Auto merge of #59114 - matthewjasper:enable-migate-2015, r=pnkfelix

Enable NLL migrate mode on the 2015 edition

## What is in this PR?

* Remove the `-Zborrowck=ast` flag option from rustc.
* The default in the 2015 edition is now `-Zborrowck=migrate`.
* The 2018 edition default is unchanged: it's still `-Zborrowck=migrate`.
* Enable two-phase borrows (currently toggled via the `-Ztwo-phase-borrows` flag) on all editions.
* Remove most dead code that handled these options.
* Update tests for the above changes.

## What is *not* in this PR?

These are left for future PRs

* Use `-Zborrowck=mir` in NLL compare mode tests (#56993)
* Remove the `-Zborrowck=compare` option (#59193)
* Remove the `-Ztwo-phase-borrows` flag. It's kept, as a flag that does nothing so that perf.rlo has time to stop using it (cc @Mark-Simulacrum)
* Remove MIR typeck as its own MIR pass - it's now run by NLL.
* Enabling `-Zborrowck=mir` by default (#58781)
* Replace `allow_bind_by_move_patterns_with_guards` and `check_for_mutation_in_guard_via_ast_walk` with just using the feature gate. (#59192)

Soundness issues that are fixed by NLL will stay open until full NLL is emitting hard errors. However, these diagnostics and completeness issues can now be closed:

Closes #18330
Closes #22323
Closes #23591
Closes #26736
Closes #27487
Closes #28092
Closes #28970
Closes #29733
Closes #30104
Closes #38915
Closes #39908
Closes #43407
Closes #47524
Closes #48540
Closes #49073
Closes #52614
Closes #55085
Closes #56093
Closes #56496
Closes #57804

cc #43234

r? @pnkfelix
cc @rust-lang/lang
cc @rust-lang/wg-compiler-nll

src/librustc/infer/mod.rs

@@ -92,8 +92,8 @@ impl SuppressRegionErrors {
     /// enabled.
     pub fn when_nll_is_enabled(tcx: TyCtxt<'_, '_, '_>) -> Self {
         match tcx.borrowck_mode() {
-            // If we're on AST or Migrate mode, report AST region errors
-            BorrowckMode::Ast | BorrowckMode::Migrate => SuppressRegionErrors { suppressed: false },
+            // If we're on Migrate mode, report AST region errors
+            BorrowckMode::Migrate => SuppressRegionErrors { suppressed: false },
 
             // If we're on MIR or Compare mode, don't report AST region errors as they should
             // be reported by NLL

src/librustc/session/config.rs

@@ -460,7 +460,6 @@ pub enum PrintRequest {
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum BorrowckMode {
-    Ast,
     Mir,
     Compare,
     Migrate,
@@ -471,7 +470,6 @@ impl BorrowckMode {
     /// 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,
@@ -481,21 +479,11 @@ impl BorrowckMode {
     /// 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?
-    pub fn use_mir(self) -> bool {
-        match self {
-            BorrowckMode::Ast => false,
-            BorrowckMode::Compare => true,
-            BorrowckMode::Mir => true,
-            BorrowckMode::Migrate => true,
-        }
-    }
 }
 
 pub enum Input {
@@ -627,7 +615,7 @@ impl Default for Options {
             incremental: None,
             debugging_opts: basic_debugging_options(),
             prints: Vec::new(),
-            borrowck_mode: BorrowckMode::Ast,
+            borrowck_mode: BorrowckMode::Migrate,
             cg: basic_codegen_options(),
             error_format: ErrorOutputType::default(),
             externs: Externs(BTreeMap::new()),
@@ -2326,10 +2314,9 @@ pub fn build_session_options_and_crate_config(
     }));
 
     let borrowck_mode = match debugging_opts.borrowck.as_ref().map(|s| &s[..]) {
-        None | Some("ast") => BorrowckMode::Ast,
+        None | Some("migrate") => BorrowckMode::Migrate,
         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

