Remove PartialOrd/Ord impl for PlaceRef

This is a new attempt at rust-lang#93315. It removes one usage
of the `Ord` impl for `DefId`, which should make it easier
to eventually remove that impl.

Author: Aaron1011

compiler/rustc_const_eval/src/transform/validate.rs

@@ -1,5 +1,6 @@
 //! Validates the MIR to ensure that invariants are upheld.
 
+use rustc_data_structures::fx::FxHashSet;
 use rustc_index::bit_set::BitSet;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::interpret::Scalar;
@@ -701,8 +702,8 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     }
                 }
                 let all_len = self.place_cache.len();
-                self.place_cache.sort_unstable();
-                self.place_cache.dedup();
+                let mut dedup = FxHashSet::default();
+                self.place_cache.retain(|p| dedup.insert(*p));
                 let has_duplicates = all_len != self.place_cache.len();
                 if has_duplicates {
                     self.fail(

compiler/rustc_middle/src/lib.rs

@@ -35,6 +35,7 @@
 #![feature(get_mut_unchecked)]
 #![feature(if_let_guard)]
 #![feature(map_first_last)]
+#![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(extern_types)]
 #![feature(new_uninit)]

compiler/rustc_middle/src/mir/mod.rs

@@ -2084,12 +2084,18 @@ rustc_index::newtype_index! {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub struct PlaceRef<'tcx> {
     pub local: Local,
     pub projection: &'tcx [PlaceElem<'tcx>],
 }
 
+// Once we stop implementing `Ord` for `DefId`,
+// this impl will be unnecessary. Until then, we'll
+// leave this impl in place to prevent re-adding a
+// dependnecy on the `Ord` impl for `DefId`
+impl<'tcx> !PartialOrd for PlaceRef<'tcx> {}
+
 impl<'tcx> Place<'tcx> {
     // FIXME change this to a const fn by also making List::empty a const fn.
     pub fn return_place() -> Place<'tcx> {

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -11,7 +11,7 @@ use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard};
 use crate::build::{BlockAnd, BlockAndExtension, Builder};
 use crate::build::{GuardFrame, GuardFrameLocal, LocalsForNode};
 use rustc_data_structures::{
-    fx::{FxIndexMap, FxIndexSet},
+    fx::{FxHashSet, FxIndexMap, FxIndexSet},
     stack::ensure_sufficient_stack,
 };
 use rustc_hir::HirId;
@@ -1741,7 +1741,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             all_fake_borrows.push(place.as_ref());
         }
 
-        all_fake_borrows.dedup();
+        // Deduplicate
+        let mut dedup = FxHashSet::default();
+        all_fake_borrows.retain(|b| dedup.insert(*b));
 
         debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);