introduce a unit testing feature rustc_evaluate_where_clauses

This attribute will cause us to invoke evaluate on every where clause of an
invoked function and to generate an error with the result.

Without this, it is very difficult to observe the effects of invoking the trait
evaluator.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -565,6 +565,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         template!(Word, List: "delay_span_bug_from_inside_query")
     ),
     rustc_attr!(TEST, rustc_dump_user_substs, AssumedUsed, template!(Word)),
+    rustc_attr!(TEST, rustc_evaluate_where_clauses, AssumedUsed, template!(Word)),
     rustc_attr!(TEST, rustc_if_this_changed, AssumedUsed, template!(Word, List: "DepNode")),
     rustc_attr!(TEST, rustc_then_this_would_need, AssumedUsed, template!(List: "DepNode")),
     rustc_attr!(

compiler/rustc_span/src/symbol.rs

@@ -1010,6 +1010,7 @@ symbols! {
         rustc_dump_program_clauses,
         rustc_dump_user_substs,
         rustc_error,
+        rustc_evaluate_where_clauses,
         rustc_expected_cgu_reuse,
         rustc_if_this_changed,
         rustc_inherit_overflow_checks,

compiler/rustc_typeck/src/check/callee.rs

@@ -6,8 +6,14 @@ use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def::{Namespace, Res};
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
-use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
-use rustc_infer::{infer, traits};
+use rustc_infer::{
+    infer,
+    traits::{self, Obligation},
+};
+use rustc_infer::{
+    infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind},
+    traits::ObligationCause,
+};
 use rustc_middle::ty::adjustment::{
     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability,
 };
@@ -17,6 +23,7 @@ use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
 use rustc_target::spec::abi;
 use rustc_trait_selection::autoderef::Autoderef;
+use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use std::iter;
 
 /// Checks that it is legal to call methods of the trait corresponding
@@ -294,7 +301,34 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Expectation<'tcx>,
     ) -> Ty<'tcx> {
         let (fn_sig, def_id) = match *callee_ty.kind() {
-            ty::FnDef(def_id, _) => (callee_ty.fn_sig(self.tcx), Some(def_id)),
+            ty::FnDef(def_id, subst) => {
+                // Unit testing: function items annotated with
+                // `#[rustc_evaluate_where_clauses]` trigger special output
+                // to let us test the trait evaluation system.
+                if self.tcx.has_attr(def_id, sym::rustc_evaluate_where_clauses) {
+                    let predicates = self.tcx.predicates_of(def_id);
+                    let predicates = predicates.instantiate(self.tcx, subst);
+                    for (predicate, predicate_span) in
+                        predicates.predicates.iter().zip(&predicates.spans)
+                    {
+                        let obligation = Obligation::new(
+                            ObligationCause::dummy_with_span(callee_expr.span),
+                            self.param_env,
+                            predicate.clone(),
+                        );
+                        let result = self.infcx.evaluate_obligation(&obligation);
+                        self.tcx
+                            .sess
+                            .struct_span_err(
+                                callee_expr.span,
+                                &format!("evaluate({:?}) = {:?}", predicate, result),
+                            )
+                            .span_label(*predicate_span, "predicate")
+                            .emit();
+                    }
+                }
+                (callee_ty.fn_sig(self.tcx), Some(def_id))
+            }
             ty::FnPtr(sig) => (sig, None),
             ref t => {
                 let mut unit_variant = None;

src/test/ui/cycle-me.rs

@@ -0,0 +1,31 @@
+#![feature(rustc_attrs)]
+
+// A (reached depth 0)
+//   ...
+//      B // depth 1 -- reached depth = 0
+//          C // depth 2 -- reached depth = 1 (should be 0)
+//              B
+//          A // depth 0
+//   D (reached depth 1)
+//      C (cache -- reached depth = 2)
+
+struct A {
+    b: B,
+    c: C,
+}
+
+struct B {
+    c: C,
+    a: Option<Box<A>>,
+}
+
+struct C {
+    b: Option<Box<B>>,
+}
+
+#[rustc_evaluate_where_clauses]
+fn test<X: Send>() {}
+
+fn main() {
+    test::<A>();
+}