@@ -70,7 +70,6 @@ use rustc_macros::HashStable;
 use syntax::ast;
 use syntax::attr;
 use syntax::source_map::MultiSpan;
-use syntax::edition::Edition;
 use syntax::feature_gate;
 use syntax::symbol::{Symbol, keywords, InternedString};
 use syntax_pos::Span;
@@ -1496,21 +1495,13 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     /// 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()
+        self.features().bind_by_move_pattern_guards
     }
 
     /// 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 {
-        // 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;
+        !self.allow_bind_by_move_patterns_with_guards()
     }
 
     /// If true, we should use the AST-based borrowck (we may *also* use
@@ -1519,12 +1510,6 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.borrowck_mode().use_ast()
     }
 
-    /// If true, we should use the MIR-based borrowck (we may *also* use
-    /// the AST-based borrowck).
-    pub fn use_mir_borrowck(self) -> bool {
-        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 {
@@ -1541,38 +1526,18 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     /// statements (which simulate the maximal effect of executing the
     /// patterns in a match arm).
     pub fn emit_read_for_match(&self) -> bool {
-        self.use_mir_borrowck() && !self.sess.opts.debugging_opts.nll_dont_emit_read_for_match
-    }
-
-    /// If true, pattern variables for use in guards on match arms
-    /// will be bound as references to the data, and occurrences of
-    /// those variables in the guard expression will implicitly
-    /// dereference those bindings. (See rust-lang/rust#27282.)
-    pub fn all_pat_vars_are_implicit_refs_within_guards(self) -> bool {
-        self.borrowck_mode().use_mir()
-    }
-
-    /// If true, we should enable two-phase borrows checks. This is
-    /// done with either: `-Ztwo-phase-borrows`, `#![feature(nll)]`,
-    /// or by opting into an edition after 2015.
-    pub fn two_phase_borrows(self) -> bool {
-        self.sess.rust_2018() || self.features().nll ||
-        self.sess.opts.debugging_opts.two_phase_borrows
+        !self.sess.opts.debugging_opts.nll_dont_emit_read_for_match
     }
 
     /// What mode(s) of borrowck should we run? AST? MIR? both?
     /// (Also considers the `#![feature(nll)]` setting.)
     pub fn borrowck_mode(&self) -> BorrowckMode {
         // Here are the main constraints we need to deal with:
         //
-        // 1. An opts.borrowck_mode of `BorrowckMode::Ast` is
+        // 1. An opts.borrowck_mode of `BorrowckMode::Migrate` 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
+        // 2. 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).
@@ -1585,24 +1550,13 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         //   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, if no `-Z borrowck=...` then use migrate mode
         //
         // * Otherwise, use the behavior requested via `-Z borrowck=...`
 
         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,
-            },
-        }
+        self.sess.opts.borrowck_mode
     }
 
     #[inline]

src/librustc_borrowck/borrowck/mod.rs

@@ -49,8 +49,6 @@ pub mod gather_loans;
 
 pub mod move_data;
 
-mod unused;
-
 #[derive(Clone, Copy)]
 pub struct LoanDataFlowOperator;
 
@@ -138,10 +136,6 @@ fn borrowck<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, owner_def_id: DefId)
         check_loans::check_loans(&mut bccx, &loan_dfcx, &flowed_moves, &all_loans, body);
     }
 
-    if !tcx.use_mir_borrowck() {
-        unused::check(&mut bccx, body);
-    }
-
     Lrc::new(BorrowCheckResult {
         used_mut_nodes: bccx.used_mut_nodes.into_inner(),
         signalled_any_error: bccx.signalled_any_error.into_inner(),

src/librustc_mir/borrow_check/borrow_set.rs

@@ -303,9 +303,8 @@ impl<'a, 'gcx, 'tcx> GatherBorrows<'a, 'gcx, 'tcx> {
     /// allowed to be split into separate Reservation and
     /// Activation phases.
     fn allow_two_phase_borrow(&self, kind: mir::BorrowKind) -> bool {
-        self.tcx.two_phase_borrows()
-            && (kind.allows_two_phase_borrow()
-                || self.tcx.sess.opts.debugging_opts.two_phase_beyond_autoref)
+        kind.allows_two_phase_borrow()
+            || self.tcx.sess.opts.debugging_opts.two_phase_beyond_autoref
     }
 
     /// If this is a two-phase borrow, then we will record it