Auto merge of #51462 - Havvy:refactor-cmp, r=nikomatsakis

Refactor: Rename ExistentialPredicate::cmp to ExistentialPredicate::stable_cmp

See https://github.com/rust-lang/rust/pull/51276#discussion_r193549404 for
rationale.

Because stable_cmp takes three arguments and Ord::cmp takes two, I am confident that there is no shadowing happening here.

r? @nikomatsakis

src/librustc/ty/context.rs

@@ -2514,7 +2514,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     pub fn intern_existential_predicates(self, eps: &[ExistentialPredicate<'tcx>])
         -> &'tcx Slice<ExistentialPredicate<'tcx>> {
         assert!(!eps.is_empty());
-        assert!(eps.windows(2).all(|w| w[0].cmp(self, &w[1]) != Ordering::Greater));
+        assert!(eps.windows(2).all(|w| w[0].stable_cmp(self, &w[1]) != Ordering::Greater));
         self._intern_existential_predicates(eps)
     }
 

src/librustc/ty/sty.rs

@@ -496,7 +496,9 @@ pub enum ExistentialPredicate<'tcx> {
 }
 
 impl<'a, 'gcx, 'tcx> ExistentialPredicate<'tcx> {
-    pub fn cmp(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>, other: &Self) -> Ordering {
+    /// Compares via an ordering that will not change if modules are reordered or other changes are
+    /// made to the tree. In particular, this ordering is preserved across incremental compilations.
+    pub fn stable_cmp(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>, other: &Self) -> Ordering {
         use self::ExistentialPredicate::*;
         match (*self, *other) {
             (Trait(_), Trait(_)) => Ordering::Equal,

src/librustc_typeck/astconv.rs

@@ -718,7 +718,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
             .chain(existential_projections
                    .map(|x| ty::ExistentialPredicate::Projection(*x.skip_binder())))
             .collect::<AccumulateVec<[_; 8]>>();
-        v.sort_by(|a, b| a.cmp(tcx, b));
+        v.sort_by(|a, b| a.stable_cmp(tcx, b));
         let existential_predicates = ty::Binder::bind(tcx.mk_existential_predicates(v.into_iter()));