src/test/ui/traits/issue-83538-tainted-cache-after-cycle.rs

@@ -0,0 +1,66 @@
+// Regression test for issue #83538. The problem here is that we have
+// two cycles:
+//
+// * `Ty` embeds `Box<Ty>` indirectly, which depends on `Global: 'static`, which is OkModuloRegions.
+// * But `Ty` also references `First`, which has a cycle on itself. That should just be `Ok`.
+//
+// But our caching mechanism was blending both cycles and giving the incorrect result.
+
+#![feature(rustc_attrs)]
+#![allow(bad_style)]
+
+struct First {
+    b: Vec<First>,
+}
+
+pub struct Second {
+    d: Vec<First>,
+}
+
+struct Third<f> {
+    g: Vec<f>,
+}
+
+enum Ty {
+    j(Fourth, Fifth, Sixth),
+}
+
+struct Fourth {
+    o: Vec<Ty>,
+}
+
+struct Fifth {
+    bounds: First,
+}
+
+struct Sixth {
+    p: Box<Ty>,
+}
+
+#[rustc_evaluate_where_clauses]
+fn forward()
+where
+    Vec<First>: Unpin,
+    Third<Ty>: Unpin,
+{
+}
+
+#[rustc_evaluate_where_clauses]
+fn reverse()
+where
+    Third<Ty>: Unpin,
+    Vec<First>: Unpin,
+{
+}
+
+fn main() {
+    // The only ERROR included here is the one that is totally wrong:
+
+    forward();
+    //~^ ERROR evaluate(Binder(TraitPredicate(<std::vec::Vec<First> as std::marker::Unpin>), [])) = Ok(EvaluatedToOkModuloRegions)
+    //~| ERROR evaluate(Binder(TraitPredicate(<Third<Ty> as std::marker::Unpin>), [])) = Ok(EvaluatedToOkModuloRegions)
+
+    reverse();
+    //~^ ERROR evaluate(Binder(TraitPredicate(<std::vec::Vec<First> as std::marker::Unpin>), [])) = Ok(EvaluatedToOkModuloRegions)
+    //~| ERROR evaluate(Binder(TraitPredicate(<Third<Ty> as std::marker::Unpin>), [])) = Ok(EvaluatedToOkModuloRegions)
+}

src/test/ui/traits/issue-83538-tainted-cache-after-cycle.stderr

@@ -0,0 +1,38 @@
+error: evaluate(Binder(TraitPredicate(<std::vec::Vec<First> as std::marker::Unpin>), [])) = Ok(EvaluatedToOkModuloRegions)
+  --> $DIR/issue-83538-tainted-cache-after-cycle.rs:59:5
+   |
+LL |     Vec<First>: Unpin,
+   |                 ----- predicate
+...
+LL |     forward();
+   |     ^^^^^^^
+
+error: evaluate(Binder(TraitPredicate(<Third<Ty> as std::marker::Unpin>), [])) = Ok(EvaluatedToOkModuloRegions)
+  --> $DIR/issue-83538-tainted-cache-after-cycle.rs:59:5
+   |
+LL |     Third<Ty>: Unpin,
+   |                ----- predicate
+...
+LL |     forward();
+   |     ^^^^^^^
+
+error: evaluate(Binder(TraitPredicate(<Third<Ty> as std::marker::Unpin>), [])) = Ok(EvaluatedToOkModuloRegions)
+  --> $DIR/issue-83538-tainted-cache-after-cycle.rs:63:5
+   |
+LL |     Third<Ty>: Unpin,
+   |                ----- predicate
+...
+LL |     reverse();
+   |     ^^^^^^^
+
+error: evaluate(Binder(TraitPredicate(<std::vec::Vec<First> as std::marker::Unpin>), [])) = Ok(EvaluatedToOkModuloRegions)
+  --> $DIR/issue-83538-tainted-cache-after-cycle.rs:63:5
+   |
+LL |     Vec<First>: Unpin,
+   |                 ----- predicate
+...
+LL |     reverse();
+   |     ^^^^^^^
+
+error: aborting due to 4 previous errors
+