Auto merge of #63059 - Centril:sound-bind-by-move, r=matthewjasper

Make `#![feature(bind_by_move_pattern_guards)]` sound without `#[feature(nll)]`

Implements #15287 (comment) making `#![feature(bind_by_move_pattern_guards)]]` sound without also having `#![feature(nll)]`. The logic here is that if we see a `match` guard, we will refuse to downgrade NLL errors to warnings. This is in preparation for hopefully stabilizing the former feature in #63118.

As fall out from the implementation we also:
Fixes #31287
Fixes #27282

r? @matthewjasper

src/librustc/query/mod.rs

@@ -461,7 +461,7 @@ rustc_queries! {
     }
 
     TypeChecking {
-        query check_match(key: DefId) -> () {
+        query check_match(key: DefId) -> SignalledError {
             cache_on_disk_if { key.is_local() }
         }
 

src/librustc/ty/query/mod.rs

@@ -4,7 +4,7 @@ use crate::hir::def::{DefKind, Export};
 use crate::hir::{self, TraitCandidate, ItemLocalId, CodegenFnAttrs};
 use crate::infer::canonical::{self, Canonical};
 use crate::lint;
-use crate::middle::borrowck::BorrowCheckResult;
+use crate::middle::borrowck::{BorrowCheckResult, SignalledError};
 use crate::middle::cstore::{ExternCrate, LinkagePreference, NativeLibrary, ForeignModule};
 use crate::middle::cstore::{NativeLibraryKind, DepKind, CrateSource};
 use crate::middle::privacy::AccessLevels;

src/librustc_ast_borrowck/borrowck/mod.rs

@@ -66,6 +66,13 @@ fn borrowck(tcx: TyCtxt<'_>, owner_def_id: DefId) -> &BorrowCheckResult {
 
     debug!("borrowck(body_owner_def_id={:?})", owner_def_id);
 
+    let signalled_error = tcx.check_match(owner_def_id);
+    if let SignalledError::SawSomeError = signalled_error {
+        return tcx.arena.alloc(BorrowCheckResult {
+            signalled_any_error: SignalledError::SawSomeError,
+        })
+    }
+
     let owner_id = tcx.hir().as_local_hir_id(owner_def_id).unwrap();
 
