Minor tweaks to make some normalization (adjacent) code less confusing

compiler/rustc_hir_typeck/src/writeback.rs

@@ -1,6 +1,12 @@
-// Type resolution: the phase that finds all the types in the AST with
-// unresolved type variables and replaces "ty_var" types with their
-// generic parameters.
+//! During type inference, partially inferred terms are
+//! represented using inference variables (ty::Infer). These don't appear in
+//! the final [`ty::TypeckResults`] since all of the types should have been
+//! inferred once typeck is done.
+//!
+//! When type inference is running however, having to update the typeck results
+//! every time a new type is inferred would be unreasonably slow, so instead all
+//! of the replacement happens at the end in [`FnCtxt::resolve_type_vars_in_body`],
+//! which creates a new `TypeckResults` which doesn't contain any inference variables.
 
 use std::mem;
 
@@ -27,15 +33,6 @@ use crate::FnCtxt;
 ///////////////////////////////////////////////////////////////////////////
 // Entry point
 
-// During type inference, partially inferred types are
-// represented using Type variables (ty::Infer). These don't appear in
-// the final TypeckResults since all of the types should have been
-// inferred once typeck is done.
-// When type inference is running however, having to update the typeck
-// typeck results every time a new type is inferred would be unreasonably slow,
-// so instead all of the replacement happens at the end in
-// resolve_type_vars_in_body, which creates a new TypeTables which
-// doesn't contain any inference types.
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub(crate) fn resolve_type_vars_in_body(
         &self,
@@ -90,14 +87,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 }
 
-///////////////////////////////////////////////////////////////////////////
-// The Writeback context. This visitor walks the HIR, checking the
-// fn-specific typeck results to find references to types or regions. It
-// resolves those regions to remove inference variables and writes the
-// final result back into the master typeck results in the tcx. Here and
-// there, it applies a few ad-hoc checks that were not convenient to
-// do elsewhere.
-
+/// The Writeback context. This visitor walks the HIR, checking the
+/// fn-specific typeck results to find inference variables. It resolves
+/// those inference variables and writes the final result into the
+/// `TypeckResults`. It also applies a few ad-hoc checks that were not
+/// convenient to do elsewhere.
 struct WritebackCx<'cx, 'tcx> {
     fcx: &'cx FnCtxt<'cx, 'tcx>,
 
@@ -897,7 +891,7 @@ impl<'cx, 'tcx> Resolver<'cx, 'tcx> {
             let cause = ObligationCause::misc(self.span.to_span(tcx), body_id);
             let at = self.fcx.at(&cause, self.fcx.param_env);
             let universes = vec![None; outer_exclusive_binder(value).as_usize()];
-            match solve::deeply_normalize_with_skipped_universes_and_ambiguous_goals(
+            match solve::deeply_normalize_with_skipped_universes_and_ambiguous_coroutine_goals(
                 at, value, universes,
             ) {
                 Ok((value, goals)) => {

compiler/rustc_trait_selection/src/solve.rs

@@ -11,6 +11,6 @@ pub use fulfill::{FulfillmentCtxt, NextSolverError};
 pub(crate) use normalize::deeply_normalize_for_diagnostics;
 pub use normalize::{
     deeply_normalize, deeply_normalize_with_skipped_universes,
-    deeply_normalize_with_skipped_universes_and_ambiguous_goals,
+    deeply_normalize_with_skipped_universes_and_ambiguous_coroutine_goals,
 };
 pub use select::InferCtxtSelectExt;

compiler/rustc_trait_selection/src/solve/normalize.rs

@@ -45,9 +45,11 @@ where
     T: TypeFoldable<TyCtxt<'tcx>>,
     E: FromSolverError<'tcx, NextSolverError<'tcx>>,
 {
-    let (value, goals) =
-        deeply_normalize_with_skipped_universes_and_ambiguous_goals(at, value, universes)?;
-    assert_eq!(goals, vec![]);
+    let (value, coroutine_goals) =
+        deeply_normalize_with_skipped_universes_and_ambiguous_coroutine_goals(
+            at, value, universes,
+        )?;
+    assert_eq!(coroutine_goals, vec![]);
 
