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/librustc/middle/borrowck.rs

@@ -15,9 +15,15 @@ use util::nodemap::FxHashSet;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher,
                                            StableHasherResult};
 
+#[derive(Copy, Clone, Debug, RustcEncodable, RustcDecodable)]
+pub enum SignalledError { SawSomeError, NoErrorsSeen }
+
+impl_stable_hash_for!(enum self::SignalledError { SawSomeError, NoErrorsSeen });
+
 #[derive(Debug, RustcEncodable, RustcDecodable)]
 pub struct BorrowCheckResult {
     pub used_mut_nodes: FxHashSet<HirId>,
+    pub signalled_any_error: SignalledError,
 }
 
 impl<'a> HashStable<StableHashingContext<'a>> for BorrowCheckResult {
@@ -26,7 +32,9 @@ impl<'a> HashStable<StableHashingContext<'a>> for BorrowCheckResult {
                                           hasher: &mut StableHasher<W>) {
         let BorrowCheckResult {
             ref used_mut_nodes,
+            ref signalled_any_error,
         } = *self;
         used_mut_nodes.hash_stable(hcx, hasher);
+        signalled_any_error.hash_stable(hcx, hasher);
     }
 }

src/librustc/session/config.rs

@@ -455,15 +455,28 @@ pub enum BorrowckMode {
     Ast,
     Mir,
     Compare,
+    Migrate,
 }
 
 impl BorrowckMode {
+    /// Should we run the MIR-based borrow check, but also fall back
+    /// on the AST borrow check if the MIR-based one errors.
+    pub fn migrate(self) -> bool {
+        match self {
+            BorrowckMode::Ast => false,
+            BorrowckMode::Compare => false,
+            BorrowckMode::Mir => false,
+            BorrowckMode::Migrate => true,
+        }
+    }
+
     /// Should we emit the AST-based borrow checker errors?
     pub fn use_ast(self) -> bool {
         match self {
             BorrowckMode::Ast => true,
             BorrowckMode::Compare => true,
             BorrowckMode::Mir => false,
+            BorrowckMode::Migrate => false,
         }
     }
     /// Should we emit the MIR-based borrow checker errors?
@@ -472,6 +485,7 @@ impl BorrowckMode {
             BorrowckMode::Ast => false,
             BorrowckMode::Compare => true,
             BorrowckMode::Mir => true,
+            BorrowckMode::Migrate => true,
         }
     }
 }
@@ -1127,7 +1141,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
     emit_end_regions: bool = (false, parse_bool, [UNTRACKED],
         "emit EndRegion as part of MIR; enable transforms that solely process EndRegion"),
     borrowck: Option<String> = (None, parse_opt_string, [UNTRACKED],
-        "select which borrowck is used (`ast`, `mir`, or `compare`)"),
+        "select which borrowck is used (`ast`, `mir`, `migrate`, or `compare`)"),
     two_phase_borrows: bool = (false, parse_bool, [UNTRACKED],
         "use two-phase reserved/active distinction for `&mut` borrows in MIR borrowck"),
     two_phase_beyond_autoref: bool = (false, parse_bool, [UNTRACKED],
@@ -2168,6 +2182,7 @@ pub fn build_session_options_and_crate_config(
         None | Some("ast") => BorrowckMode::Ast,
         Some("mir") => BorrowckMode::Mir,
         Some("compare") => BorrowckMode::Compare,
+        Some("migrate") => BorrowckMode::Migrate,
         Some(m) => early_error(error_format, &format!("unknown borrowck mode `{}`", m)),
     };
 

src/librustc/ty/context.rs

@@ -74,6 +74,7 @@ use rustc_target::spec::abi;
 use syntax::ast::{self, NodeId};
 use syntax::attr;
 use syntax::codemap::MultiSpan;
+use syntax::edition::Edition;
 use syntax::feature_gate;
 use syntax::symbol::{Symbol, keywords, InternedString};
 use syntax_pos::Span;
@@ -1366,6 +1367,12 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.borrowck_mode().use_mir()
     }
 
+    /// If true, we should use the MIR-based borrow check, but also
+    /// fall back on the AST borrow check if the MIR-based one errors.
+    pub fn migrate_borrowck(self) -> bool {
+        self.borrowck_mode().migrate()
+    }
+
     /// If true, make MIR codegen for `match` emit a temp that holds a
     /// borrow of the input to the match expression.
     pub fn generate_borrow_of_any_match_input(&self) -> bool {
@@ -1397,18 +1404,51 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     /// What mode(s) of borrowck should we run? AST? MIR? both?
     /// (Also considers the `#![feature(nll)]` setting.)
     pub fn borrowck_mode(&self) -> BorrowckMode {
-        match self.sess.opts.borrowck_mode {
-            mode @ BorrowckMode::Mir |
-            mode @ BorrowckMode::Compare => mode,
+        // Here are the main constraints we need to deal with:
+        //
+        // 1. An opts.borrowck_mode of `BorrowckMode::Ast` is
+        //    synonymous with no `-Z borrowck=...` flag at all.
+        //    (This is arguably a historical accident.)
+        //
+        // 2. `BorrowckMode::Migrate` is the limited migration to
+        //    NLL that we are deploying with the 2018 edition.
+        //
+        // 3. We want to allow developers on the Nightly channel
+        //    to opt back into the "hard error" mode for NLL,
+        //    (which they can do via specifying `#![feature(nll)]`
+        //    explicitly in their crate).
+        //
+        // So, this precedence list is how pnkfelix chose to work with
+        // the above constraints:
+        //
+        // * `#![feature(nll)]` *always* means use NLL with hard
+        //   errors. (To simplify the code here, it now even overrides
+        //   a user's attempt to specify `-Z borrowck=compare`, which
+        //   we arguably do not need anymore and should remove.)
+        //
+        // * Otherwise, if no `-Z borrowck=...` flag was given (or
+        //   if `borrowck=ast` was specified), then use the default
+        //   as required by the edition.
+        //
+        // * Otherwise, use the behavior requested via `-Z borrowck=...`
 
