Rollup merge of #64221 - Centril:nll-no-migrate-2015, r=matthewjasper

 Rust 2015: No longer downgrade NLL errors

As per decision on a language team meeting as described in #63565 (comment), in Rust 2015, we refuse to downgrade NLL errors, that AST borrowck accepts, into warnings and keep them as hard errors.

The remaining work to throw out AST borrowck and adjust some tests still remains after this PR.

Fixes #38899
Fixes #53432
Fixes #45157
Fixes #31567
Fixes #27868
Fixes #47366

r? @matthewjasper

src/librustc_mir/borrow_check/conflict_errors.rs

@@ -105,9 +105,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 format!("{} occurs due to use{}", desired_action.as_noun(), use_spans.describe()),
             );
 
-            // This error should not be downgraded to a warning,
-            // even in migrate mode.
-            self.disable_error_downgrading();
             err.buffer(&mut self.errors_buffer);
         } else {
             if let Some((reported_place, _)) = self.move_error_reported.get(&move_out_indices) {

src/librustc_mir/borrow_check/mod.rs

@@ -7,7 +7,6 @@ use rustc::hir::def_id::DefId;
 use rustc::infer::InferCtxt;
 use rustc::lint::builtin::UNUSED_MUT;
 use rustc::lint::builtin::{MUTABLE_BORROW_RESERVATION_CONFLICT};
-use rustc::middle::borrowck::SignalledError;
 use rustc::mir::{AggregateKind, BasicBlock, BorrowCheckResult, BorrowKind};
 use rustc::mir::{
     ClearCrossCrate, Local, Location, Body, Mutability, Operand, Place, PlaceBase, PlaceElem,
@@ -18,7 +17,7 @@ use rustc::mir::{Terminator, TerminatorKind};
 use rustc::ty::query::Providers;
 use rustc::ty::{self, TyCtxt};
 
-use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, Level};
+use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder};
 use rustc_data_structures::bit_set::BitSet;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::graph::dominators::Dominators;
@@ -259,10 +258,6 @@ fn do_mir_borrowck<'a, 'tcx>(
         move_error_reported: BTreeMap::new(),
         uninitialized_error_reported: Default::default(),
         errors_buffer,
-        // Only downgrade errors on Rust 2015 and refuse to do so on Rust 2018.
-        // FIXME(Centril): In Rust 1.40.0, refuse doing so on 2015 as well and
-        // proceed to throwing out the migration infrastructure.
-        disable_error_downgrading: body.span.rust_2018(),
         nonlexical_regioncx: regioncx,
         used_mut: Default::default(),
         used_mut_upvars: SmallVec::new(),
@@ -374,33 +369,6 @@ fn do_mir_borrowck<'a, 'tcx>(
     if !mbcx.errors_buffer.is_empty() {
         mbcx.errors_buffer.sort_by_key(|diag| diag.span.primary_span());
 
-        if !mbcx.disable_error_downgrading && tcx.migrate_borrowck() {
-            // When borrowck=migrate, check if AST-borrowck would
-            // error on the given code.
-
-            // rust-lang/rust#55492, rust-lang/rust#58776 check the base def id
-            // for errors. AST borrowck is responsible for aggregating
-            // `signalled_any_error` from all of the nested closures here.
-            let base_def_id = tcx.closure_base_def_id(def_id);
-
-            match tcx.borrowck(base_def_id).signalled_any_error {
-                SignalledError::NoErrorsSeen => {
-                    // if AST-borrowck signalled no errors, then
-                    // downgrade all the buffered MIR-borrowck errors
-                    // to warnings.
-
-                    for err in mbcx.errors_buffer.iter_mut() {
-                        downgrade_if_error(err);
-                    }
-                }
-                SignalledError::SawSomeError => {
-                    // if AST-borrowck signalled a (cancelled) error,
-                    // then we will just emit the buffered
-                    // MIR-borrowck errors as normal.
-                }
-            }
-        }
-
         for diag in mbcx.errors_buffer.drain(..) {
             mbcx.infcx.tcx.sess.diagnostic().emit_diagnostic(&diag);
         }
@@ -416,21 +384,6 @@ fn do_mir_borrowck<'a, 'tcx>(
     result
 }
 
