Rollup merge of #136429 - fmease:gci-fix-def-site-checks, r=BoxyUwU

GCI: At their def site, actually wfcheck the where-clause & always eval free lifetime-generic constants

* 1st commit: Partially addresses [#136204](https://github.com/rust-lang/rust/issues/136204) by turning const eval errors from post to pre-mono for free lifetime-generic constants.
  * As the linked issue/comment states, on master there's a difference between `const _: () = panic!();` (pre-mono error) and `const _<'a>: () = panic!();` (post-mono error) which feels wrong.
  * With this PR, both become pre-mono ones!
* 2nd commit: Oof, yeah, I missed that in the initial impl!

This doesn't fully address #136204 because I still haven't figured out how & where to properly & best suppress const eval of free constants whose predicates don't hold at the def site. The motivating example is `const _UNUSED: () = () where for<'_delay> String: Copy;` which can also be found over at the tracking issue #113521.

r? compiler-errors or reassign

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -298,11 +298,9 @@ fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) -> Result<()
             check_item_fn(tcx, def_id, ident, item.span, sig.decl)
         }
         hir::ItemKind::Static(_, ty, ..) => {
-            check_item_type(tcx, def_id, ty.span, UnsizedHandling::Forbid)
-        }
-        hir::ItemKind::Const(_, ty, ..) => {
-            check_item_type(tcx, def_id, ty.span, UnsizedHandling::Forbid)
+            check_static_item(tcx, def_id, ty.span, UnsizedHandling::Forbid)
         }
+        hir::ItemKind::Const(_, ty, ..) => check_const_item(tcx, def_id, ty.span, item.span),
         hir::ItemKind::Struct(_, _, hir_generics) => {
             let res = check_type_defn(tcx, item, false);
             check_variances_for_type_defn(tcx, item, hir_generics);
@@ -366,7 +364,7 @@ fn check_foreign_item<'tcx>(
             check_item_fn(tcx, def_id, item.ident, item.span, sig.decl)
         }
         hir::ForeignItemKind::Static(ty, ..) => {
-            check_item_type(tcx, def_id, ty.span, UnsizedHandling::AllowIfForeignTail)
+            check_static_item(tcx, def_id, ty.span, UnsizedHandling::AllowIfForeignTail)
         }
         hir::ForeignItemKind::Type => Ok(()),
     }
@@ -1331,14 +1329,13 @@ enum UnsizedHandling {
     AllowIfForeignTail,
 }
 
-fn check_item_type(
+#[instrument(level = "debug", skip(tcx, ty_span, unsized_handling))]
+fn check_static_item(
     tcx: TyCtxt<'_>,
     item_id: LocalDefId,
     ty_span: Span,
     unsized_handling: UnsizedHandling,
 ) -> Result<(), ErrorGuaranteed> {
-    debug!("check_item_type: {:?}", item_id);
-
     enter_wf_checking_ctxt(tcx, ty_span, item_id, |wfcx| {
         let ty = tcx.type_of(item_id).instantiate_identity();
         let item_ty = wfcx.deeply_normalize(ty_span, Some(WellFormedLoc::Ty(item_id)), ty);
@@ -1388,6 +1385,34 @@ fn check_item_type(
     })
 }
 
+fn check_const_item(
+    tcx: TyCtxt<'_>,
+    def_id: LocalDefId,
+    ty_span: Span,
+    item_span: Span,
+) -> Result<(), ErrorGuaranteed> {
+    enter_wf_checking_ctxt(tcx, ty_span, def_id, |wfcx| {
+        let ty = tcx.type_of(def_id).instantiate_identity();
+        let ty = wfcx.deeply_normalize(ty_span, Some(WellFormedLoc::Ty(def_id)), ty);
+
+        wfcx.register_wf_obligation(ty_span, Some(WellFormedLoc::Ty(def_id)), ty.into());
+        wfcx.register_bound(
+            traits::ObligationCause::new(
+                ty_span,
+                wfcx.body_def_id,
+                ObligationCauseCode::SizedConstOrStatic,
+            ),
+            wfcx.param_env,
+            ty,
+            tcx.require_lang_item(LangItem::Sized, None),
+        );
+
+        check_where_clauses(wfcx, item_span, def_id);
+
+        Ok(())
+    })
+}
+
 #[instrument(level = "debug", skip(tcx, hir_self_ty, hir_trait_ref))]
 fn check_impl<'tcx>(
     tcx: TyCtxt<'tcx>,

