hir: Do not introduce dummy type names for extern blocks in def paths

Use a separate nameless `DefPathData` variant instead

compiler/rustc_hir/src/definitions.rs

@@ -267,6 +267,8 @@ pub enum DefPathData {
     // Different kinds of items and item-like things:
     /// An impl.
     Impl,
+    /// An `extern` block.
+    ForeignMod,
     /// Something in the type namespace.
     TypeNs(Symbol),
     /// Something in the value namespace.
@@ -469,7 +471,9 @@ impl DefPathData {
         match *self {
             TypeNs(name) | ValueNs(name) | MacroNs(name) | LifetimeNs(name) => Some(name),
 
-            Impl | CrateRoot | Misc | ClosureExpr | Ctor | AnonConst | ImplTrait => None,
+            Impl | ForeignMod | CrateRoot | Misc | ClosureExpr | Ctor | AnonConst | ImplTrait => {
+                None
+            }
         }
     }
 
@@ -482,6 +486,7 @@ impl DefPathData {
             // Note that this does not show up in user print-outs.
             CrateRoot => DefPathDataName::Anon { namespace: kw::Crate },
             Impl => DefPathDataName::Anon { namespace: kw::Impl },
+            ForeignMod => DefPathDataName::Anon { namespace: kw::Extern },
             Misc => DefPathDataName::Anon { namespace: sym::misc },
             ClosureExpr => DefPathDataName::Anon { namespace: sym::closure },
             Ctor => DefPathDataName::Anon { namespace: sym::constructor },

compiler/rustc_lint/src/context.rs

@@ -1040,8 +1040,8 @@ impl<'tcx> LateContext<'tcx> {
             ) -> Result<Self::Path, Self::Error> {
                 let mut path = print_prefix(self)?;
 
-                // Skip `::{{constructor}}` on tuple/unit structs.
-                if let DefPathData::Ctor = disambiguated_data.data {
+                // Skip `::{{extern}}` blocks and `::{{constructor}}` on tuple/unit structs.
+                if let DefPathData::ForeignMod | DefPathData::Ctor = disambiguated_data.data {
                     return Ok(path);
                 }
 

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1740,30 +1740,26 @@ impl<F: fmt::Write> Printer<'tcx> for FmtPrinter<'_, 'tcx, F> {
     ) -> Result<Self::Path, Self::Error> {
         self = print_prefix(self)?;
 
-        // Skip `::{{constructor}}` on tuple/unit structs.
-        if let DefPathData::Ctor = disambiguated_data.data {
+        // Skip `::{{extern}}` blocks and `::{{constructor}}` on tuple/unit structs.
+        if let DefPathData::ForeignMod | DefPathData::Ctor = disambiguated_data.data {
             return Ok(self);
         }
 
-        // FIXME(eddyb) `name` should never be empty, but it
-        // currently is for `extern { ... }` "foreign modules".
         let name = disambiguated_data.data.name();
-        if name != DefPathDataName::Named(kw::Empty) {
-            if !self.empty_path {
-                write!(self, "::")?;
-            }
+        if !self.empty_path {
+            write!(self, "::")?;
+        }
 
-            if let DefPathDataName::Named(name) = name {
-                if Ident::with_dummy_span(name).is_raw_guess() {
-                    write!(self, "r#")?;
-                }
+        if let DefPathDataName::Named(name) = name {
+            if Ident::with_dummy_span(name).is_raw_guess() {
+                write!(self, "r#")?;
             }
+        }
 
-            let verbose = self.tcx.sess.verbose();
-            disambiguated_data.fmt_maybe_verbose(&mut self, verbose)?;
+        let verbose = self.tcx.sess.verbose();
+        disambiguated_data.fmt_maybe_verbose(&mut self, verbose)?;
 
-            self.empty_path = false;
-        }
+        self.empty_path = false;
 
