Auto merge of #133321 - compiler-errors:const-checker, r=wesleywiser

Get rid of HIR const checker

As far as I can tell, the HIR const checker was implemented in #66170 because we were not able to issue useful const error messages in the MIR const checker.

This seems to have changed in the last 5 years, probably due to work like #90532. I've tweaked the diagnostics slightly and think the error messages have gotten *better* in fact.

Thus I think the HIR const checker has reached the end of its usefulness, and we can retire it.

cc `@RalfJung`

Author: bors

Diff for: compiler/rustc_const_eval/messages.ftl

@@ -110,7 +110,7 @@ const_eval_extern_type_field = `extern type` field does not have a known offset
 const_eval_fn_ptr_call =
     function pointers need an RFC before allowed to be called in {const_eval_const_context}s
 const_eval_for_loop_into_iter_non_const =
-    cannot convert `{$ty}` into an iterator in {const_eval_const_context}s
+    cannot use `for` loop on `{$ty}` in {const_eval_const_context}s
 
 const_eval_frame_note = {$times ->
     [0] {const_eval_frame_note_inner}
@@ -324,11 +324,11 @@ const_eval_ptr_as_bytes_1 =
     this code performed an operation that depends on the underlying bytes representing a pointer
 const_eval_ptr_as_bytes_2 =
     the absolute address of a pointer is not known at compile-time, so such operations are not supported
-const_eval_question_branch_non_const =
-    `?` cannot determine the branch of `{$ty}` in {const_eval_const_context}s
 
+const_eval_question_branch_non_const =
+    `?` is not allowed on `{$ty}` in {const_eval_const_context}s
 const_eval_question_from_residual_non_const =
-    `?` cannot convert from residual of `{$ty}` in {const_eval_const_context}s
+    `?` is not allowed on `{$ty}` in {const_eval_const_context}s
 
 const_eval_range = in the range {$lo}..={$hi}
 const_eval_range_lower = greater or equal to {$lo}

Diff for: compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -180,8 +180,10 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                     };
                 }
 
-                let mut err = match kind {
-                    CallDesugaringKind::ForLoopIntoIter => {
+                // Don't point at the trait if this is a desugaring...
+                // FIXME(const_trait_impl): we could perhaps do this for `Iterator`.
+                match kind {
+                    CallDesugaringKind::ForLoopIntoIter | CallDesugaringKind::ForLoopNext => {
                         error!(NonConstForLoopIntoIter)
                     }
                     CallDesugaringKind::QuestionBranch => {
@@ -196,10 +198,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                     CallDesugaringKind::Await => {
                         error!(NonConstAwait)
                     }
-                };
-
-                diag_trait(&mut err, self_ty, kind.trait_def_id(tcx));
-                err
+                }
             }
             CallKind::FnCall { fn_trait_id, self_ty } => {
                 let note = match self_ty.kind() {

Diff for: compiler/rustc_interface/src/passes.rs

@@ -830,7 +830,6 @@ fn run_required_analyses(tcx: TyCtxt<'_>) {
                     tcx.ensure().check_mod_attrs(module);
                     tcx.ensure().check_mod_naked_functions(module);
                     tcx.ensure().check_mod_unstable_api_usage(module);
-                    tcx.ensure().check_mod_const_bodies(module);
                 });
             },
             {

Diff for: compiler/rustc_middle/src/query/mod.rs

@@ -958,11 +958,6 @@ rustc_queries! {
         desc { |tcx| "checking for unstable API usage in {}", describe_as_module(key, tcx) }
     }
 
-    /// Checks the const bodies in the module for illegal operations (e.g. `if` or `loop`).
-    query check_mod_const_bodies(key: LocalModDefId) {
-        desc { |tcx| "checking consts in {}", describe_as_module(key, tcx) }
-    }
-
     /// Checks the loops in the module.
     query check_mod_loops(key: LocalModDefId) {
         desc { |tcx| "checking loops in {}", describe_as_module(key, tcx) }

Diff for: compiler/rustc_middle/src/util/call_kind.rs

@@ -14,6 +14,8 @@ use crate::ty::{AssocItemContainer, GenericArgsRef, Instance, Ty, TyCtxt, Typing
 pub enum CallDesugaringKind {
     /// for _ in x {} calls x.into_iter()
     ForLoopIntoIter,
+    /// for _ in x {} calls iter.next()
+    ForLoopNext,
     /// x? calls x.branch()
     QuestionBranch,
     /// x? calls type_of(x)::from_residual()
@@ -28,6 +30,7 @@ impl CallDesugaringKind {
     pub fn trait_def_id(self, tcx: TyCtxt<'_>) -> DefId {
         match self {
             Self::ForLoopIntoIter => tcx.get_diagnostic_item(sym::IntoIterator).unwrap(),
+            Self::ForLoopNext => tcx.require_lang_item(LangItem::Iterator, None),
             Self::QuestionBranch | Self::TryBlockFromOutput => {
                 tcx.require_lang_item(LangItem::Try, None)
             }
@@ -121,6 +124,10 @@ pub fn call_kind<'tcx>(
             && fn_call_span.desugaring_kind() == Some(DesugaringKind::ForLoop)
         {
             Some((CallDesugaringKind::ForLoopIntoIter, method_args.type_at(0)))
+        } else if tcx.is_lang_item(method_did, LangItem::IteratorNext)
+            && fn_call_span.desugaring_kind() == Some(DesugaringKind::ForLoop)
+        {
+            Some((CallDesugaringKind::ForLoopNext, method_args.type_at(0)))
         } else if fn_call_span.desugaring_kind() == Some(DesugaringKind::QuestionMark) {
             if tcx.is_lang_item(method_did, LangItem::TryTraitBranch) {
                 Some((CallDesugaringKind::QuestionBranch, method_args.type_at(0)))

Diff for: compiler/rustc_passes/messages.ftl

@@ -709,8 +709,6 @@ passes_should_be_applied_to_trait =
     attribute should be applied to a trait
     .label = not a trait
 
-passes_skipping_const_checks = skipping const checks
-
 passes_stability_promotable =
     attribute cannot be applied to an expression
 

Diff for: compiler/rustc_passes/src/check_const.rs

@@ -1670,13 +1670,6 @@ pub(crate) struct ProcMacroBadSig {
     pub kind: ProcMacroKind,
 }
 
-#[derive(Diagnostic)]
-#[diag(passes_skipping_const_checks)]
-pub(crate) struct SkippingConstChecks {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(LintDiagnostic)]
 #[diag(passes_unreachable_due_to_uninhabited)]
 pub(crate) struct UnreachableDueToUninhabited<'desc, 'tcx> {

Diff for: compiler/rustc_passes/src/errors.rs

@@ -19,7 +19,6 @@ use rustc_middle::query::Providers;
 
 pub mod abi_test;
 mod check_attr;
-mod check_const;
 pub mod dead;
 mod debugger_visualizer;
 mod diagnostic_items;
@@ -43,7 +42,6 @@ rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 pub fn provide(providers: &mut Providers) {
     check_attr::provide(providers);
-    check_const::provide(providers);
     dead::provide(providers);
     debugger_visualizer::provide(providers);
     diagnostic_items::provide(providers);