Auto merge of #123645 - matthiaskrgr:rollup-yd8d7f1, r=matthiaskrgr

Rollup of 9 pull requests

Successful merges:

 - #122781 (Fix argument ABI for overaligned structs on ppc64le)
 - #123367 (Safe Transmute: Compute transmutability from `rustc_target::abi::Layout`)
 - #123518 (Fix `ByMove` coroutine-closure shim (for 2021 precise closure capturing behavior))
 - #123547 (bootstrap: remove unused pub fns)
 - #123564 (Don't emit divide-by-zero panic paths in `StepBy::len`)
 - #123578 (Restore `pred_known_to_hold_modulo_regions`)
 - #123591 (Remove unnecessary cast from `LLVMRustGetInstrProfIncrementIntrinsic`)
 - #123632 (parser: reduce visibility of unnecessary public `UnmatchedDelim`)
 - #123635 (CFI: Fix ICE in KCFI non-associated function pointers)

r? `@ghost`
`@rustbot` modify labels: rollup

4 files changed, 105 insertions, 19 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 10c03387a5b..837b784f272 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -40,7 +40,7 @@ pub struct ImplCandidate<'tcx> {
 
 enum GetSafeTransmuteErrorAndReason {
     Silent,
-    Error { err_msg: String, safe_transmute_explanation: String },
+    Error { err_msg: String, safe_transmute_explanation: Option<String> },
 }
 
 struct UnsatisfiedConst(pub bool);
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
index afdc2c5a7f7..1b8b09ddda1 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
@@ -558,7 +558,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                                 GetSafeTransmuteErrorAndReason::Error {
                                     err_msg,
                                     safe_transmute_explanation,
-                                } => (err_msg, Some(safe_transmute_explanation)),
+                                } => (err_msg, safe_transmute_explanation),
                             }
                         } else {
                             (err_msg, None)
@@ -3068,28 +3068,33 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             return GetSafeTransmuteErrorAndReason::Silent;
         };
 
+        let dst = trait_ref.args.type_at(0);
+        let src = trait_ref.args.type_at(1);
+        let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
+
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
             obligation.cause,
             src_and_dst,
             assume,
         ) {
             Answer::No(reason) => {
-                let dst = trait_ref.args.type_at(0);
-                let src = trait_ref.args.type_at(1);
-                let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
                 let safe_transmute_explanation = match reason {
                     rustc_transmute::Reason::SrcIsNotYetSupported => {
-                        format!("analyzing the transmutability of `{src}` is not yet supported.")
+                        format!("analyzing the transmutability of `{src}` is not yet supported")
                     }
 
                     rustc_transmute::Reason::DstIsNotYetSupported => {
-                        format!("analyzing the transmutability of `{dst}` is not yet supported.")
+                        format!("analyzing the transmutability of `{dst}` is not yet supported")
                     }
 
                     rustc_transmute::Reason::DstIsBitIncompatible => {
                         format!("at least one value of `{src}` isn't a bit-valid value of `{dst}`")
                     }
 
+                    rustc_transmute::Reason::DstUninhabited => {
+                        format!("`{dst}` is uninhabited")
+                    }
+
                     rustc_transmute::Reason::DstMayHaveSafetyInvariants => {
                         format!("`{dst}` may carry safety invariants")
                     }
@@ -3135,14 +3140,23 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         format!("`{dst}` has an unknown layout")
                     }
                 };
-                GetSafeTransmuteErrorAndReason::Error { err_msg, safe_transmute_explanation }
+                GetSafeTransmuteErrorAndReason::Error {
+                    err_msg,
+                    safe_transmute_explanation: Some(safe_transmute_explanation),
+                }
             }
             // Should never get a Yes at this point! We already ran it before, and did not get a Yes.
             Answer::Yes => span_bug!(
                 span,
                 "Inconsistent rustc_transmute::is_transmutable(...) result, got Yes",
             ),
-            other => span_bug!(span, "Unsupported rustc_transmute::Answer variant: `{other:?}`"),
+            // Reached when a different obligation (namely `Freeze`) causes the
+            // transmutability analysis to fail. In this case, silence the
+            // transmutability error message in favor of that more specific
+            // error.
+            Answer::If(_) => {
+                GetSafeTransmuteErrorAndReason::Error { err_msg, safe_transmute_explanation: None }
+            }
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 2c8116b779b..98d5b466cd0 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -119,7 +119,9 @@ pub fn predicates_for_generics<'tcx>(
 