compiler/rustc_hir_analysis/src/lib.rs

@@ -203,7 +203,9 @@ pub fn check_crate(tcx: TyCtxt<'_>) {
                 tcx.ensure_ok().eval_static_initializer(item_def_id);
                 check::maybe_check_static_with_link_section(tcx, item_def_id);
             }
-            DefKind::Const if tcx.generics_of(item_def_id).is_empty() => {
+            DefKind::Const if !tcx.generics_of(item_def_id).own_requires_monomorphization() => {
+                // FIXME(generic_const_items): Passing empty instead of identity args is fishy but
+                //                             seems to be fine for now. Revisit this!
                 let instance = ty::Instance::new_raw(item_def_id.into(), ty::GenericArgs::empty());
                 let cid = GlobalId { instance, promoted: None };
                 let typing_env = ty::TypingEnv::fully_monomorphized();

compiler/rustc_middle/src/mir/interpret/queries.rs

@@ -9,7 +9,6 @@ use super::{
     ReportedErrorInfo,
 };
 use crate::mir;
-use crate::query::TyCtxtEnsureOk;
 use crate::ty::{self, GenericArgs, TyCtxt, TypeVisitableExt};
 
 impl<'tcx> TyCtxt<'tcx> {
@@ -197,24 +196,3 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 }
-
-impl<'tcx> TyCtxtEnsureOk<'tcx> {
-    /// Evaluates a constant without providing any generic parameters. This is useful to evaluate consts
-    /// that can't take any generic arguments like const items or enum discriminants. If a
-    /// generic parameter is used within the constant `ErrorHandled::TooGeneric` will be returned.
-    #[instrument(skip(self), level = "debug")]
-    pub fn const_eval_poly(self, def_id: DefId) {
-        // In some situations def_id will have generic parameters within scope, but they aren't allowed
-        // to be used. So we can't use `Instance::mono`, instead we feed unresolved generic parameters
-        // into `const_eval` which will return `ErrorHandled::TooGeneric` if any of them are
-        // encountered.
-        let args = GenericArgs::identity_for_item(self.tcx, def_id);
-        let instance = ty::Instance::new_raw(def_id, self.tcx.erase_regions(args));
-        let cid = GlobalId { instance, promoted: None };
-        let typing_env = ty::TypingEnv::post_analysis(self.tcx, def_id);
-        // Const-eval shouldn't depend on lifetimes at all, so we can erase them, which should
-        // improve caching of queries.
-        let inputs = self.tcx.erase_regions(typing_env.as_query_input(cid));
-        self.eval_to_const_value_raw(inputs)
-    }
-}

compiler/rustc_monomorphize/src/collector.rs

@@ -1483,7 +1483,7 @@ impl<'v> RootCollector<'_, 'v> {
 
                 // But even just declaring them must collect the items they refer to
                 // unless their generics require monomorphization.
-                if !self.tcx.generics_of(id.owner_id).requires_monomorphization(self.tcx)
+                if !self.tcx.generics_of(id.owner_id).own_requires_monomorphization()
                     && let Ok(val) = self.tcx.const_eval_poly(id.owner_id.to_def_id())
                 {
                     collect_const_value(self.tcx, val, self.output);

tests/ui/generic-const-items/def-site-eval.fail.stderr

@@ -1,5 +1,5 @@
-error[E0080]: evaluation of `_::<'_>` failed
-  --> $DIR/def-site-eval.rs:14:20
+error[E0080]: evaluation of constant value failed
+  --> $DIR/def-site-eval.rs:13:20
    |
 LL | const _<'_a>: () = panic!();
    |                    ^^^^^^^^ evaluation panicked: explicit panic

tests/ui/generic-const-items/def-site-eval.rs

@@ -1,16 +1,15 @@
 //! Test that we only evaluate free const items (their def site to be clear)
 //! whose generics don't require monomorphization.
 #![feature(generic_const_items)]
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 
 //@ revisions: fail pass
-//@[fail] build-fail (we require monomorphization)
-//@[pass] build-pass (we require monomorphization)
+//@[pass] check-pass
 
 const _<_T>: () = panic!();
 const _<const _N: usize>: () = panic!();
 
 #[cfg(fail)]
-const _<'_a>: () = panic!(); //[fail]~ ERROR evaluation of `_::<'_>` failed
+const _<'_a>: () = panic!(); //[fail]~ ERROR evaluation of constant value failed
 
 fn main() {}

tests/ui/generic-const-items/def-site-predicates-wf.rs

@@ -0,0 +1,9 @@
+//! Ensure that we check the predicates for well-formedness at the definition site.
+#![feature(generic_const_items)]
+#![expect(incomplete_features)]
+
+const _: () = ()
+where
+    Vec<str>: Sized; //~ ERROR the size for values of type `str` cannot be known at compilation time
+
+fn main() {}

tests/ui/generic-const-items/def-site-predicates-wf.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:7:15
+   |
+LL |     Vec<str>: Sized;
+   |               ^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Vec`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.