Emit a note that static bounds from HRTBs are a bug

compiler/rustc_borrowck/src/constraints/mod.rs

@@ -21,10 +21,7 @@ pub(crate) struct OutlivesConstraintSet<'tcx> {
 
 impl<'tcx> OutlivesConstraintSet<'tcx> {
     pub(crate) fn push(&mut self, constraint: OutlivesConstraint<'tcx>) {
-        debug!(
-            "OutlivesConstraintSet::push({:?}: {:?} @ {:?}",
-            constraint.sup, constraint.sub, constraint.locations
-        );
+        debug!("OutlivesConstraintSet::push({:?})", constraint);
         if constraint.sup == constraint.sub {
             // 'a: 'a is pretty uninteresting
             return;
@@ -73,7 +70,7 @@ impl<'tcx> Index<OutlivesConstraintIndex> for OutlivesConstraintSet<'tcx> {
     }
 }
 
-#[derive(Clone, PartialEq, Eq)]
+#[derive(Copy, Clone, PartialEq, Eq)]
 pub struct OutlivesConstraint<'tcx> {
     // NB. The ordering here is not significant for correctness, but
     // it is for convenience. Before we dump the constraints in the

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -15,7 +15,7 @@ use rustc_middle::ty::{self, RegionVid, TyCtxt};
 use rustc_span::symbol::{kw, Symbol};
 use rustc_span::{sym, DesugaringKind, Span};
 
-use crate::region_infer::BlameConstraint;
+use crate::region_infer::{BlameConstraint, ExtraConstraintInfo};
 use crate::{
     borrow_set::BorrowData, nll::ConstraintDescription, region_infer::Cause, MirBorrowckCtxt,
     WriteKind,
@@ -38,6 +38,7 @@ pub(crate) enum BorrowExplanation<'tcx> {
         span: Span,
         region_name: RegionName,
         opt_place_desc: Option<String>,
+        extra_info: Vec<ExtraConstraintInfo>,
     },
     Unexplained,
 }
@@ -243,6 +244,7 @@ impl<'tcx> BorrowExplanation<'tcx> {
                 ref region_name,
                 ref opt_place_desc,
                 from_closure: _,
+                ref extra_info,
             } => {
                 region_name.highlight_region_name(err);
 
@@ -268,6 +270,14 @@ impl<'tcx> BorrowExplanation<'tcx> {
                     );
                 };
 
+                for extra in extra_info {
+                    match extra {
+                        ExtraConstraintInfo::PlaceholderFromPredicate(span) => {
+                            err.span_note(*span, format!("due to current limitations in the borrow checker, this implies a `'static` lifetime"));
+                        }
+                    }
+                }
+
                 self.add_lifetime_bound_suggestion_to_diagnostic(err, &category, span, region_name);
             }
             _ => {}
@@ -309,18 +319,17 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         &self,
         borrow_region: RegionVid,
         outlived_region: RegionVid,
-    ) -> (ConstraintCategory<'tcx>, bool, Span, Option<RegionName>) {
-        let BlameConstraint { category, from_closure, cause, variance_info: _ } =
-            self.regioncx.best_blame_constraint(
-                &self.body,
-                borrow_region,
-                NllRegionVariableOrigin::FreeRegion,
-                |r| self.regioncx.provides_universal_region(r, borrow_region, outlived_region),
-            );
+    ) -> (ConstraintCategory<'tcx>, bool, Span, Option<RegionName>, Vec<ExtraConstraintInfo>) {
+        let (blame_constraint, extra_info) = self.regioncx.best_blame_constraint(
+            borrow_region,
+            NllRegionVariableOrigin::FreeRegion,
+            |r| self.regioncx.provides_universal_region(r, borrow_region, outlived_region),
+        );
+        let BlameConstraint { category, from_closure, cause, .. } = blame_constraint;
 
         let outlived_fr_name = self.give_region_a_name(outlived_region);
 
-        (category, from_closure, cause.span, outlived_fr_name)
+        (category, from_closure, cause.span, outlived_fr_name, extra_info)
     }
 
     /// Returns structured explanation for *why* the borrow contains the
@@ -392,7 +401,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
             None => {
                 if let Some(region) = self.to_error_region_vid(borrow_region_vid) {
-                    let (category, from_closure, span, region_name) =
+                    let (category, from_closure, span, region_name, extra_info) =
                         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());
@@ -402,6 +411,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             span,
                             region_name,
                             opt_place_desc,
+                            extra_info,
                         }
                     } else {
                         debug!("Could not generate a region name");

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -31,7 +31,7 @@ use crate::session_diagnostics::{
 };
 
 use super::{OutlivesSuggestionBuilder, RegionName};
-use crate::region_infer::BlameConstraint;
+use crate::region_infer::{BlameConstraint, ExtraConstraintInfo};
 use crate::{
     nll::ConstraintDescription,
     region_infer::{values::RegionElement, TypeTest},
@@ -234,7 +234,6 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
                     let (_, cause) = self.regioncx.find_outlives_blame_span(
-                        &self.body,
                         longer_fr,
                         NllRegionVariableOrigin::Placeholder(placeholder),
                         error_vid,
@@ -355,10 +354,11 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     ) {
         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 
-        let BlameConstraint { category, cause, variance_info, from_closure: _ } =
-            self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, |r| {
+        let (blame_constraint, extra_info) =
+            self.regioncx.best_blame_constraint(fr, fr_origin, |r| {
                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
             });
+        let BlameConstraint { category, cause, variance_info, .. } = blame_constraint;
 
         debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info);
 
@@ -467,6 +467,14 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             }
         }
 
+        for extra in extra_info {
+            match extra {
+                ExtraConstraintInfo::PlaceholderFromPredicate(span) => {
+                    diag.span_note(span, format!("due to current limitations in the borrow checker, this implies a `'static` lifetime"));
+                }
+            }
+        }
+
         self.buffer_error(diag);
     }
 
@@ -558,6 +566,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     /// LL |     ref_obj(x)
     ///    |     ^^^^^^^^^^ `x` escapes the function body here
     /// ```
+    #[instrument(level = "debug", skip(self))]
     fn report_escaping_data_error(
         &self,
         errci: &ErrorConstraintInfo<'tcx>,