Auto merge of #67476 - mark-i-m:simplify-borrow_check-5, r=matthewjasper

Region naming refactoring [6/N]

Followup to #67474

EDIT: this PR is probably best read commit-by-commit...

The major changes in this PR include:
- moving many functions around to modules that better suit them. In particular, a lot of methods were moved from `borrow_check::diagnostics::region_errors` to `borrow_check::region_infer`, and `report_region_errors` was moved from `borrow_check` to `borrow_check::diagnostics::region_errors`.
- `borrow_check::diagnostics::{region_errors, region_name}` are now most comprised of methods on `MirBorrowckCtxt` instead of `RegionInferenceContext`, allowing us to get rid of the annoying `pub(in crate::borrow_check)` on most of the fields of the latter, along with a number of method arguments on many methods.
- I renamed `MirBorrowckCtxt.nonlexical_regioncx` to just `regioncx` because their is no lexical lifetimes any more, and the old name was annoyingly verbose, causing many lines to wrap unnecessarily.
- I got rid of `ErrorRegionNamingContext`. Region naming is implemented as inherent methods on `MirBorrowckCtxt`, so we just move the naming stuff into that struct.

The PR is rather large, but the commits are fairly self-contained (though they don't all compile). There was one minor output change to one test with `compare-mode=nll`, which I think is acceptable.

Between this PR and the last one, a net of 200 lines are removed, most of which was function parameters and context structs :tada:

Some samples:

```diff
-                        self.nonlexical_regioncx.free_region_constraint_info(
-                            &self.body,
-                            &self.local_names,
-                            &self.upvars,
-                            self.mir_def_id,
-                            self.infcx,
-                            borrow_region_vid,
-                            region,
-                        );
+                        self.free_region_constraint_info(borrow_region_vid, region);
```

```diff
-            .or_else(|| {
-                self.give_name_if_anonymous_region_appears_in_yield_ty(
-                    infcx,
-                    body,
-                    *mir_def_id,
-                    fr,
-                    renctx,
-                )
-            });
+            .or_else(|| self.give_name_if_anonymous_region_appears_in_arguments(fr))
```

r? @matthewjasper

cc @eddyb

src/librustc_mir/borrow_check/diagnostics/explain_borrow.rs

@@ -2,12 +2,13 @@
 
 use std::collections::VecDeque;
 
+use rustc::infer::NLLRegionVariableOrigin;
 use rustc::mir::{
     Body, CastKind, ConstraintCategory, FakeReadCause, Local, Location, Operand, Place, Rvalue,
     Statement, StatementKind, TerminatorKind,
 };
 use rustc::ty::adjustment::PointerCast;
-use rustc::ty::{self, TyCtxt};
+use rustc::ty::{self, RegionVid, TyCtxt};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_index::vec::IndexVec;
@@ -254,6 +255,23 @@ impl BorrowExplanation {
 }
 
 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
+    fn free_region_constraint_info(
+        &self,
+        borrow_region: RegionVid,
+        outlived_region: RegionVid,
+    ) -> (ConstraintCategory, bool, Span, Option<RegionName>) {
+        let (category, from_closure, span) = self.regioncx.best_blame_constraint(
+            &self.body,
+            borrow_region,
+            NLLRegionVariableOrigin::FreeRegion,
+            |r| self.regioncx.provides_universal_region(r, borrow_region, outlived_region),
+        );
+
+        let outlived_fr_name = self.give_region_a_name(outlived_region);
+
+        (category, from_closure, span, outlived_fr_name)
+    }
+
     /// Returns structured explanation for *why* the borrow contains the
     /// point from `location`. This is key for the "3-point errors"
     /// [described in the NLL RFC][d].
@@ -278,14 +296,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             location, borrow, kind_place
         );
 
-        let regioncx = &self.nonlexical_regioncx;
+        let regioncx = &self.regioncx;
         let body: &Body<'_> = &self.body;
         let tcx = self.infcx.tcx;
 
         let borrow_region_vid = borrow.region;
         debug!("explain_why_borrow_contains_point: borrow_region_vid={:?}", borrow_region_vid);
 
-        let region_sub = regioncx.find_sub_region_live_at(borrow_region_vid, location);
+        let region_sub = self.regioncx.find_sub_region_live_at(borrow_region_vid, location);
         debug!("explain_why_borrow_contains_point: region_sub={:?}", region_sub);
 
         match find_use::find(body, regioncx, tcx, region_sub, location) {
@@ -329,10 +347,9 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             }
 
