Fix deferred cast checks using the wrong body for determining constness

compiler/rustc_hir_typeck/src/cast.rs

@@ -32,7 +32,7 @@ use rustc_ast::util::parser::ExprPrecedence;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::codes::*;
 use rustc_errors::{Applicability, Diag, ErrorGuaranteed};
-use rustc_hir::def_id::DefId;
+use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::{self as hir, ExprKind};
 use rustc_infer::infer::DefineOpaqueTypes;
 use rustc_macros::{TypeFoldable, TypeVisitable};
@@ -63,6 +63,7 @@ pub(crate) struct CastCheck<'tcx> {
     cast_ty: Ty<'tcx>,
     cast_span: Span,
     span: Span,
+    pub body_id: LocalDefId,
 }
 
 /// The kind of pointer and associated metadata (thin, length or vtable) - we
@@ -244,7 +245,8 @@ impl<'a, 'tcx> CastCheck<'tcx> {
         span: Span,
     ) -> Result<CastCheck<'tcx>, ErrorGuaranteed> {
         let expr_span = expr.span.find_ancestor_inside(span).unwrap_or(expr.span);
-        let check = CastCheck { expr, expr_ty, expr_span, cast_ty, cast_span, span };
+        let check =
+            CastCheck { expr, expr_ty, expr_span, cast_ty, cast_span, span, body_id: fcx.body_id };
 
         // For better error messages, check for some obviously unsized
         // cases now. We do a more thorough check at the end, once

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1,5 +1,5 @@
 use std::ops::Deref;
-use std::{fmt, iter, mem};
+use std::{fmt, iter};
 
 use itertools::Itertools;
 use rustc_data_structures::fx::FxIndexSet;
@@ -72,16 +72,13 @@ pub(crate) enum DivergingBlockBehavior {
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub(in super::super) fn check_casts(&mut self) {
-        // don't hold the borrow to deferred_cast_checks while checking to avoid borrow checker errors
-        // when writing to `self.param_env`.
-        let mut deferred_cast_checks = mem::take(&mut *self.deferred_cast_checks.borrow_mut());
-
+        let mut deferred_cast_checks = self.root_ctxt.deferred_cast_checks.borrow_mut();
         debug!("FnCtxt::check_casts: {} deferred checks", deferred_cast_checks.len());
         for cast in deferred_cast_checks.drain(..) {
+            let body_id = std::mem::replace(&mut self.body_id, cast.body_id);
             cast.check(self);
+            self.body_id = body_id;
         }
-
-        *self.deferred_cast_checks.borrow_mut() = deferred_cast_checks;
     }
 
     pub(in super::super) fn check_asms(&self) {

tests/ui/traits/const-traits/issue-103677.rs

@@ -1,5 +1,9 @@
 //@ check-pass
+//@ revisions: stock cnst
+#![cfg_attr(cnst, feature(const_trait_impl))]
 
-const _: fn(&String) = |s| { &*s as &str; };
+const _: fn(&String) = |s| {
+    &*s as &str;
+};
 
 fn main() {}