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, that's embarrassing! How could I miss 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

Author: bors

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -307,7 +307,11 @@ fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) -> Result<()
             check_item_type(tcx, def_id, ty.span, UnsizedHandling::Forbid)
         }
         hir::ItemKind::Const(ty, ..) => {
-            check_item_type(tcx, def_id, ty.span, UnsizedHandling::Forbid)
+            let res = check_item_type(tcx, def_id, ty.span, UnsizedHandling::Forbid);
+            res.and(enter_wf_checking_ctxt(tcx, item.span, def_id, |wfcx| {
+                check_where_clauses(wfcx, item.span, def_id);
+                Ok(())
+            }))
         }
         hir::ItemKind::Struct(_, hir_generics) => {
             let res = check_type_defn(tcx, item, false);

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::Span;
 use rustc_trait_selection::traits;
 
@@ -168,11 +167,8 @@ pub fn check_crate(tcx: TyCtxt<'_>) {
         let def_kind = tcx.def_kind(item_def_id);
         match def_kind {
             DefKind::Static { .. } => tcx.ensure().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().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().const_eval_poly(item_def_id.into())
             }
             _ => (),
         }

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.current.stderr

@@ -0,0 +1,22 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:11:5
+   |
+LL |     <Vec<str> as Discard>::Output:;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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[E0080]: evaluation of constant value failed
+  --> $DIR/def-site-predicates-wf.rs:9:1
+   |
+LL | / const _: () = ()
+LL | | where
+LL | |     <Vec<str> as Discard>::Output:;
+   | |___________________________________^ entering unreachable code
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0080, E0277.
+For more information about an error, try `rustc --explain E0080`.

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

@@ -0,0 +1,32 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:11:5
+   |
+LL |     <Vec<str> as Discard>::Output:;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:17:36
+   |
+LL |     <Vec<str> as Discard>::Output: 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[E0080]: evaluation of constant value failed
+  --> $DIR/def-site-predicates-wf.rs:9:1
+   |
+LL | / const _: () = ()
+LL | | where
+LL | |     <Vec<str> as Discard>::Output:;
+   | |___________________________________^ entering unreachable code
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0080, E0277.
+For more information about an error, try `rustc --explain E0080`.

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

@@ -0,0 +1,22 @@
+//! Ensure that we check the predicates at the definition site for well-formedness.
+#![feature(generic_const_items)]
+#![allow(incomplete_features)]
+
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+
+const _: () = ()
+where
+    <Vec<str> as Discard>::Output:; //~ ERROR the size for values of type `str` cannot be known at compilation time
+//~^^^ ERROR evaluation of constant value failed
+
+// FIXME(#100041): This should error in the current solver, too.
+const _: () = ()
+where
+    <Vec<str> as Discard>::Output: Sized; //[next]~ ERROR the size for values of type `str` cannot be known at compilation time
+
+trait Discard { type Output; }
+impl<T> Discard for T { type Output = (); }
+
+fn main() {}