Auto merge of #113188 - matthiaskrgr:rollup-j3abaks, r=matthiaskrgr

Rollup of 6 pull requests

Successful merges:

 - #107624 (Stabilize `const_cstr_methods`)
 - #111403 (suggest `slice::swap` for `mem::swap(&mut x[0], &mut x[1])` borrowck error)
 - #113071 (Account for late-bound vars from parent arg-position impl trait)
 - #113165 (Encode item bounds for `DefKind::ImplTraitPlaceholder`)
 - #113171 (Properly implement variances_of for RPITIT GAT)
 - #113177 (Use structured suggestion when telling user about `for<'a>`)

r? `@ghost`
`@rustbot` modify labels: rollup

3 files changed, 89 insertions, 15 deletions

diff --git a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
index c8b405228a5..97a3e01c52a 100644
--- a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
+++ b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
@@ -1344,12 +1344,10 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                 Scope::Binder {
                     where_bound_origin: Some(hir::PredicateOrigin::ImplTrait), ..
                 } => {
-                    let mut err = self.tcx.sess.struct_span_err(
-                        lifetime_ref.ident.span,
-                        "`impl Trait` can only mention lifetimes bound at the fn or impl level",
-                    );
-                    err.span_note(self.tcx.def_span(region_def_id), "lifetime declared here");
-                    err.emit();
+                    self.tcx.sess.emit_err(errors::LateBoundInApit::Lifetime {
+                        span: lifetime_ref.ident.span,
+                        param_span: self.tcx.def_span(region_def_id),
+                    });
                     return;
                 }
                 Scope::Root { .. } => break,
@@ -1379,6 +1377,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
         let mut late_depth = 0;
         let mut scope = self.scope;
         let mut crossed_anon_const = false;
+
         let result = loop {
             match *scope {
                 Scope::Body { s, .. } => {
@@ -1446,6 +1445,50 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
             return;
         }
 
+        // We may fail to resolve higher-ranked ty/const vars that are mentioned by APIT.
+        // AST-based resolution does not care for impl-trait desugaring, which are the
+        // responsibility of lowering. This may create a mismatch between the resolution
+        // AST found (`param_def_id`) which points to HRTB, and what HIR allows.
+        // ```
+        // fn foo(x: impl for<T> Trait<Assoc = impl Trait2<T>>) {}
+        // ```
+        //
+        // In such case, walk back the binders to diagnose it properly.
+        let mut scope = self.scope;
+        loop {
+            match *scope {
+                Scope::Binder {
+                    where_bound_origin: Some(hir::PredicateOrigin::ImplTrait), ..
+                } => {
+                    let guar = self.tcx.sess.emit_err(match self.tcx.def_kind(param_def_id) {
+                        DefKind::TyParam => errors::LateBoundInApit::Type {
+                            span: self.tcx.hir().span(hir_id),
+                            param_span: self.tcx.def_span(param_def_id),
+                        },
+                        DefKind::ConstParam => errors::LateBoundInApit::Const {
+                            span: self.tcx.hir().span(hir_id),
+                            param_span: self.tcx.def_span(param_def_id),
+                        },
+                        kind => {
+                            bug!("unexpected def-kind: {}", kind.descr(param_def_id.to_def_id()))
+                        }
+                    });
+                    self.map.defs.insert(hir_id, ResolvedArg::Error(guar));
+                    return;
+                }
+                Scope::Root { .. } => break,
+                Scope::Binder { s, .. }
+                | Scope::Body { s, .. }
+                | Scope::Elision { s, .. }
+                | Scope::ObjectLifetimeDefault { s, .. }
+                | Scope::Supertrait { s, .. }
+                | Scope::TraitRefBoundary { s, .. }
+                | Scope::AnonConstBoundary { s } => {
+                    scope = s;
+                }
+            }
+        }
+
         self.tcx.sess.delay_span_bug(
             self.tcx.hir().span(hir_id),
             format!("could not resolve {param_def_id:?}"),
diff --git a/compiler/rustc_hir_analysis/src/errors.rs b/compiler/rustc_hir_analysis/src/errors.rs
index 7dce1272f96..cb840592edd 100644
--- a/compiler/rustc_hir_analysis/src/errors.rs
+++ b/compiler/rustc_hir_analysis/src/errors.rs
@@ -875,3 +875,28 @@ pub(crate) enum ReturnTypeNotationIllegalParam {
         param_span: Span,
     },
 }
+
+#[derive(Diagnostic)]
+pub(crate) enum LateBoundInApit {
+    #[diag(hir_analysis_late_bound_type_in_apit)]
+    Type {
+        #[primary_span]
+        span: Span,
+        #[label]
+        param_span: Span,
+    },
+    #[diag(hir_analysis_late_bound_const_in_apit)]
+    Const {
+        #[primary_span]
+        span: Span,
+        #[label]
+        param_span: Span,
+    },
+    #[diag(hir_analysis_late_bound_lifetime_in_apit)]
+    Lifetime {
+        #[primary_span]
+        span: Span,
+        #[label]
+        param_span: Span,
+    },
+}
diff --git a/compiler/rustc_hir_analysis/src/variance/mod.rs b/compiler/rustc_hir_analysis/src/variance/mod.rs
index 23d8da88a45..49aee6b59a2 100644
--- a/compiler/rustc_hir_analysis/src/variance/mod.rs
+++ b/compiler/rustc_hir_analysis/src/variance/mod.rs
@@ -7,7 +7,7 @@ use rustc_arena::DroplessArena;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
-use rustc_middle::ty::{self, CrateVariancesMap, SubstsRef, Ty, TyCtxt};
+use rustc_middle::ty::{self, CrateVariancesMap, ImplTraitInTraitData, SubstsRef, Ty, TyCtxt};
 use rustc_middle::ty::{TypeSuperVisitable, TypeVisitable};
 use std::ops::ControlFlow;
 
@@ -51,20 +51,26 @@ fn variances_of(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Variance] {
         | DefKind::Struct
         | DefKind::Union
         | DefKind::Variant
-        | DefKind::Ctor(..) => {}
+        | DefKind::Ctor(..) => {
+            // These are inferred.
+            let crate_map = tcx.crate_variances(());
+            return crate_map.variances.get(&item_def_id.to_def_id()).copied().unwrap_or(&[]);
+        }
         DefKind::OpaqueTy | DefKind::ImplTraitPlaceholder => {
             return variance_of_opaque(tcx, item_def_id);
         }
-        _ => {
-            // Variance not relevant.
-            span_bug!(tcx.def_span(item_def_id), "asked to compute variance for wrong kind of item")
+        DefKind::AssocTy => {
+            if let Some(ImplTraitInTraitData::Trait { .. }) =
+                tcx.opt_rpitit_info(item_def_id.to_def_id())
+            {
+                return variance_of_opaque(tcx, item_def_id);
+            }
         }
+        _ => {}
     }
 
-    // Everything else must be inferred.
-
-    let crate_map = tcx.crate_variances(());
-    crate_map.variances.get(&item_def_id.to_def_id()).copied().unwrap_or(&[])
+    // Variance not relevant.
+    span_bug!(tcx.def_span(item_def_id), "asked to compute variance for wrong kind of item");
 }
 
 #[instrument(level = "trace", skip(tcx), ret)]