Make synthetic RPITIT assoc ty name handling more rigorous.

Currently it relies on special treatment of `kw::Empty`, which is really
easy to get wrong. This commit makes the special case clearer in the
type system by using `Option`. It's a bit clumsy, but the synthetic name
handling itself is a bit clumsy; better to make it explicit than sneak
it in.

Fixes rust-lang#133426.

Author: nnethercote

Diff for: compiler/rustc_hir/src/def.rs

@@ -254,7 +254,7 @@ impl DefKind {
     }
 
     // Some `DefKind`s require a name, some don't. Panics if one is needed but
-    // not provided.
+    // not provided. (`AssocTy` is an exception, see below.)
     pub fn def_path_data(self, name: Option<Symbol>) -> DefPathData {
         match self {
             DefKind::Mod
@@ -266,9 +266,13 @@ impl DefKind {
             | DefKind::TyAlias
             | DefKind::ForeignTy
             | DefKind::TraitAlias
-            | DefKind::AssocTy
             | DefKind::TyParam
-            | DefKind::ExternCrate => DefPathData::TypeNs(name.unwrap()),
+            | DefKind::ExternCrate => DefPathData::TypeNs(Some(name.unwrap())),
+
+            // An associated type names will be missing for an RPITIT. It will
+            // later be given a name with `synthetic` in it, if necessary.
+            DefKind::AssocTy => DefPathData::TypeNs(name),
+
             // It's not exactly an anon const, but wrt DefPathData, there
             // is no difference.
             DefKind::Static { nested: true, .. } => DefPathData::AnonConst,

Diff for: compiler/rustc_hir/src/definitions.rs

@@ -271,8 +271,9 @@ pub enum DefPathData {
     Use,
     /// A global asm item.
     GlobalAsm,
-    /// Something in the type namespace.
-    TypeNs(Symbol),
+    /// Something in the type namespace. Will be empty for RPITIT associated
+    /// types, which are given a synthetic name later, if necessary.
+    TypeNs(Option<Symbol>),
     /// Something in the value namespace.
     ValueNs(Symbol),
     /// Something in the macro namespace.
@@ -410,8 +411,9 @@ impl DefPathData {
     pub fn get_opt_name(&self) -> Option<Symbol> {
         use self::DefPathData::*;
         match *self {
-            TypeNs(name) if name == kw::Empty => None,
-            TypeNs(name) | ValueNs(name) | MacroNs(name) | LifetimeNs(name) => Some(name),
+            TypeNs(name) => name,
+
+            ValueNs(name) | MacroNs(name) | LifetimeNs(name) => Some(name),
 
             Impl | ForeignMod | CrateRoot | Use | GlobalAsm | Closure | Ctor | AnonConst
             | OpaqueTy => None,
@@ -421,12 +423,14 @@ impl DefPathData {
     pub fn name(&self) -> DefPathDataName {
         use self::DefPathData::*;
         match *self {
-            TypeNs(name) if name == kw::Empty => {
-                DefPathDataName::Anon { namespace: sym::synthetic }
-            }
-            TypeNs(name) | ValueNs(name) | MacroNs(name) | LifetimeNs(name) => {
-                DefPathDataName::Named(name)
+            TypeNs(name) => {
+                if let Some(name) = name {
+                    DefPathDataName::Named(name)
+                } else {
+                    DefPathDataName::Anon { namespace: sym::synthetic }
+                }
             }
+            ValueNs(name) | MacroNs(name) | LifetimeNs(name) => DefPathDataName::Named(name),
             // Note that this does not show up in user print-outs.
             CrateRoot => DefPathDataName::Anon { namespace: kw::Crate },
             Impl => DefPathDataName::Anon { namespace: kw::Impl },

Diff for: compiler/rustc_middle/src/ty/print/pretty.rs

@@ -569,7 +569,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
             // the children of the visible parent (as was done when computing
             // `visible_parent_map`), looking for the specific child we currently have and then
             // have access to the re-exported name.
-            DefPathData::TypeNs(ref mut name) if Some(visible_parent) != actual_parent => {
+            DefPathData::TypeNs(Some(ref mut name)) if Some(visible_parent) != actual_parent => {
                 // Item might be re-exported several times, but filter for the one
                 // that's public and whose identifier isn't `_`.
                 let reexport = self
@@ -590,7 +590,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
             }
             // Re-exported `extern crate` (#43189).
             DefPathData::CrateRoot => {
-                data = DefPathData::TypeNs(self.tcx().crate_name(def_id.krate));
+                data = DefPathData::TypeNs(Some(self.tcx().crate_name(def_id.krate)));
             }
             _ => {}
         }

Diff for: compiler/rustc_middle/src/ty/significant_drop_order.rs

@@ -23,9 +23,11 @@ fn true_significant_drop_ty<'tcx>(
 
             match key.disambiguated_data.data {
                 rustc_hir::definitions::DefPathData::CrateRoot => {
-                    name_rev.push(tcx.crate_name(did.krate))
+                    name_rev.push(tcx.crate_name(did.krate));
+                }
+                rustc_hir::definitions::DefPathData::TypeNs(symbol) => {
+                    name_rev.push(symbol.unwrap());
                 }
-                rustc_hir::definitions::DefPathData::TypeNs(symbol) => name_rev.push(symbol),
                 _ => return None,
             }
             if let Some(parent) = key.parent {