-fn downgrade_if_error(diag: &mut Diagnostic) {
-    if diag.is_error() {
-        diag.level = Level::Warning;
-        diag.warn(
-            "this error has been downgraded to a warning for backwards \
-            compatibility with previous releases",
-        ).warn(
-            "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`"
-        );
-    }
-}
-
 crate struct MirBorrowckCtxt<'cx, 'tcx> {
     crate infcx: &'cx InferCtxt<'cx, 'tcx>,
     body: &'cx Body<'tcx>,
@@ -491,9 +444,6 @@ crate struct MirBorrowckCtxt<'cx, 'tcx> {
     uninitialized_error_reported: FxHashSet<PlaceRef<'cx, 'tcx>>,
     /// Errors to be reported buffer
     errors_buffer: Vec<Diagnostic>,
-    /// If there are no errors reported by the HIR borrow checker, we downgrade
-    /// all NLL errors to warnings. Setting this flag disables downgrading.
-    disable_error_downgrading: bool,
     /// This field keeps track of all the local variables that are declared mut and are mutated.
     /// Used for the warning issued by an unused mutable local variable.
     used_mut: FxHashSet<Local>,
@@ -934,12 +884,6 @@ impl InitializationRequiringAction {
 }
 
 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
-    /// If there are no errors reported by the HIR borrow checker, we downgrade
-    /// all NLL errors to warnings. Calling this disables downgrading.
-    crate fn disable_error_downgrading(&mut self)  {
-        self.disable_error_downgrading = true;
-    }
-
     /// Checks an access to the given place to see if it is allowed. Examines the set of borrows
     /// that are in scope, as well as which paths have been initialized, to ensure that (a) the
     /// place is initialized and (b) it is not borrowed in some way that would prevent this

src/test/ui/borrowck/borrowck-anon-fields-variant.rs

@@ -15,9 +15,7 @@ fn distinct_variant() {
     // reference.
     let b = match y {
       Foo::Y(_, ref mut b) => b,
-      //~^ WARNING cannot use `y`
-      //~| WARNING this error has been downgraded to a warning
-      //~| WARNING this warning will become a hard error in the future
+      //~^ ERROR cannot use `y`
       Foo::X => panic!()
     };
 

src/test/ui/borrowck/borrowck-anon-fields-variant.stderr

@@ -1,4 +1,4 @@
-warning[E0503]: cannot use `y` because it was mutably borrowed
+error[E0503]: cannot use `y` because it was mutably borrowed
   --> $DIR/borrowck-anon-fields-variant.rs:17:7
    |
 LL |       Foo::Y(ref mut a, _) => a,
@@ -9,13 +9,9 @@ LL |       Foo::Y(_, ref mut b) => b,
 ...
 LL |     *a += 1;
    |     ------- borrow later used here
-   |
-   = 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[E0503]: cannot use `y` because it was mutably borrowed
-  --> $DIR/borrowck-anon-fields-variant.rs:37:7
+  --> $DIR/borrowck-anon-fields-variant.rs:35:7
    |
 LL |       Foo::Y(ref mut a, _) => a,
    |              --------- borrow of `y.0` occurs here
@@ -27,7 +23,7 @@ LL |     *a += 1;
    |     ------- borrow later used here
 
 error[E0499]: cannot borrow `y.0` as mutable more than once at a time
-  --> $DIR/borrowck-anon-fields-variant.rs:37:14
+  --> $DIR/borrowck-anon-fields-variant.rs:35:14
    |
 LL |       Foo::Y(ref mut a, _) => a,
    |              --------- first mutable borrow occurs here
@@ -38,7 +34,7 @@ LL |       Foo::Y(ref mut b, _) => b,
 LL |     *a += 1;
    |     ------- first borrow later used here
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0499, E0503.
 For more information about an error, try `rustc --explain E0499`.