
+  --> $DIR/borrowck-mutate-in-guard.rs:22:38
+   |
+LL |         Enum::A(_) if { let y = &mut x; *y = Enum::B(false); false } => 1,
+   |                                      ^ borrowed mutably in pattern guard
+   |
+   = help: add #![feature(bind_by_move_pattern_guards)] to the crate attributes to enable
+
+error[E0302]: cannot assign in a pattern guard
+  --> $DIR/borrowck-mutate-in-guard.rs:22:41
+   |
+LL |         Enum::A(_) if { let y = &mut x; *y = Enum::B(false); false } => 1,
+   |                                         ^^^^^^^^^^^^^^^^^^^ assignment in pattern guard
+
+error: aborting due to 3 previous errors
+
+Some errors occurred: E0301, E0302.
+For more information about an error, try `rustc --explain E0301`.

src/test/ui/error-codes/E0008.nll.stderr

@@ -0,0 +1,11 @@
+error[E0008]: cannot bind by-move into a pattern guard
+  --> $DIR/E0008.rs:13:14
+   |
+LL |         Some(s) if s.len() == 0 => {},
+   |              ^ moves value into pattern guard
+   |
+   = help: add #![feature(bind_by_move_pattern_guards)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0008`.

src/test/ui/error-codes/E0301.nll.stderr

@@ -0,0 +1,11 @@
+error[E0301]: cannot mutably borrow in a pattern guard
+  --> $DIR/E0301.rs:14:19
+   |
+LL |         option if option.take().is_none() => {}, //~ ERROR E0301
+   |                   ^^^^^^ borrowed mutably in pattern guard
+   |
+   = help: add #![feature(bind_by_move_pattern_guards)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0301`.

src/test/ui/rfc-0107-bind-by-move-pattern-guards/bind-by-move-no-guards.rs

@@ -0,0 +1,21 @@
+// Adaptation of existing ui test (from way back in
+// rust-lang/rust#2329), that starts passing with this feature in
+// place.
+
+// compile-pass
+
+#![feature(nll)]
+#![feature(bind_by_move_pattern_guards)]
+
+use std::sync::mpsc::channel;
+
+fn main() {
+    let (tx, rx) = channel();
+    let x = Some(rx);
+    tx.send(false);
+    match x {
+        Some(z) if z.recv().unwrap() => { panic!() },
+        Some(z) => { assert!(!z.recv().unwrap()); },
+        None => panic!()
+    }
+}