Auto merge of #109753 - compiler-errors:replenish-region-constraints, r=aliemjay

Clone region var origins instead of taking them in borrowck

Fixes an issue with the new solver where reporting a borrow-checker error ICEs because it calls `InferCtxt::evaluate_obligation`.

This also removes a handful of unnecessary `tcx.infer_ctxt().build()` calls that are only there to mitigate this same exact issue, but with the old solver.

Fixes compiler-errors/next-solver-hir-issues#12.

----

This implements `@aliemjay's` solution where we just don't *take* the region constraints, but clone them. This potentially makes it easier to write a bug about taking region constraints twice or never at all, but again, not many folks are touching this code.

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -10,7 +10,6 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::intravisit::{walk_block, walk_expr, Visitor};
 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, LangItem};
-use rustc_infer::infer::TyCtxtInferExt;
 use rustc_infer::traits::ObligationCause;
 use rustc_middle::hir::nested_filter::OnlyBodies;
 use rustc_middle::mir::tcx::PlaceTy;
@@ -643,11 +642,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             let Some(default_trait) = tcx.get_diagnostic_item(sym::Default) else {
                 return false;
             };
-            // Regions are already solved, so we must use a fresh InferCtxt,
+            self.infcx
-            // but the type has region variables, so erase those.
+                .type_implements_trait(default_trait, [ty], param_env)
-            tcx.infer_ctxt()
-                .build()
-                .type_implements_trait(default_trait, [tcx.erase_regions(ty)], param_env)
                 .must_apply_modulo_regions()
         };
 
@@ -740,13 +736,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     fn suggest_cloning(&self, err: &mut Diagnostic, ty: Ty<'tcx>, span: Span) {
         let tcx = self.infcx.tcx;
         // Try to find predicates on *generic params* that would allow copying `ty`
-        let infcx = tcx.infer_ctxt().build();
-
         if let Some(clone_trait_def) = tcx.lang_items().clone_trait()
-            && infcx
+            && self.infcx
                 .type_implements_trait(
                     clone_trait_def,
-                    [tcx.erase_regions(ty)],
+                    [ty],
                     self.param_env,
                 )
                 .must_apply_modulo_regions()
@@ -770,12 +764,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             .and_then(|def_id| tcx.hir().get_generics(def_id))
         else { return; };
         // Try to find predicates on *generic params* that would allow copying `ty`
-        let infcx = tcx.infer_ctxt().build();
+        let ocx = ObligationCtxt::new(&self.infcx);
-        let ocx = ObligationCtxt::new(&infcx);
         let copy_did = tcx.require_lang_item(LangItem::Copy, Some(span));
         let cause = ObligationCause::misc(span, self.mir_def_id());
 
-        ocx.register_bound(cause, self.param_env, infcx.tcx.erase_regions(ty), copy_did);
+        ocx.register_bound(cause, self.param_env, ty, copy_did);
         let errors = ocx.select_all_or_error();
 
         // Only emit suggestion if all required predicates are on generic
@@ -2219,7 +2212,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             let tcx = self.infcx.tcx;
 
             let return_ty = self.regioncx.universal_regions().unnormalized_output_ty;
-            let return_ty = tcx.erase_regions(return_ty);
 
             // to avoid panics
             if let Some(iter_trait) = tcx.get_diagnostic_item(sym::Iterator)