             None => {
-                if let Some(region) = regioncx.to_error_region_vid(borrow_region_vid) {
-                    let (category, from_closure, span, region_name) = self
-                        .nonlexical_regioncx
-                        .free_region_constraint_info(self, borrow_region_vid, region);
+                if let Some(region) = self.to_error_region_vid(borrow_region_vid) {
+                    let (category, from_closure, span, region_name) =
+                        self.free_region_constraint_info(borrow_region_vid, region);
                     if let Some(region_name) = region_name {
                         let opt_place_desc = self.describe_place(borrow.borrowed_place.as_ref());
                         BorrowExplanation::MustBeValidFor {
@@ -345,14 +362,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     } else {
                         debug!(
                             "explain_why_borrow_contains_point: \
-                                Could not generate a region name"
+                             Could not generate a region name"
                         );
                         BorrowExplanation::Unexplained
                     }
                 } else {
                     debug!(
                         "explain_why_borrow_contains_point: \
-                            Could not generate an error region vid"
+                         Could not generate an error region vid"
                     );
                     BorrowExplanation::Unexplained
                 }

src/librustc_mir/borrow_check/diagnostics/mod.rs

@@ -32,7 +32,7 @@ mod region_errors;
 crate use mutability_errors::AccessKind;
 crate use outlives_suggestion::OutlivesSuggestionBuilder;
 crate use region_errors::{ErrorConstraintInfo, RegionErrorKind, RegionErrors};
-crate use region_name::{RegionErrorNamingCtx, RegionName, RegionNameSource};
+crate use region_name::{RegionName, RegionNameSource};
 
 pub(super) struct IncludingDowncast(pub(super) bool);
 

src/librustc_mir/borrow_check/diagnostics/outlives_suggestion.rs

@@ -12,7 +12,7 @@ use smallvec::SmallVec;
 
 use crate::borrow_check::MirBorrowckCtxt;
 
-use super::{ErrorConstraintInfo, RegionErrorNamingCtx, RegionName, RegionNameSource};
+use super::{ErrorConstraintInfo, RegionName, RegionNameSource};
 
 /// The different things we could suggest.
 enum SuggestedConstraint {
@@ -77,19 +77,15 @@ impl OutlivesSuggestionBuilder {
     fn region_vid_to_name(
         &self,
         mbcx: &MirBorrowckCtxt<'_, '_>,
-        renctx: &mut RegionErrorNamingCtx,
         region: RegionVid,
     ) -> Option<RegionName> {
-        mbcx.nonlexical_regioncx
-            .give_region_a_name(mbcx, renctx, region)
-            .filter(Self::region_name_is_suggestable)
+        mbcx.give_region_a_name(region).filter(Self::region_name_is_suggestable)
     }
 
     /// Compiles a list of all suggestions to be printed in the final big suggestion.
     fn compile_all_suggestions(
         &self,
         mbcx: &MirBorrowckCtxt<'_, '_>,
-        renctx: &mut RegionErrorNamingCtx,
     ) -> SmallVec<[SuggestedConstraint; 2]> {
         let mut suggested = SmallVec::new();
 
@@ -98,7 +94,7 @@ impl OutlivesSuggestionBuilder {
         let mut unified_already = FxHashSet::default();
 
         for (fr, outlived) in &self.constraints_to_add {
-            let fr_name = if let Some(fr_name) = self.region_vid_to_name(mbcx, renctx, *fr) {
+            let fr_name = if let Some(fr_name) = self.region_vid_to_name(mbcx, *fr) {
                 fr_name
             } else {
                 continue;
@@ -107,9 +103,7 @@ impl OutlivesSuggestionBuilder {
             let outlived = outlived
                 .iter()
                 // if there is a `None`, we will just omit that constraint
-                .filter_map(|fr| {
-                    self.region_vid_to_name(mbcx, renctx, *fr).map(|rname| (fr, rname))
-                })
+                .filter_map(|fr| self.region_vid_to_name(mbcx, *fr).map(|rname| (fr, rname)))
                 .collect::<Vec<_>>();
 
             // No suggestable outlived lifetimes.
@@ -173,12 +167,11 @@ impl OutlivesSuggestionBuilder {
         &mut self,
         mbcx: &MirBorrowckCtxt<'_, '_>,
         errci: &ErrorConstraintInfo,
-        renctx: &mut RegionErrorNamingCtx,
         diag: &mut DiagnosticBuilder<'_>,
     ) {
         // Emit an intermediate note.
-        let fr_name = self.region_vid_to_name(mbcx, renctx, errci.fr);
-        let outlived_fr_name = self.region_vid_to_name(mbcx, renctx, errci.outlived_fr);
+        let fr_name = self.region_vid_to_name(mbcx, errci.fr);
+        let outlived_fr_name = self.region_vid_to_name(mbcx, errci.outlived_fr);
 
         if let (Some(fr_name), Some(outlived_fr_name)) = (fr_name, outlived_fr_name) {
             if let RegionNameSource::Static = outlived_fr_name.source {
@@ -194,11 +187,7 @@ impl OutlivesSuggestionBuilder {
 
     /// If there is a suggestion to emit, add a diagnostic to the buffer. This is the final
     /// suggestion including all collected constraints.
-    crate fn add_suggestion(
-        &self,
-        mbcx: &mut MirBorrowckCtxt<'_, '_>,
-        renctx: &mut RegionErrorNamingCtx,
-    ) {
+    crate fn add_suggestion(&self, mbcx: &mut MirBorrowckCtxt<'_, '_>) {
         // No constraints to add? Done.
         if self.constraints_to_add.is_empty() {
             debug!("No constraints to suggest.");
@@ -215,7 +204,7 @@ impl OutlivesSuggestionBuilder {
         }
 
         // Get all suggestable constraints.
-        let suggested = self.compile_all_suggestions(mbcx, renctx);
+        let suggested = self.compile_all_suggestions(mbcx);
 
         // If there are no suggestable constraints...
         if suggested.is_empty() {