Rollup merge of #109641 - compiler-errors:dont-elaborate-non-obl, r=oli-obk

Don't elaborate non-obligations into obligations

It's suspicious to elaborate a `PolyTraitRef` or `Predicate` into an `Obligation`, since the former does not have a param-env associated with it, but the latter does. This is a footgun that, while not being misused *currently* in the compiler, easily could be misused by someone less familiar with the elaborator's inner workings.

This PR just changes the API -- ideally, the elaborator wouldn't even have to deal with obligations if we're not elaborating obligations, but that would require a bit more abstraction than I could be bothered with today.

src/tools/clippy/clippy_lints/src/needless_pass_by_value.rs

@@ -124,9 +124,9 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
 
         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
             .filter(|p| !p.is_global())
-            .filter_map(|obligation| {
+            .filter_map(|pred| {
                 // Note that we do not want to deal with qualified predicates here.
-                match obligation.predicate.kind().no_bound_vars() {
+                match pred.kind().no_bound_vars() {
                     Some(ty::PredicateKind::Clause(ty::Clause::Trait(pred))) if pred.def_id() != sized_trait => {
                         Some(pred)
                     },

src/tools/clippy/clippy_utils/src/lib.rs

@@ -2106,7 +2106,6 @@ pub fn fn_has_unsatisfiable_preds(cx: &LateContext<'_>, did: DefId) -> bool {
     traits::impossible_predicates(
         cx.tcx,
         traits::elaborate_predicates(cx.tcx, predicates)
-            .map(|o| o.predicate)
             .collect::<Vec<_>>(),
     )
 }