 /// Determines whether the type `ty` is known to meet `bound` and
 /// returns true if so. Returns false if `ty` either does not meet
-/// `bound` or is not known to meet bound.
+/// `bound` or is not known to meet bound (note that this is
+/// conservative towards *no impl*, which is the opposite of the
+/// `evaluate` methods).
 pub fn type_known_to_meet_bound_modulo_regions<'tcx>(
     infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
@@ -127,8 +129,50 @@ pub fn type_known_to_meet_bound_modulo_regions<'tcx>(
     def_id: DefId,
 ) -> bool {
     let trait_ref = ty::TraitRef::new(infcx.tcx, def_id, [ty]);
-    let obligation = Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, trait_ref);
-    infcx.predicate_must_hold_modulo_regions(&obligation)
+    pred_known_to_hold_modulo_regions(infcx, param_env, trait_ref)
+}
+
+/// FIXME(@lcnr): this function doesn't seem right and shouldn't exist?
+///
+/// Ping me on zulip if you want to use this method and need help with finding
+/// an appropriate replacement.
+#[instrument(level = "debug", skip(infcx, param_env, pred), ret)]
+fn pred_known_to_hold_modulo_regions<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    pred: impl ToPredicate<'tcx>,
+) -> bool {
+    let obligation = Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, pred);
+
+    let result = infcx.evaluate_obligation_no_overflow(&obligation);
+    debug!(?result);
+
+    if result.must_apply_modulo_regions() {
+        true
+    } else if result.may_apply() {
+        // Sometimes obligations are ambiguous because the recursive evaluator
+        // is not smart enough, so we fall back to fulfillment when we're not certain
+        // that an obligation holds or not. Even still, we must make sure that
+        // the we do no inference in the process of checking this obligation.
+        let goal = infcx.resolve_vars_if_possible((obligation.predicate, obligation.param_env));
+        infcx.probe(|_| {
+            let ocx = ObligationCtxt::new(infcx);
+            ocx.register_obligation(obligation);
+
+            let errors = ocx.select_all_or_error();
+            match errors.as_slice() {
+                // Only known to hold if we did no inference.
+                [] => infcx.shallow_resolve(goal) == goal,
+
+                errors => {
+                    debug!(?errors);
+                    false
+                }
+            }
+        })
+    } else {
+        false
+    }
 }
 
 #[instrument(level = "debug", skip(tcx, elaborated_env))]
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index f98a1714a3f..25ba985397e 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -314,12 +314,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     .flat_map(|cond| flatten_answer_tree(tcx, obligation, predicate, cond))
                     .collect(),
                 Condition::IfTransmutable { src, dst } => {
-                    let trait_def_id = obligation.predicate.def_id();
+                    let transmute_trait = obligation.predicate.def_id();
                     let assume_const = predicate.trait_ref.args.const_at(2);
-                    let make_obl = |from_ty, to_ty| {
-                        let trait_ref1 = ty::TraitRef::new(
+                    let make_transmute_obl = |from_ty, to_ty| {
+                        let trait_ref = ty::TraitRef::new(
                             tcx,
-                            trait_def_id,
+                            transmute_trait,
                             [
                                 ty::GenericArg::from(to_ty),
                                 ty::GenericArg::from(from_ty),
@@ -331,17 +331,45 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             obligation.cause.clone(),
                             obligation.recursion_depth + 1,
                             obligation.param_env,
-                            trait_ref1,
+                            trait_ref,
                         )
                     };
 
+                    let make_freeze_obl = |ty| {
+                        let trait_ref = ty::TraitRef::new(
+                            tcx,
+                            tcx.lang_items().freeze_trait().unwrap(),
+                            [ty::GenericArg::from(ty)],
+                        );
+                        Obligation::with_depth(
+                            tcx,
+                            obligation.cause.clone(),
+                            obligation.recursion_depth + 1,
+                            obligation.param_env,
+                            trait_ref,
+                        )
+                    };
+
+                    let mut obls = vec![];
+
+                    // If the source is a shared reference, it must be `Freeze`;
+                    // otherwise, transmuting could lead to data races.
+                    if src.mutability == Mutability::Not {
+                        obls.extend([make_freeze_obl(src.ty), make_freeze_obl(dst.ty)])
+                    }
+
                     // If Dst is mutable, check bidirectionally.
                     // For example, transmuting bool -> u8 is OK as long as you can't update that u8
                     // to be > 1, because you could later transmute the u8 back to a bool and get UB.
                     match dst.mutability {
-                        Mutability::Not => vec![make_obl(src.ty, dst.ty)],
-                        Mutability::Mut => vec![make_obl(src.ty, dst.ty), make_obl(dst.ty, src.ty)],
+                        Mutability::Not => obls.push(make_transmute_obl(src.ty, dst.ty)),
+                        Mutability::Mut => obls.extend([
+                            make_transmute_obl(src.ty, dst.ty),
+                            make_transmute_obl(dst.ty, src.ty),
+                        ]),
                     }
+
+                    obls
                 }
             }
         }