     Ok(value)
 }
@@ -59,9 +61,9 @@ where
 /// entered before passing `value` to the function. This is currently needed for
 /// `normalize_erasing_regions`, which skips binders as it walks through a type.
 ///
-/// This returns a set of stalled obligations if the typing mode of the underlying infcx
-/// has any stalled coroutine def ids.
-pub fn deeply_normalize_with_skipped_universes_and_ambiguous_goals<'tcx, T, E>(
+/// This returns a set of stalled obligations involving coroutines if the typing mode of
+/// the underlying infcx has any stalled coroutine def ids.
+pub fn deeply_normalize_with_skipped_universes_and_ambiguous_coroutine_goals<'tcx, T, E>(
     at: At<'_, 'tcx>,
     value: T,
     universes: Vec<Option<UniverseIndex>>,
@@ -71,19 +73,24 @@ where
     E: FromSolverError<'tcx, NextSolverError<'tcx>>,
 {
     let fulfill_cx = FulfillmentCtxt::new(at.infcx);
-    let mut folder =
-        NormalizationFolder { at, fulfill_cx, depth: 0, universes, stalled_goals: vec![] };
+    let mut folder = NormalizationFolder {
+        at,
+        fulfill_cx,
+        depth: 0,
+        universes,
+        stalled_coroutine_goals: vec![],
+    };
     let value = value.try_fold_with(&mut folder)?;
     let errors = folder.fulfill_cx.select_all_or_error(at.infcx);
-    if errors.is_empty() { Ok((value, folder.stalled_goals)) } else { Err(errors) }
+    if errors.is_empty() { Ok((value, folder.stalled_coroutine_goals)) } else { Err(errors) }
 }
 
 struct NormalizationFolder<'me, 'tcx, E> {
     at: At<'me, 'tcx>,
     fulfill_cx: FulfillmentCtxt<'tcx, E>,
     depth: usize,
     universes: Vec<Option<UniverseIndex>>,
-    stalled_goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
+    stalled_coroutine_goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 }
 
 impl<'tcx, E> NormalizationFolder<'_, 'tcx, E>
@@ -182,7 +189,7 @@ where
             return Err(errors);
         }
 
-        self.stalled_goals.extend(
+        self.stalled_coroutine_goals.extend(
             self.fulfill_cx
                 .drain_stalled_obligations_for_coroutines(self.at.infcx)
                 .into_iter()
@@ -298,13 +305,13 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for DeeplyNormalizeForDiagnosticsFolder<'_,
 
     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
         let infcx = self.at.infcx;
-        let result =
-            infcx.commit_if_ok(|_| {
-                deeply_normalize_with_skipped_universes_and_ambiguous_goals::<
-                    _,
-                    ScrubbedTraitError<'tcx>,
-                >(self.at, ty, vec![None; ty.outer_exclusive_binder().as_usize()])
-            });
+        let result: Result<_, Vec<ScrubbedTraitError<'tcx>>> = infcx.commit_if_ok(|_| {
+            deeply_normalize_with_skipped_universes_and_ambiguous_coroutine_goals(
+                self.at,
+                ty,
+                vec![None; ty.outer_exclusive_binder().as_usize()],
+            )
+        });
         match result {
             Ok((ty, _)) => ty,
             Err(_) => ty.super_fold_with(self),
@@ -313,13 +320,13 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for DeeplyNormalizeForDiagnosticsFolder<'_,
 
     fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
         let infcx = self.at.infcx;
-        let result =
-            infcx.commit_if_ok(|_| {
-                deeply_normalize_with_skipped_universes_and_ambiguous_goals::<
-                    _,
-                    ScrubbedTraitError<'tcx>,
-                >(self.at, ct, vec![None; ct.outer_exclusive_binder().as_usize()])
-            });
+        let result: Result<_, Vec<ScrubbedTraitError<'tcx>>> = infcx.commit_if_ok(|_| {
+            deeply_normalize_with_skipped_universes_and_ambiguous_coroutine_goals(
+                self.at,
+                ct,
+                vec![None; ct.outer_exclusive_binder().as_usize()],
+            )
+        });
         match result {
             Ok((ct, _)) => ct,
             Err(_) => ct.super_fold_with(self),

compiler/rustc_trait_selection/src/traits/normalize.rs

@@ -260,11 +260,14 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
             }
 
             ty::Projection if !data.has_escaping_bound_vars() => {
-                // This branch is *mostly* just an optimization: when we don't
-                // have escaping bound vars, we don't need to replace them with
-                // placeholders (see branch below). *Also*, we know that we can
-                // register an obligation to *later* project, since we know
-                // there won't be bound vars there.
+                // When we don't have escaping bound vars we can normalize ambig aliases
+                // to inference variables (done in `normalize_projection_ty`). This would
+                // be wrong if there were escaping bound vars as even if we instantiated
+                // the bound vars with placeholders, we wouldn't be able to map them back
+                // after normalization succeeded.
+                //
+                // Also, as an optimization: when we don't have escaping bound vars, we don't
+                // need to replace them with placeholders (see branch below).
                 let data = data.fold_with(self);
                 let normalized_ty = project::normalize_projection_ty(
                     self.selcx,