Auto merge of rust-lang#136429 - fmease:gci-fix-def-site-checks, r=<try>

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

1st commit: Partially addresses rust-lang#136204 by turning const eval errors from post to pre-mono for free lifetime-generic constants.
Re. 2nd commit: Oof, yeah, I missed that in the initial impl!

This doesn't fully address rust-lang#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 `#![feature(trivial_bounds)] const _UNUSED: () = () where String: Copy;` which can also be found over at the tracking issue rust-lang#113521.

r? compiler-errors or reassign

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -306,9 +306,7 @@ fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) -> Result<()
         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)
-        }
+        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);
@@ -1280,6 +1278,7 @@ enum UnsizedHandling {
     AllowIfForeignTail,
 }
 
+// FIXME(fmease): Rename to check_static_item once LTAs don't use it anymore (#136432)
 fn check_item_type(
     tcx: TyCtxt<'_>,
     item_id: LocalDefId,
@@ -1338,6 +1337,34 @@ 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.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::WellFormed(None),
+            ),
+            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

@@ -97,9 +97,8 @@ use rustc_abi::ExternAbi;
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_middle::middle;
-use rustc_middle::mir::interpret::GlobalId;
 use rustc_middle::query::Providers;
-use rustc_middle::ty::{self, Const, Ty, TyCtxt};
+use rustc_middle::ty::{Const, Ty, TyCtxt};
 use rustc_span::{ErrorGuaranteed, Span};
 use rustc_trait_selection::traits;
 
@@ -172,11 +171,8 @@ pub fn check_crate(tcx: TyCtxt<'_>) {
         let def_kind = tcx.def_kind(item_def_id);
         match def_kind {
             DefKind::Static { .. } => tcx.ensure_ok().eval_static_initializer(item_def_id),
-            DefKind::Const if tcx.generics_of(item_def_id).is_empty() => {
-                let instance = ty::Instance::new(item_def_id.into(), ty::GenericArgs::empty());
-                let cid = GlobalId { instance, promoted: None };
-                let typing_env = ty::TypingEnv::fully_monomorphized();
-                tcx.ensure_ok().eval_to_const_value_raw(typing_env.as_query_input(cid));
+            DefKind::Const if !tcx.generics_of(item_def_id).requires_monomorphization(tcx) => {
+                tcx.ensure_ok().const_eval_poly(item_def_id.into())
             }
             _ => (),
         }

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

@@ -209,7 +209,7 @@ impl<'tcx> TyCtxtEnsureOk<'tcx> {
         // 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(def_id, self.tcx.erase_regions(args));
+        let instance = ty::Instance::new(def_id, 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

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

@@ -1,8 +1,8 @@
 error[E0080]: evaluation of `_::<'_>` failed
-  --> $DIR/def-site-eval.rs:14:20
+  --> $DIR/def-site-eval.rs:13:20
    |
 LL | const _<'_a>: () = panic!();
-   |                    ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/def-site-eval.rs:14:20
+   |                    ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/def-site-eval.rs:13:20
    |
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 

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

@@ -4,7 +4,6 @@
 #![allow(incomplete_features)]
 
 //@ revisions: fail pass
-//@[fail] build-fail (we require monomorphization)
 //@[pass] build-pass (we require monomorphization)
 
 const _<_T>: () = panic!();

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)]
+#![allow(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`.