     match tcx.hir().get(owner_id) {

src/librustc_mir/error_codes.rs

@@ -1989,7 +1989,7 @@ When matching on a variable it cannot be mutated in the match guards, as this
 could cause the match to be non-exhaustive:
 
 ```compile_fail,E0510
-#![feature(nll, bind_by_move_pattern_guards)]
+#![feature(bind_by_move_pattern_guards)]
 let mut x = Some(0);
 match x {
     None => (),

src/librustc_mir/hair/pattern/check_match.rs

@@ -4,6 +4,7 @@ use super::_match::WitnessPreference::*;
 
 use super::{Pattern, PatternContext, PatternError, PatternKind};
 
+use rustc::middle::borrowck::SignalledError;
 use rustc::middle::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor};
 use rustc::middle::expr_use_visitor::{LoanCause, MutateMode};
 use rustc::middle::expr_use_visitor as euv;
@@ -26,21 +27,24 @@ use std::slice;
 
 use syntax_pos::{Span, DUMMY_SP, MultiSpan};
 
-pub(crate) fn check_match(tcx: TyCtxt<'_>, def_id: DefId) {
+crate fn check_match(tcx: TyCtxt<'_>, def_id: DefId) -> SignalledError {
     let body_id = if let Some(id) = tcx.hir().as_local_hir_id(def_id) {
         tcx.hir().body_owned_by(id)
     } else {
-        return;
+        return SignalledError::NoErrorsSeen;
     };
 
-    MatchVisitor {
+    let mut visitor = MatchVisitor {
         tcx,
         body_owner: def_id,
         tables: tcx.body_tables(body_id),
         region_scope_tree: &tcx.region_scope_tree(def_id),
         param_env: tcx.param_env(def_id),
         identity_substs: InternalSubsts::identity_for_item(tcx, def_id),
-    }.visit_body(tcx.hir().body(body_id));
+        signalled_error: SignalledError::NoErrorsSeen,
+    };
+    visitor.visit_body(tcx.hir().body(body_id));
+    visitor.signalled_error
 }
 
 fn create_e0004(sess: &Session, sp: Span, error_message: String) -> DiagnosticBuilder<'_> {
@@ -54,6 +58,7 @@ struct MatchVisitor<'a, 'tcx> {
     param_env: ty::ParamEnv<'tcx>,
     identity_substs: SubstsRef<'tcx>,
     region_scope_tree: &'a region::ScopeTree,
+    signalled_error: SignalledError,
 }
 
 impl<'a, 'tcx> Visitor<'tcx> for MatchVisitor<'a, 'tcx> {
@@ -64,11 +69,8 @@ impl<'a, 'tcx> Visitor<'tcx> for MatchVisitor<'a, 'tcx> {
     fn visit_expr(&mut self, ex: &'tcx hir::Expr) {
         intravisit::walk_expr(self, ex);
 
-        match ex.node {
-            hir::ExprKind::Match(ref scrut, ref arms, source) => {
-                self.check_match(scrut, arms, source);
-            }
-            _ => {}
+        if let hir::ExprKind::Match(ref scrut, ref arms, source) = ex.node {
+            self.check_match(scrut, arms, source);
         }
     }
 
@@ -130,26 +132,27 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
 }
 
 impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
-    fn check_patterns(&self, has_guard: bool, pats: &[P<Pat>]) {
+    fn check_patterns(&mut self, has_guard: bool, pats: &[P<Pat>]) {
         check_legality_of_move_bindings(self, has_guard, pats);
         for pat in pats {
             check_legality_of_bindings_in_at_patterns(self, pat);
         }
     }
 
     fn check_match(
-        &self,
+        &mut self,
         scrut: &hir::Expr,
         arms: &'tcx [hir::Arm],
-        source: hir::MatchSource)
-    {
+        source: hir::MatchSource
+    ) {
         for arm in arms {
             // First, check legality of move bindings.
             self.check_patterns(arm.guard.is_some(), &arm.pats);
 
             // Second, if there is a guard on each arm, make sure it isn't
             // assigning or borrowing anything mutably.
             if let Some(ref guard) = arm.guard {
+                self.signalled_error = SignalledError::SawSomeError;
                 if !self.tcx.features().bind_by_move_pattern_guards {
                     check_for_mutation_in_guard(self, &guard);
                 }
@@ -548,7 +551,7 @@ fn maybe_point_at_variant(
 
 // Legality of move bindings checking
 fn check_legality_of_move_bindings(
-    cx: &MatchVisitor<'_, '_>,
+    cx: &mut MatchVisitor<'_, '_>,
     has_guard: bool,
     pats: &[P<Pat>],
 ) {
@@ -565,7 +568,12 @@ fn check_legality_of_move_bindings(
         })
     }
     let span_vec = &mut Vec::new();
-    let check_move = |p: &Pat, sub: Option<&Pat>, span_vec: &mut Vec<Span>| {
+    let check_move = |
+        cx: &mut MatchVisitor<'_, '_>,
+        p: &Pat,
+        sub: Option<&Pat>,
+        span_vec: &mut Vec<Span>,
+    | {
         // check legality of moving out of the enum
 
         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
@@ -574,15 +582,17 @@ fn check_legality_of_move_bindings(
                              "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 && !cx.tcx.features().bind_by_move_pattern_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() {
-                err.help("add `#![feature(bind_by_move_pattern_guards)]` to the \
-                          crate attributes to enable");
+        } else if has_guard {
+            if !cx.tcx.features().bind_by_move_pattern_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() {
+                    err.help("add `#![feature(bind_by_move_pattern_guards)]` to the \
+                            crate attributes to enable");
+                }
+                err.emit();
             }
-            err.emit();
         } else if let Some(_by_ref_span) = by_ref_span {
             span_vec.push(p.span);
         }
@@ -596,7 +606,7 @@ fn check_legality_of_move_bindings(
                         ty::BindByValue(..) => {
                             let pat_ty = cx.tables.node_type(p.hir_id);
                             if !pat_ty.is_copy_modulo_regions(cx.tcx, cx.param_env, pat.span) {
-                                check_move(p, sub.as_ref().map(|p| &**p), span_vec);
+                                check_move(cx, p, sub.as_ref().map(|p| &**p), span_vec);
                             }
                         }
                         _ => {}

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

@@ -1,14 +1,14 @@
-warning[E0507]: cannot move out of `foo` in pattern guard
-  --> $DIR/borrowck-migrate-to-nll.rs:26:18
+warning[E0502]: cannot borrow `*block.current` as immutable because it is also borrowed as mutable
+  --> $DIR/borrowck-migrate-to-nll.rs:28:21
    |
-LL |                 (|| { let bar = foo; bar.take() })();
-   |                  ^^             ---
-   |                  |              |
