Rollup merge of #106718 - lcnr:solver-cycles, r=compiler-errors

finish trait solver skeleton work

### 648d661b4e0fcf55f7082894f577377eb451db4b

The previous implementation didn't remove provisional entries which depended on the current goal if we're forced to rerun in case the provisional result of that entry is different from the new result. For reference, see https://rust-lang.github.io/chalk/book/recursive/search_graph.html.

We should also treat inductive cycles as overflow, not ordinary ambiguity.

### 219a5de2517cebfe20a2c3417bd302f7c12db70c 6a1912be539dd5a3b3c10be669787c4bf0c1868a

These two commits move canonicalization to the start of the queries which simplifies a bunch of stuff. I originally intended to keep stuff canonicalized for a while because I expected us to add a additional caches the trait solver, either for candidate assembly or for projections. We ended up not adding (and expect to not need) any of them so this just ends up being easier to understand.

### d78d5ad0979e965afde6500bccfa119b47063506

adds a special `eq` for the solver which doesn't care about obligations or spans

### 18704e6a78b7703e1bbb3856f015cb76c0a07a06

implements https://rust-lang.zulipchat.com/#narrow/stream/364551-t-types.2Ftrait-system-refactor/topic/projection.20cache

r? `@compiler-errors`

Author: Dylan-DPC

Cargo.lock

@@ -4783,6 +4783,7 @@ dependencies = [
  "rustc_middle",
  "rustc_parse_format",
  "rustc_query_system",
+ "rustc_serialize",
  "rustc_session",
  "rustc_span",
  "rustc_target",

compiler/rustc_middle/src/infer/canonical.rs

@@ -339,6 +339,12 @@ TrivialTypeTraversalAndLiftImpls! {
 }
 
 impl<'tcx> CanonicalVarValues<'tcx> {
+    /// Creates dummy var values which should not be used in a
+    /// canonical response.
+    pub fn dummy() -> CanonicalVarValues<'tcx> {
+        CanonicalVarValues { var_values: Default::default() }
+    }
+
     #[inline]
     pub fn len(&self) -> usize {
         self.var_values.len()

compiler/rustc_middle/src/ty/sty.rs

@@ -1113,17 +1113,6 @@ impl<'tcx, T> Binder<'tcx, T> {
         if self.0.has_escaping_bound_vars() { None } else { Some(self.skip_binder()) }
     }
 
-    pub fn no_bound_vars_ignoring_escaping(self, tcx: TyCtxt<'tcx>) -> Option<T>
-    where
-        T: TypeFoldable<'tcx>,
-    {
-        if !self.0.has_escaping_bound_vars() {
-            Some(self.skip_binder())
-        } else {
-            self.0.try_fold_with(&mut SkipBindersAt { index: ty::INNERMOST, tcx }).ok()
-        }
-    }
-
     /// Splits the contents into two things that share the same binder
     /// level as the original, returning two distinct binders.
     ///

compiler/rustc_trait_selection/Cargo.toml

@@ -19,6 +19,7 @@ rustc_infer = { path = "../rustc_infer" }
 rustc_lint_defs = { path = "../rustc_lint_defs" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_query_system = { path = "../rustc_query_system" }
+rustc_serialize = { path = "../rustc_serialize" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }

compiler/rustc_trait_selection/src/lib.rs

@@ -21,6 +21,7 @@
 #![feature(never_type)]
 #![feature(result_option_inspect)]
 #![feature(type_alias_impl_trait)]
+#![feature(min_specialization)]
 #![recursion_limit = "512"] // For rustdoc
 
 #[macro_use]