Remove `vec_linked_list`.

It provides a way to effectively embed a linked list within an
`IndexVec` and also iterate over that list. It's written in a very
generic way, involving two traits `Links` and `LinkElem`. But the
`Links` trait is only impl'd for `IndexVec` and `&IndexVec`, and the
whole thing is only used in one module within `rustc_borrowck`. So I
think it's over-engineered and hard to read. Plus it has no comments.

This commit removes it, and adds a (non-generic) local iterator for the
use within `rustc_borrowck`. Much simpler.

3 files changed, 32 insertions, 81 deletions

diff --git a/compiler/rustc_borrowck/src/type_check/liveness/local_use_map.rs b/compiler/rustc_borrowck/src/type_check/liveness/local_use_map.rs
index da5456692ab..ccd9fb25739 100644
--- a/compiler/rustc_borrowck/src/type_check/liveness/local_use_map.rs
+++ b/compiler/rustc_borrowck/src/type_check/liveness/local_use_map.rs
@@ -1,4 +1,3 @@
-use rustc_data_structures::vec_linked_list as vll;
 use rustc_index::IndexVec;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::{Body, Local, Location};
@@ -37,9 +36,12 @@ pub(crate) struct LocalUseMap {
     /// we add for each local variable.
     first_drop_at: IndexVec<Local, Option<AppearanceIndex>>,
 
-    appearances: IndexVec<AppearanceIndex, Appearance>,
+    appearances: Appearances,
 }
 
+// The `Appearance::next` field effectively embeds a linked list within `Appearances`.
+type Appearances = IndexVec<AppearanceIndex, Appearance>;
+
 struct Appearance {
     point_index: PointIndex,
     next: Option<AppearanceIndex>,
@@ -49,14 +51,34 @@ rustc_index::newtype_index! {
     pub struct AppearanceIndex {}
 }
 
-impl vll::LinkElem for Appearance {
-    type LinkIndex = AppearanceIndex;
+fn appearances_iter(
+    first: Option<AppearanceIndex>,
+    appearances: &Appearances,
+) -> impl Iterator<Item = AppearanceIndex> + '_ {
+    AppearancesIter { appearances, current: first }
+}
+
+// Iterates over `Appearances` by following `next` fields.
+struct AppearancesIter<'a> {
+    appearances: &'a Appearances,
+    current: Option<AppearanceIndex>,
+}
 
-    fn next(elem: &Self) -> Option<AppearanceIndex> {
-        elem.next
+impl<'a> Iterator for AppearancesIter<'a> {
+    type Item = AppearanceIndex;
+
+    fn next(&mut self) -> Option<AppearanceIndex> {
+        if let Some(c) = self.current {
+            self.current = self.appearances[c].next;
+            Some(c)
+        } else {
+            None
+        }
     }
 }
 
+//-----------------------------------------------------------------------------
+
 impl LocalUseMap {
     pub(crate) fn build(
         live_locals: &[Local],
@@ -86,17 +108,17 @@ impl LocalUseMap {
     }
 
     pub(crate) fn defs(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
-        vll::iter(self.first_def_at[local], &self.appearances)
+        appearances_iter(self.first_def_at[local], &self.appearances)
             .map(move |aa| self.appearances[aa].point_index)
     }
 
     pub(crate) fn uses(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
-        vll::iter(self.first_use_at[local], &self.appearances)
+        appearances_iter(self.first_use_at[local], &self.appearances)
             .map(move |aa| self.appearances[aa].point_index)
     }
 
     pub(crate) fn drops(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
-        vll::iter(self.first_drop_at[local], &self.appearances)
+        appearances_iter(self.first_drop_at[local], &self.appearances)
             .map(move |aa| self.appearances[aa].point_index)
     }
 }
@@ -146,7 +168,7 @@ impl LocalUseMapBuild<'_> {
     fn insert(
         elements: &DenseLocationMap,
         first_appearance: &mut Option<AppearanceIndex>,
-        appearances: &mut IndexVec<AppearanceIndex, Appearance>,
+        appearances: &mut Appearances,
         location: Location,
     ) {
         let point_index = elements.point_from_location(location);
diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index 2e5f3806b12..407ee0453e5 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -85,7 +85,6 @@ pub mod temp_dir;
 pub mod transitive_relation;
 pub mod unhash;
 pub mod unord;
-pub mod vec_linked_list;
 pub mod work_queue;
 
 mod atomic_ref;
diff --git a/compiler/rustc_data_structures/src/vec_linked_list.rs b/compiler/rustc_data_structures/src/vec_linked_list.rs
deleted file mode 100644
index fda72c9a3b2..00000000000
--- a/compiler/rustc_data_structures/src/vec_linked_list.rs
+++ /dev/null
@@ -1,70 +0,0 @@
-use rustc_index::{Idx, IndexVec};
-
-pub fn iter<Ls>(
-    first: Option<Ls::LinkIndex>,
-    links: &Ls,
-) -> impl Iterator<Item = Ls::LinkIndex> + '_
-where
-    Ls: Links,
-{
-    VecLinkedListIterator { links, current: first }
-}
-
-pub struct VecLinkedListIterator<Ls>
-where
-    Ls: Links,
-{
-    links: Ls,
-    current: Option<Ls::LinkIndex>,
-}
-
-impl<Ls> Iterator for VecLinkedListIterator<Ls>
-where
-    Ls: Links,
-{
-    type Item = Ls::LinkIndex;
-
-    fn next(&mut self) -> Option<Ls::LinkIndex> {
-        if let Some(c) = self.current {
-            self.current = <Ls as Links>::next(&self.links, c);
-            Some(c)
-        } else {
-            None
-        }
-    }
-}
-
-pub trait Links {
-    type LinkIndex: Copy;
-
-    fn next(links: &Self, index: Self::LinkIndex) -> Option<Self::LinkIndex>;
-}
-
-impl<Ls> Links for &Ls
-where
-    Ls: Links,
-{
-    type LinkIndex = Ls::LinkIndex;
-
-    fn next(links: &Self, index: Ls::LinkIndex) -> Option<Ls::LinkIndex> {
-        <Ls as Links>::next(links, index)
-    }
-}
-
-pub trait LinkElem {
-    type LinkIndex: Copy;
-
-    fn next(elem: &Self) -> Option<Self::LinkIndex>;
-}
-
-impl<L, E> Links for IndexVec<L, E>
-where
-    E: LinkElem<LinkIndex = L>,
-    L: Idx,
-{
-    type LinkIndex = L;
-
-    fn next(links: &Self, index: L) -> Option<L> {
-        <E as LinkElem>::next(&links[index])
-    }
-}