-            mode @ BorrowckMode::Ast => {
-                if self.features().nll {
-                    BorrowckMode::Mir
-                } else {
-                    mode
-                }
-            }
+        if self.features().nll { return BorrowckMode::Mir; }
 
+        match self.sess.opts.borrowck_mode {
+            mode @ BorrowckMode::Mir |
+            mode @ BorrowckMode::Compare |
+            mode @ BorrowckMode::Migrate => mode,
+
+            BorrowckMode::Ast => match self.sess.edition() {
+                Edition::Edition2015 => BorrowckMode::Ast,
+                Edition::Edition2018 => BorrowckMode::Migrate,
+
+                // For now, future editions mean Migrate. (But it
+                // would make a lot of sense for it to be changed to
+                // `BorrowckMode::Mir`, depending on how we plan to
+                // time the forcing of full migration to NLL.)
+                _ => BorrowckMode::Migrate,
+            },
         }
     }
 

src/librustc_borrowck/borrowck/check_loans.rs

@@ -447,10 +447,12 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                                       .region_scope_tree
                                       .yield_in_scope_for_expr(scope,
                                                                cmt.hir_id,
-                                                               self.bccx.body) {
+                                                               self.bccx.body)
+        {
             self.bccx.cannot_borrow_across_generator_yield(borrow_span,
                                                            yield_span,
                                                            Origin::Ast).emit();
+            self.bccx.signal_error();
         }
     }
 
@@ -507,9 +509,13 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
             new_loan, old_loan, old_loan, new_loan).err();
 
         match (err_old_new, err_new_old) {
-            (Some(mut err), None) | (None, Some(mut err)) => err.emit(),
+            (Some(mut err), None) | (None, Some(mut err)) => {
+                err.emit();
+                self.bccx.signal_error();
+            }
             (Some(mut err_old), Some(mut err_new)) => {
                 err_old.emit();
+                self.bccx.signal_error();
                 err_new.cancel();
             }
             (None, None) => return true,
@@ -695,6 +701,7 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                         loan_span, &self.bccx.loan_path_to_string(&loan_path),
                         Origin::Ast)
                     .emit();
+                self.bccx.signal_error();
             }
         }
     }
@@ -745,6 +752,7 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                 };
 
                 err.emit();
+                self.bccx.signal_error();
             }
         }
     }
@@ -914,5 +922,6 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
         self.bccx.cannot_assign_to_borrowed(
             span, loan.span, &self.bccx.loan_path_to_string(loan_path), Origin::Ast)
             .emit();
+        self.bccx.signal_error();
     }
 }

src/librustc_borrowck/borrowck/gather_loans/move_error.rs

@@ -99,6 +99,7 @@ fn report_move_errors<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>, errors: &Vec<Move
                            "captured outer variable");
         }
         err.emit();
+        bccx.signal_error();
     }
 }
 

src/librustc_borrowck/borrowck/mod.rs

@@ -28,7 +28,7 @@ use rustc::middle::dataflow::DataFlowContext;
 use rustc::middle::dataflow::BitwiseOperator;
 use rustc::middle::dataflow::DataFlowOperator;
 use rustc::middle::dataflow::KillFrom;
-use rustc::middle::borrowck::BorrowCheckResult;
+use rustc::middle::borrowck::{BorrowCheckResult, SignalledError};
 use rustc::hir::def_id::{DefId, LocalDefId};
 use rustc::middle::expr_use_visitor as euv;
 use rustc::middle::mem_categorization as mc;
@@ -42,7 +42,7 @@ use rustc_mir::util::borrowck_errors::{BorrowckErrors, Origin};
 use rustc_mir::util::suggest_ref_mut;
 use rustc::util::nodemap::FxHashSet;
 
-use std::cell::RefCell;
+use std::cell::{Cell, RefCell};
 use std::fmt;
 use std::rc::Rc;
 use rustc_data_structures::sync::Lrc;
@@ -90,7 +90,7 @@ pub struct AnalysisData<'a, 'tcx: 'a> {
 fn borrowck<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, owner_def_id: DefId)
     -> Lrc<BorrowCheckResult>
 {
-    assert!(tcx.use_ast_borrowck());
+    assert!(tcx.use_ast_borrowck() || tcx.migrate_borrowck());
 
     debug!("borrowck(body_owner_def_id={:?})", owner_def_id);
 