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -10,7 +10,7 @@ use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, Namespace};
 use rustc_hir::GeneratorKind;
 use rustc_index::vec::IndexSlice;
-use rustc_infer::infer::{LateBoundRegionConversionTime, TyCtxtInferExt};
+use rustc_infer::infer::LateBoundRegionConversionTime;
 use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::{
     AggregateKind, Constant, FakeReadCause, Local, LocalInfo, LocalKind, Location, Operand, Place,
@@ -1042,15 +1042,12 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     if let Some((CallDesugaringKind::ForLoopIntoIter, _)) = desugaring {
                         let ty = moved_place.ty(self.body, tcx).ty;
                         let suggest = match tcx.get_diagnostic_item(sym::IntoIterator) {
-                            Some(def_id) => {
+                            Some(def_id) => type_known_to_meet_bound_modulo_regions(
-                                let infcx = self.infcx.tcx.infer_ctxt().build();
+                                &self.infcx,
-                                type_known_to_meet_bound_modulo_regions(
+                                self.param_env,
-                                    &infcx,
+                                tcx.mk_imm_ref(tcx.lifetimes.re_erased, ty),
-                                    self.param_env,
+                                def_id,
-                                    tcx.mk_imm_ref(tcx.lifetimes.re_erased, tcx.erase_regions(ty)),
+                            ),
-                                    def_id,
-                                )
-                            }
                             _ => false,
                         };
                         if suggest {
@@ -1094,20 +1091,18 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             is_partial,
                             is_loop_message,
                         });
-                        let infcx = tcx.infer_ctxt().build();
                         // Erase and shadow everything that could be passed to the new infcx.
-                        let ty = tcx.erase_regions(moved_place.ty(self.body, tcx).ty);
+                        let ty = moved_place.ty(self.body, tcx).ty;
-                        let method_substs = tcx.erase_regions(method_substs);
 
                         if let ty::Adt(def, substs) = ty.kind()
                             && Some(def.did()) == tcx.lang_items().pin_type()
                             && let ty::Ref(_, _, hir::Mutability::Mut) = substs.type_at(0).kind()
-                            && let self_ty = infcx.instantiate_binder_with_fresh_vars(
+                            && let self_ty = self.infcx.instantiate_binder_with_fresh_vars(
                                 fn_call_span,
                                 LateBoundRegionConversionTime::FnCall,
                                 tcx.fn_sig(method_did).subst(tcx, method_substs).input(0),
                             )
-                            && infcx.can_eq(self.param_env, ty, self_ty)
+                            && self.infcx.can_eq(self.param_env, ty, self_ty)
                         {
                             err.eager_subdiagnostic(
                                 &self.infcx.tcx.sess.parse_sess.span_diagnostic,
@@ -1123,7 +1118,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                                 self.param_env,
                                 ty::Binder::dummy(trait_ref),
                             )
-                            && infcx.predicate_must_hold_modulo_regions(&o)
+                            && self.infcx.predicate_must_hold_modulo_regions(&o)
                         {
                             err.span_suggestion_verbose(
                                 fn_call_span.shrink_to_lo(),

compiler/rustc_borrowck/src/nll.rs

@@ -235,7 +235,7 @@ pub(crate) fn compute_regions<'cx, 'tcx>(
     // Create the region inference context, taking ownership of the
     // region inference data that was contained in `infcx`, and the
     // base constraints generated by the type-check.
-    let var_origins = infcx.take_region_var_origins();
+    let var_origins = infcx.get_region_var_origins();
     let MirTypeckRegionConstraints {
         placeholder_indices,
         placeholder_index_to_region: _,

compiler/rustc_infer/src/infer/mod.rs

@@ -1228,11 +1228,11 @@ impl<'tcx> InferCtxt<'tcx> {
     /// hence that `resolve_regions_and_report_errors` can never be
     /// called. This is used only during NLL processing to "hand off" ownership
     /// of the set of region variables into the NLL region context.
-    pub fn take_region_var_origins(&self) -> VarInfos {
+    pub fn get_region_var_origins(&self) -> VarInfos {
         let mut inner = self.inner.borrow_mut();
         let (var_infos, data) = inner
             .region_constraint_storage
-            .take()
+            .clone()
             .expect("regions already resolved")
             .with_log(&mut inner.undo_log)
             .into_infos_and_data();

tests/ui/traits/new-solver/borrowck-error.rs

@@ -0,0 +1,11 @@
+// compile-flags: -Ztrait-solver=next
+
+use std::collections::HashMap;
+
+fn foo() -> &'static HashMap<i32, i32>
+{
+    &HashMap::new()
+    //~^ ERROR cannot return reference to temporary value
+}
+
+fn main() {}

tests/ui/traits/new-solver/borrowck-error.stderr

@@ -0,0 +1,12 @@
+error[E0515]: cannot return reference to temporary value
+  --> $DIR/borrowck-error.rs:7:5
+   |
+LL |     &HashMap::new()
+   |     ^--------------
+   |     ||
+   |     |temporary value created here
+   |     returns a reference to data owned by the current function
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0515`.