Diff for: compiler/rustc_ty_utils/src/assoc.rs

@@ -252,8 +252,8 @@ fn associated_type_for_impl_trait_in_trait(
     assert_eq!(tcx.def_kind(trait_def_id), DefKind::Trait);
 
     let span = tcx.def_span(opaque_ty_def_id);
-    // FIXME: `kw::Empty` gets special treatment by `DefPathData`'s methods.
-    let trait_assoc_ty = tcx.at(span).create_def(trait_def_id, Some(kw::Empty), DefKind::AssocTy);
+    // No name because this is a synthetic associated type.
+    let trait_assoc_ty = tcx.at(span).create_def(trait_def_id, None, DefKind::AssocTy);
 
     let local_def_id = trait_assoc_ty.def_id();
     let def_id = local_def_id.to_def_id();
@@ -305,9 +305,8 @@ fn associated_type_for_impl_trait_in_impl(
         hir::FnRetTy::DefaultReturn(_) => tcx.def_span(impl_fn_def_id),
         hir::FnRetTy::Return(ty) => ty.span,
     };
-    // FIXME: `kw::Empty` gets special treatment by `DefPathData`'s methods.
-    let impl_assoc_ty =
-        tcx.at(span).create_def(impl_local_def_id, Some(kw::Empty), DefKind::AssocTy);
+    // No name because this is a synthetic associated type.
+    let impl_assoc_ty = tcx.at(span).create_def(impl_local_def_id, None, DefKind::AssocTy);
 
     let local_def_id = impl_assoc_ty.def_id();
     let def_id = local_def_id.to_def_id();

Diff for: src/tools/clippy/clippy_utils/src/lib.rs

@@ -3489,7 +3489,7 @@ fn maybe_get_relative_path(from: &DefPath, to: &DefPath, max_super: usize) -> St
                 // a::b::c  ::d::sym refers to
                 // e::f::sym:: ::
                 // result should be super::super::super::super::e::f
-                if let DefPathData::TypeNs(s) = l {
+                if let DefPathData::TypeNs(Some(s)) = l {
                     path.push(s.to_string());
                 }
                 if let DefPathData::TypeNs(_) = r {
@@ -3500,7 +3500,7 @@ fn maybe_get_relative_path(from: &DefPath, to: &DefPath, max_super: usize) -> St
             // a::b::sym:: ::    refers to
             // c::d::e  ::f::sym
             // when looking at `f`
-            Left(DefPathData::TypeNs(sym)) => path.push(sym.to_string()),
+            Left(DefPathData::TypeNs(Some(sym))) => path.push(sym.to_string()),
             // consider:
             // a::b::c  ::d::sym refers to
             // e::f::sym:: ::
@@ -3514,7 +3514,7 @@ fn maybe_get_relative_path(from: &DefPath, to: &DefPath, max_super: usize) -> St
         // `super` chain would be too long, just use the absolute path instead
         once(String::from("crate"))
             .chain(to.data.iter().filter_map(|el| {
-                if let DefPathData::TypeNs(sym) = el.data {
+                if let DefPathData::TypeNs(Some(sym)) = el.data {
                     Some(sym.to_string())
                 } else {
                     None

Diff for: tests/crashes/133426.rs

@@ -0,0 +1,20 @@
+//! Test for the crash in #133426, caused by an empty symbol being used for a
+//! type name.
+
+#![allow(incomplete_features)]
+#![feature(never_patterns)]
+
+fn a(
+    _: impl Iterator<
+        Item = [(); {
+            match *todo!() { ! }; //~ ERROR type `!` cannot be dereferenced
+        }],
+    >,
+) {
+}
+
+fn b(_: impl Iterator<Item = { match 0 { ! } }>) {}
+//~^ ERROR associated const equality is incomplete
+//~| ERROR expected type, found constant
+
+fn main() {}

Diff for: tests/ui/lowering/no-name-for-DefPath-issue-133426.rs

@@ -0,0 +1,31 @@
+error[E0658]: associated const equality is incomplete
+  --> $DIR/no-name-for-DefPath-issue-133426.rs:16:23
+   |
+LL | fn b(_: impl Iterator<Item = { match 0 { ! } }>) {}
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #92827 <https://github.com/rust-lang/rust/issues/92827> for more information
+   = help: add `#![feature(associated_const_equality)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0614]: type `!` cannot be dereferenced
+  --> $DIR/no-name-for-DefPath-issue-133426.rs:10:19
+   |
+LL |             match *todo!() { ! };
+   |                   ^^^^^^^^ can't be dereferenced
+
+error: expected type, found constant
+  --> $DIR/no-name-for-DefPath-issue-133426.rs:16:30
+   |
+LL | fn b(_: impl Iterator<Item = { match 0 { ! } }>) {}
+   |                       ----   ^^^^^^^^^^^^^^^^^ unexpected constant
+   |                       |
+   |                       expected a type because of this associated type
+   |
+note: the associated type is defined here
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0614, E0658.
+For more information about an error, try `rustc --explain E0614`.