Fast path for WF goals in new solver

compiler/rustc_infer/src/infer/mod.rs

@@ -1046,6 +1046,13 @@ impl<'tcx> InferCtxt<'tcx> {
         }
     }
 
+    pub fn shallow_resolve_term(&self, term: ty::Term<'tcx>) -> ty::Term<'tcx> {
+        match term.kind() {
+            ty::TermKind::Ty(ty) => self.shallow_resolve(ty).into(),
+            ty::TermKind::Const(ct) => self.shallow_resolve_const(ct).into(),
+        }
+    }
+
     pub fn root_var(&self, var: ty::TyVid) -> ty::TyVid {
         self.inner.borrow_mut().type_variables().root_var(var)
     }

compiler/rustc_middle/src/ty/consts.rs

@@ -144,6 +144,19 @@ impl<'tcx> Const<'tcx> {
         let reported = tcx.dcx().span_delayed_bug(span, msg);
         Const::new_error(tcx, reported)
     }
+
+    pub fn is_trivially_wf(self) -> bool {
+        match self.kind() {
+            ty::ConstKind::Param(_) | ty::ConstKind::Placeholder(_) | ty::ConstKind::Bound(..) => {
+                true
+            }
+            ty::ConstKind::Infer(_)
+            | ty::ConstKind::Unevaluated(..)
+            | ty::ConstKind::Value(_)
+            | ty::ConstKind::Error(_)
+            | ty::ConstKind::Expr(_) => false,
+        }
+    }
 }
 
 impl<'tcx> rustc_type_ir::inherent::Const<TyCtxt<'tcx>> for Const<'tcx> {

compiler/rustc_middle/src/ty/mod.rs

@@ -651,6 +651,13 @@ impl<'tcx> Term<'tcx> {
         }
     }
 
+    pub fn is_trivially_wf(&self, tcx: TyCtxt<'tcx>) -> bool {
+        match self.kind() {
+            TermKind::Ty(ty) => ty.is_trivially_wf(tcx),
+            TermKind::Const(ct) => ct.is_trivially_wf(),
+        }
+    }
+
     /// Iterator that walks `self` and any types reachable from
     /// `self`, in depth-first order. Note that just walks the types
     /// that appear in `self`, it does not descend into the fields of

compiler/rustc_middle/src/ty/sty.rs

@@ -1888,6 +1888,50 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    pub fn is_trivially_wf(self, tcx: TyCtxt<'tcx>) -> bool {
+        match *self.kind() {
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Str
+            | ty::Never
+            | ty::Param(_)
+            | ty::Placeholder(_)
+            | ty::Bound(..) => true,
+
+            ty::Slice(ty) => ty.is_trivially_wf(tcx) && ty.is_trivially_sized(tcx),
+            ty::RawPtr(ty, _) => ty.is_trivially_wf(tcx),
+
+            ty::FnPtr(sig_tys, _) => {
+                sig_tys.skip_binder().inputs_and_output.iter().all(|ty| ty.is_trivially_wf(tcx))
+            }
+            ty::Ref(_, ty, _) => ty.is_global() && ty.is_trivially_wf(tcx),
+
+            ty::Infer(infer) => match infer {
+                ty::TyVar(_) => false,
+                ty::IntVar(_) | ty::FloatVar(_) => true,
+                ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => true,
+            },
+
+            ty::Adt(_, _)
+            | ty::Tuple(_)
+            | ty::Array(..)
+            | ty::Foreign(_)
+            | ty::Pat(_, _)
+            | ty::FnDef(..)
+            | ty::UnsafeBinder(..)
+            | ty::Dynamic(..)
+            | ty::Closure(..)
+            | ty::CoroutineClosure(..)
+            | ty::Coroutine(..)
+            | ty::CoroutineWitness(..)
+            | ty::Alias(..)
+            | ty::Error(_) => false,
+        }
+    }
+
     /// If `self` is a primitive, return its [`Symbol`].
     pub fn primitive_symbol(self) -> Option<Symbol> {
         match self.kind() {

compiler/rustc_trait_selection/src/solve/delegate.rs

@@ -136,6 +136,15 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
                     None
                 }
             }
+            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
+                if arg.is_trivially_wf(self.tcx) {
+                    Some(Certainty::Yes)
+                } else if self.shallow_resolve_term(arg).is_infer() {
+                    Some(Certainty::AMBIGUOUS)
+                } else {
+                    None
+                }
+            }
             _ => None,
         }
     }

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -12,7 +12,7 @@ use rustc_middle::bug;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{self, Binder, Const, GenericArgsRef, TypeVisitableExt, TypingMode};
-use thin_vec::ThinVec;
+use thin_vec::{ThinVec, thin_vec};
 use tracing::{debug, debug_span, instrument};
 
 use super::effects::{self, HostEffectObligation};
@@ -336,7 +336,7 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
         let infcx = self.selcx.infcx;
 
         if sizedness_fast_path(infcx.tcx, obligation.predicate) {
-            return ProcessResult::Changed(thin_vec::thin_vec![]);
+            return ProcessResult::Changed(thin_vec![]);
         }
 
         if obligation.predicate.has_aliases() {
@@ -541,6 +541,10 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 }
 
                 ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(term)) => {
+                    if term.is_trivially_wf(self.selcx.tcx()) {
+                        return ProcessResult::Changed(thin_vec![]);
+                    }
+
                     match wf::obligations(
                         self.selcx.infcx,
                         obligation.param_env,

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -660,6 +660,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
 
                 ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(term)) => {
+                    if term.is_trivially_wf(self.tcx()) {
+                        return Ok(EvaluatedToOk);
+                    }
+
                     // So, there is a bit going on here. First, `WellFormed` predicates
                     // are coinductive, like trait predicates with auto traits.
                     // This means that we need to detect if we have recursively