         Ok(self)
     }

compiler/rustc_resolve/src/def_collector.rs

@@ -92,14 +92,14 @@ impl<'a, 'b> visit::Visitor<'a> for DefCollector<'a, 'b> {
         // information we encapsulate into, the better
         let def_data = match &i.kind {
             ItemKind::Impl { .. } => DefPathData::Impl,
+            ItemKind::ForeignMod(..) => DefPathData::ForeignMod,
             ItemKind::Mod(..)
             | ItemKind::Trait(..)
             | ItemKind::TraitAlias(..)
             | ItemKind::Enum(..)
             | ItemKind::Struct(..)
             | ItemKind::Union(..)
             | ItemKind::ExternCrate(..)
-            | ItemKind::ForeignMod(..)
             | ItemKind::TyAlias(..) => DefPathData::TypeNs(i.ident.name),
             ItemKind::Static(..) | ItemKind::Const(..) | ItemKind::Fn(..) => {
                 DefPathData::ValueNs(i.ident.name)

compiler/rustc_symbol_mangling/src/legacy.rs

@@ -311,8 +311,8 @@ impl<'tcx> Printer<'tcx> for &mut SymbolPrinter<'tcx> {
     ) -> Result<Self::Path, Self::Error> {
         self = print_prefix(self)?;
 
-        // Skip `::{{constructor}}` on tuple/unit structs.
-        if let DefPathData::Ctor = disambiguated_data.data {
+        // Skip `::{{extern}}` blocks and `::{{constructor}}` on tuple/unit structs.
+        if let DefPathData::ForeignMod | DefPathData::Ctor = disambiguated_data.data {
             return Ok(self);
         }
 

compiler/rustc_symbol_mangling/src/v0.rs

@@ -771,6 +771,10 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
         disambiguated_data: &DisambiguatedDefPathData,
     ) -> Result<Self::Path, Self::Error> {
         let ns = match disambiguated_data.data {
+            // FIXME: It shouldn't be necessary to add anything for extern block segments,
+            // but we add 't' for backward compatibility.
+            DefPathData::ForeignMod => 't',
+
             // Uppercase categories are more stable than lowercase ones.
             DefPathData::TypeNs(_) => 't',
             DefPathData::ValueNs(_) => 'v',

src/librustdoc/clean/inline.rs

@@ -8,6 +8,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
+use rustc_hir::definitions::DefPathData;
 use rustc_hir::Mutability;
 use rustc_metadata::creader::{CStore, LoadedMacro};
 use rustc_middle::ty::{self, TyCtxt};
@@ -165,9 +166,8 @@ crate fn record_extern_fqn(cx: &mut DocContext<'_>, did: DefId, kind: ItemType)
     let crate_name = cx.tcx.crate_name(did.krate).to_string();
 
     let relative = cx.tcx.def_path(did).data.into_iter().filter_map(|elem| {
-        // extern blocks have an empty name
-        let s = elem.data.to_string();
-        if !s.is_empty() { Some(s) } else { None }
+        // Filter out extern blocks
+        (elem.data != DefPathData::ForeignMod).then(|| elem.data.to_string())
     });
     let fqn = if let ItemType::Macro = kind {
         // Check to see if it is a macro 2.0 or built-in macro

src/librustdoc/visit_ast.rs

@@ -5,6 +5,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
+use rustc_hir::definitions::DefPathData;
 use rustc_hir::Node;
 use rustc_hir::CRATE_HIR_ID;
 use rustc_middle::middle::privacy::AccessLevel;
@@ -45,9 +46,8 @@ impl Module<'hir> {
 fn def_id_to_path(tcx: TyCtxt<'_>, did: DefId) -> Vec<String> {
     let crate_name = tcx.crate_name(did.krate).to_string();
     let relative = tcx.def_path(did).data.into_iter().filter_map(|elem| {
-        // extern blocks have an empty name
-        let s = elem.data.to_string();
-        if !s.is_empty() { Some(s) } else { None }
+        // Filter out extern blocks
+        (elem.data != DefPathData::ForeignMod).then(|| elem.data.to_string())
     });
     std::iter::once(crate_name).chain(relative).collect()
 }