-   |                  |              move occurs because `foo` has type `&mut std::option::Option<&i32>`, which does not implement the `Copy` trait
-   |                  |              move occurs due to use in closure
-   |                  move out of `foo` occurs here
+LL |     let x = &mut block;
+   |             ---------- mutable borrow occurs here
+LL |     let p: &'a u8 = &*block.current;
+   |                     ^^^^^^^^^^^^^^^ immutable borrow occurs here
+LL |     // (use `x` and `p` so enabling NLL doesn't assign overly short lifetimes)
+LL |     drop(x);
+   |          - mutable borrow later used here
    |
-   = note: variables bound in patterns cannot be moved from until after the end of the pattern guard
    = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
    = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
    = note: for more information, try `rustc --explain E0729`

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

@@ -1,4 +1,4 @@
-// This is a test of the borrowck migrate mode. It leverages #27282, a
+// This is a test of the borrowck migrate mode. It leverages #38899, a
 // bug that is fixed by NLL: this code is (unsoundly) accepted by
 // AST-borrowck, but is correctly rejected by the NLL borrowck.
 //
@@ -18,15 +18,17 @@
 //[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."),
-    }
+pub struct Block<'a> {
+    current: &'a u8,
+    unrelated: &'a u8,
 }
+
+fn bump<'a>(mut block: &mut Block<'a>) {
+    let x = &mut block;
+    let p: &'a u8 = &*block.current;
+    // (use `x` and `p` so enabling NLL doesn't assign overly short lifetimes)
+    drop(x);
+    drop(p);
+}
+
+fn main() {}

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

@@ -1,14 +1,14 @@
-warning[E0507]: cannot move out of `foo` in pattern guard
-  --> $DIR/borrowck-migrate-to-nll.rs:26:18
+warning[E0502]: cannot borrow `*block.current` as immutable because it is also borrowed as mutable
+  --> $DIR/borrowck-migrate-to-nll.rs:28:21
    |
-LL |                 (|| { let bar = foo; bar.take() })();
-   |                  ^^             ---
-   |                  |              |
-   |                  |              move occurs because `foo` has type `&mut std::option::Option<&i32>`, which does not implement the `Copy` trait
-   |                  |              move occurs due to use in closure
-   |                  move out of `foo` occurs here
+LL |     let x = &mut block;
+   |             ---------- mutable borrow occurs here
+LL |     let p: &'a u8 = &*block.current;
+   |                     ^^^^^^^^^^^^^^^ immutable borrow occurs here
+LL |     // (use `x` and `p` so enabling NLL doesn't assign overly short lifetimes)
+LL |     drop(x);
+   |          - mutable borrow later used here
    |
-   = note: variables bound in patterns cannot be moved from until after the end of the pattern guard
    = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
    = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
    = note: for more information, try `rustc --explain E0729`

src/test/ui/borrowck/borrowck-mutate-in-guard.rs

@@ -9,15 +9,11 @@ fn foo() -> isize {
     match x {
         Enum::A(_) if { x = Enum::B(false); false } => 1,
         //~^ ERROR cannot assign in a pattern guard
-        //~| WARN cannot assign `x` in match guard
-        //~| WARN this error has been downgraded to a warning for backwards compatibility
-        //~| WARN this represents potential undefined behavior in your code and this warning will
+        //~| ERROR cannot assign `x` in match guard
         Enum::A(_) if { let y = &mut x; *y = Enum::B(false); false } => 1,
         //~^ ERROR cannot mutably borrow in a pattern guard
         //~| ERROR cannot assign in a pattern guard
-        //~| WARN cannot mutably borrow `x` in match guard
-        //~| WARN this error has been downgraded to a warning for backwards compatibility
-        //~| WARN this represents potential undefined behavior in your code and this warning will
+        //~| ERROR cannot mutably borrow `x` in match guard
         Enum::A(p) => *p,
         Enum::B(_) => 2,
     }

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

@@ -5,45 +5,37 @@ 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:15:38
+  --> $DIR/borrowck-mutate-in-guard.rs:13:38
    |
 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:15:41
+  --> $DIR/borrowck-mutate-in-guard.rs:13:41
    |
 LL |         Enum::A(_) if { let y = &mut x; *y = Enum::B(false); false } => 1,
    |                                         ^^^^^^^^^^^^^^^^^^^ assignment in pattern guard
 
-warning[E0510]: cannot assign `x` in match guard
+error[E0510]: cannot assign `x` in match guard
   --> $DIR/borrowck-mutate-in-guard.rs:10:25
    |
 LL |     match x {
    |           - value is immutable in match guard
 LL |         Enum::A(_) if { x = Enum::B(false); false } => 1,
    |                         ^^^^^^^^^^^^^^^^^^ cannot assign
-   |
-   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
-   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
-   = note: for more information, try `rustc --explain E0729`
 
-warning[E0510]: cannot mutably borrow `x` in match guard
-  --> $DIR/borrowck-mutate-in-guard.rs:15:33
+error[E0510]: cannot mutably borrow `x` in match guard
+  --> $DIR/borrowck-mutate-in-guard.rs:13:33
    |
 LL |     match x {
    |           - value is immutable in match guard
 ...
 LL |         Enum::A(_) if { let y = &mut x; *y = Enum::B(false); false } => 1,
    |                                 ^^^^^^ cannot mutably borrow
-   |
-   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
-   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
-   = note: for more information, try `rustc --explain E0729`
 
-error: aborting due to 3 previous errors
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0301, E0302, E0510.
 For more information about an error, try `rustc --explain E0301`.

src/test/ui/borrowck/issue-27282-mutation-in-guard.rs

@@ -0,0 +1,13 @@
+fn main() {
+    match Some(&4) {
+        None => {},
+        ref mut foo
+            if {
+                (|| { let bar = foo; bar.take() })();
+                //~^ ERROR cannot move out of `foo` in pattern guard
+                false
+            } => {},
+        Some(ref _s) => println!("Note this arm is bogus; the `Some` became `None` in the guard."),
+        _ => println!("Here is some supposedly unreachable code."),
+    }
+}