Rollup merge of #124918 - nnethercote:FIXME-lcnr, r=lcnr

Eliminate some `FIXME(lcnr)` comments

In some cases this involved changing code. In some cases the comment was able to removed or replaced.

r? ``@lcnr``

compiler/rustc_borrowck/src/renumber.rs

@@ -26,9 +26,7 @@ pub fn renumber_mir<'tcx>(
     renumberer.visit_body(body);
 }
 
-// FIXME(@lcnr): A lot of these variants overlap and it seems like
-// this type is only used to decide which region should be used
-// as representative. This should be cleaned up.
+// The fields are used only for debugging output in `sccs_info`.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
 pub(crate) enum RegionCtxt {
     Location(Location),

compiler/rustc_infer/src/errors/note_and_explain.rs

@@ -69,16 +69,8 @@ impl<'a> DescriptionCtx<'a> {
 
             ty::RePlaceholder(_) | ty::ReError(_) => return None,
 
-            // FIXME(#13998) RePlaceholder should probably print like
-            // ReLateParam rather than dumping Debug output on the user.
-            //
-            // We shouldn't really be having unification failures with ReVar
-            // and ReBound though.
-            //
-            // FIXME(@lcnr): figure out why we have to handle `ReBound`
-            // here, this feels somewhat off.
             ty::ReVar(_) | ty::ReBound(..) | ty::ReErased => {
-                (alt_span, "revar", format!("{region:?}"))
+                bug!("unexpected region for DescriptionCtx: {:?}", region);
             }
         };
         Some(DescriptionCtx { span, kind, arg })

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -172,11 +172,9 @@ pub(super) fn note_and_explain_region<'tcx>(
 
         ty::ReError(_) => return,
 
-        // We shouldn't really be having unification failures with ReVar
-        // and ReBound though.
-        //
-        // FIXME(@lcnr): Figure out whether this is reachable and if so, why.
-        ty::ReVar(_) | ty::ReBound(..) | ty::ReErased => (format!("lifetime `{region}`"), alt_span),
+        ty::ReVar(_) | ty::ReBound(..) | ty::ReErased => {
+            bug!("unexpected region for note_and_explain_region: {:?}", region);
+        }
     };
 
     emit_msg_span(err, prefix, description, span, suffix);

compiler/rustc_infer/src/infer/mod.rs

@@ -528,8 +528,8 @@ pub enum RegionVariableOrigin {
 
     /// Region variables created as the values for early-bound regions.
     ///
-    /// FIXME(@lcnr): This can also store a `DefId`, similar to
-    /// `TypeVariableOriginKind::TypeParameterDefinition`.
+    /// FIXME(@lcnr): This should also store a `DefId`, similar to
+    /// `TypeVariableOrigin`.
     RegionParameterDefinition(Span, Symbol),
 
     /// Region variables created when instantiating a binder with

compiler/rustc_middle/src/ty/normalize_erasing_regions.rs

@@ -100,8 +100,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// codegen, we need to normalize the contents.
     // FIXME(@lcnr): This method should not be necessary, we now normalize
     // inside of binders. We should be able to only use
-    // `tcx.instantiate_bound_regions_with_erased`. Same for the `try_X`
-    // variant.
+    // `tcx.instantiate_bound_regions_with_erased`.
     #[tracing::instrument(level = "debug", skip(self, param_env))]
     pub fn normalize_erasing_late_bound_regions<T>(
         self,
@@ -115,26 +114,6 @@ impl<'tcx> TyCtxt<'tcx> {
         self.normalize_erasing_regions(param_env, value)
     }
 
-    /// If you have a `Binder<'tcx, T>`, you can do this to strip out the
-    /// late-bound regions and then normalize the result, yielding up
-    /// a `T` (with regions erased). This is appropriate when the
-    /// binder is being instantiated at the call site.
-    ///
-    /// N.B., currently, higher-ranked type bounds inhibit
-    /// normalization. Therefore, each time we erase them in
-    /// codegen, we need to normalize the contents.
-    pub fn try_normalize_erasing_late_bound_regions<T>(
-        self,
-        param_env: ty::ParamEnv<'tcx>,
-        value: ty::Binder<'tcx, T>,
-    ) -> Result<T, NormalizationError<'tcx>>
-    where
-        T: TypeFoldable<TyCtxt<'tcx>>,
-    {
-        let value = self.instantiate_bound_regions_with_erased(value);
-        self.try_normalize_erasing_regions(param_env, value)
-    }
-
     /// Monomorphizes a type from the AST by first applying the
     /// in-scope instantiations and then normalizing any associated
     /// types.

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -3443,8 +3443,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         self.dcx().try_steal_replace_and_emit_err(self.tcx.def_span(def_id), StashKey::Cycle, err)
     }
 
-    // FIXME(@lcnr): This function could be changed to trait `TraitRef` directly
-    // instead of using a `Binder`.
     fn report_signature_mismatch_error(
         &self,
         obligation: &PredicateObligation<'tcx>,

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -693,7 +693,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let vtable_base = vtable_trait_first_method_offset(
             tcx,
-            (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
+            unnormalized_upcast_trait_ref,
+            ty::Binder::dummy(object_trait_ref),
         );
 
         Ok(ImplSource::Builtin(BuiltinImplSource::Object { vtable_base: vtable_base }, nested))

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -320,16 +320,11 @@ fn vtable_entries<'tcx>(
 }
 
 /// Find slot base for trait methods within vtable entries of another trait
-// FIXME(@lcnr): This isn't a query, so why does it take a tuple as its argument.
 pub(super) fn vtable_trait_first_method_offset<'tcx>(
     tcx: TyCtxt<'tcx>,
-    key: (
-        ty::PolyTraitRef<'tcx>, // trait_to_be_found
-        ty::PolyTraitRef<'tcx>, // trait_owning_vtable
-    ),
+    trait_to_be_found: ty::PolyTraitRef<'tcx>,
+    trait_owning_vtable: ty::PolyTraitRef<'tcx>,
 ) -> usize {
-    let (trait_to_be_found, trait_owning_vtable) = key;
-
     // #90177
     let trait_to_be_found_erased = tcx.erase_regions(trait_to_be_found);