@@ -105,6 +105,7 @@ fn borrowck<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, owner_def_id: DefId)
             // and do not need borrowchecking.
             return Lrc::new(BorrowCheckResult {
                 used_mut_nodes: FxHashSet(),
+                signalled_any_error: SignalledError::NoErrorsSeen,
             })
         }
         _ => { }
@@ -121,6 +122,7 @@ fn borrowck<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, owner_def_id: DefId)
         owner_def_id,
         body,
         used_mut_nodes: RefCell::new(FxHashSet()),
+        signalled_any_error: Cell::new(SignalledError::NoErrorsSeen),
     };
 
     // Eventually, borrowck will always read the MIR, but at the
@@ -154,6 +156,7 @@ fn borrowck<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, owner_def_id: DefId)
 
     Lrc::new(BorrowCheckResult {
         used_mut_nodes: bccx.used_mut_nodes.into_inner(),
+        signalled_any_error: bccx.signalled_any_error.into_inner(),
     })
 }
 
@@ -234,6 +237,7 @@ pub fn build_borrowck_dataflow_data_for_fn<'a, 'tcx>(
         owner_def_id,
         body,
         used_mut_nodes: RefCell::new(FxHashSet()),
+        signalled_any_error: Cell::new(SignalledError::NoErrorsSeen),
     };
 
     let dataflow_data = build_borrowck_dataflow_data(&mut bccx, true, body_id, |_| cfg);
@@ -257,6 +261,15 @@ pub struct BorrowckCtxt<'a, 'tcx: 'a> {
     body: &'tcx hir::Body,
 
     used_mut_nodes: RefCell<FxHashSet<HirId>>,
+
+    signalled_any_error: Cell<SignalledError>,
+}
+
+
+impl<'a, 'tcx: 'a> BorrowckCtxt<'a, 'tcx> {
+    fn signal_error(&self) {
+        self.signalled_any_error.set(SignalledError::SawSomeError);
+    }
 }
 
 impl<'a, 'b, 'tcx: 'b> BorrowckErrors<'a> for &'a BorrowckCtxt<'b, 'tcx> {
@@ -645,6 +658,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     .span_label(use_span, format!("use of possibly uninitialized `{}`",
                                                   self.loan_path_to_string(lp)))
                     .emit();
+                self.signal_error();
                 return;
             }
             _ => {
@@ -760,6 +774,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
         // not considered particularly helpful.
 
         err.emit();
+        self.signal_error();
     }
 
     pub fn report_partial_reinitialization_of_uninitialized_structure(
@@ -770,6 +785,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                                                       &self.loan_path_to_string(lp),
                                                       Origin::Ast)
             .emit();
+        self.signal_error();
     }
 
     pub fn report_reassigned_immutable_variable(&self,
@@ -787,6 +803,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                                                 self.loan_path_to_string(lp)));
         }
         err.emit();
+        self.signal_error();
     }
 
     pub fn struct_span_err_with_code<S: Into<MultiSpan>>(&self,
@@ -908,6 +925,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     self.tcx.hir.hir_to_node_id(err.cmt.hir_id)
                 );
                 db.emit();
+                self.signal_error();
             }
             err_out_of_scope(super_scope, sub_scope, cause) => {
                 let msg = match opt_loan_path(&err.cmt) {
@@ -1022,6 +1040,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                 }
 
                 db.emit();
+                self.signal_error();
             }
             err_borrowed_pointer_too_short(loan_scope, ptr_scope) => {
                 let descr = self.cmt_to_path_or_string(err.cmt);
@@ -1047,6 +1066,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     "");
 
                 db.emit();
+                self.signal_error();
             }
         }
     }
@@ -1125,6 +1145,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
             err.help("closures behind references must be called via `&mut`");
         }
         err.emit();
+        self.signal_error();
     }
 
     /// Given a type, if it is an immutable reference, return a suggestion to make it mutable
@@ -1307,6 +1328,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                                        cmt_path_or_string),
                              suggestion)
             .emit();
+        self.signal_error();
     }
 
     fn region_end_span(&self, region: ty::Region<'tcx>) -> Option<Span> {

src/librustc_errors/diagnostic.rs

@@ -99,6 +99,25 @@ impl Diagnostic {
         }
     }
 
+    pub fn is_error(&self) -> bool {
+        match self.level {
+            Level::Bug |
+            Level::Fatal |
+            Level::PhaseFatal |
+            Level::Error |
+            Level::FailureNote => {
+                true
+            }
+
+            Level::Warning |
+            Level::Note |
+            Level::Help |
+            Level::Cancelled => {
+                false
+            }
+        }
+    }
+
     /// Cancel the diagnostic (a structured diagnostic must either be emitted or
     /// canceled or it will panic when dropped).
     pub fn cancel(&mut self) {