Auto merge of #100726 - jswrenn:transmute, r=oli-obk

safe transmute: use `Assume` struct to provide analysis options

This task was left as a TODO in #92268; resolving it brings [`BikeshedIntrinsicFrom`](https://doc.rust-lang.org/nightly/core/mem/trait.BikeshedIntrinsicFrom.html) more in line with the API defined in [MCP411](https://github.com/rust-lang/compiler-team/issues/411).

**Before:**
```rust
pub unsafe trait BikeshedIntrinsicFrom<
    Src,
    Context,
    const ASSUME_ALIGNMENT: bool,
    const ASSUME_LIFETIMES: bool,
    const ASSUME_VALIDITY: bool,
    const ASSUME_VISIBILITY: bool,
> where
    Src: ?Sized,
{}
```
**After:**
```rust
pub unsafe trait BikeshedIntrinsicFrom<Src, Context, const ASSUME: Assume = { Assume::NOTHING }>
where
    Src: ?Sized,
{}
```

`Assume::visibility` has also been renamed to `Assume::safety`, as library safety invariants are what's actually being assumed; visibility is just the mechanism by which it is currently checked (and that may change).

r? `@oli-obk`

---

Related:
- https://github.com/rust-lang/compiler-team/issues/411
- https://github.com/rust-lang/rust/issues/99571

compiler/rustc_transmute/src/layout/dfa.rs

@@ -104,7 +104,6 @@ where
     }
 
     #[instrument(level = "debug")]
-    #[cfg_attr(feature = "rustc", allow(rustc::potential_query_instability))]
     pub(crate) fn from_nfa(nfa: Nfa<R>) -> Self {
         let Nfa { transitions: nfa_transitions, start: nfa_start, accepting: nfa_accepting } = nfa;
 

compiler/rustc_transmute/src/layout/nfa.rs

@@ -119,8 +119,6 @@ where
 
         let mut transitions: Map<State, Map<Transition<R>, Set<State>>> = self.transitions;
 
-        // the iteration order doesn't matter
-        #[cfg_attr(feature = "rustc", allow(rustc::potential_query_instability))]
         for (source, transition) in other.transitions {
             let fix_state = |state| if state == other.start { self.accepting } else { state };
             let entry = transitions.entry(fix_state(source)).or_default();
@@ -142,8 +140,6 @@ where
 
         let mut transitions: Map<State, Map<Transition<R>, Set<State>>> = self.transitions.clone();
 
-        // the iteration order doesn't matter
-        #[cfg_attr(feature = "rustc", allow(rustc::potential_query_instability))]
         for (&(mut source), transition) in other.transitions.iter() {
             // if source is starting state of `other`, replace with starting state of `self`
             if source == other.start {
@@ -152,8 +148,6 @@ where
             let entry = transitions.entry(source).or_default();
             for (edge, destinations) in transition {
                 let entry = entry.entry(edge.clone()).or_default();
-                // the iteration order doesn't matter
-                #[cfg_attr(feature = "rustc", allow(rustc::potential_query_instability))]
                 for &(mut destination) in destinations {
                     // if dest is accepting state of `other`, replace with accepting state of `self`
                     if destination == other.accepting {

compiler/rustc_transmute/src/lib.rs

@@ -6,11 +6,7 @@
 #[macro_use]
 extern crate tracing;
 
-#[cfg(feature = "rustc")]
-pub(crate) use rustc_data_structures::fx::{FxHashMap as Map, FxHashSet as Set};
-
-#[cfg(not(feature = "rustc"))]
-pub(crate) use std::collections::{HashMap as Map, HashSet as Set};
+pub(crate) use rustc_data_structures::fx::{FxIndexMap as Map, FxIndexSet as Set};
 
 pub(crate) mod layout;
 pub(crate) mod maybe_transmutable;
@@ -19,8 +15,8 @@ pub(crate) mod maybe_transmutable;
 pub struct Assume {
     pub alignment: bool,
     pub lifetimes: bool,
+    pub safety: bool,
     pub validity: bool,
-    pub visibility: bool,
 }
 
 /// The type encodes answers to the question: "Are these types transmutable?"
@@ -62,11 +58,17 @@ pub enum Reason {
 
 #[cfg(feature = "rustc")]
 mod rustc {
+    use super::*;
+
+    use rustc_hir::lang_items::LangItem;
     use rustc_infer::infer::InferCtxt;
     use rustc_macros::{TypeFoldable, TypeVisitable};
     use rustc_middle::traits::ObligationCause;
     use rustc_middle::ty::Binder;
+    use rustc_middle::ty::Const;
+    use rustc_middle::ty::ParamEnv;
     use rustc_middle::ty::Ty;
+    use rustc_middle::ty::TyCtxt;
 
     /// The source and destination types of a transmutation.
     #[derive(TypeFoldable, TypeVisitable, Debug, Clone, Copy)]
@@ -106,6 +108,54 @@ mod rustc {
             .answer()
         }
     }
+
+    impl Assume {
+        /// Constructs an `Assume` from a given const-`Assume`.
+        pub fn from_const<'tcx>(
+            tcx: TyCtxt<'tcx>,
+            param_env: ParamEnv<'tcx>,
+            c: Const<'tcx>,
+        ) -> Self {
+            use rustc_middle::ty::ScalarInt;
+            use rustc_middle::ty::TypeVisitable;
+            use rustc_span::symbol::sym;
+
+            let c = c.eval(tcx, param_env);
+
+            if let Some(err) = c.error_reported() {
+                return Self { alignment: true, lifetimes: true, safety: true, validity: true };
+            }
+
+            let adt_def = c.ty().ty_adt_def().expect("The given `Const` must be an ADT.");
+
+            assert_eq!(
+                tcx.require_lang_item(LangItem::TransmuteOpts, None),
+                adt_def.did(),
+                "The given `Const` was not marked with the `{}` lang item.",
+                LangItem::TransmuteOpts.name(),
+            );
+
+            let variant = adt_def.non_enum_variant();
+            let fields = c.to_valtree().unwrap_branch();
+
+            let get_field = |name| {
+                let (field_idx, _) = variant
+                    .fields
+                    .iter()
+                    .enumerate()
+                    .find(|(_, field_def)| name == field_def.name)
+                    .expect(&format!("There were no fields named `{name}`."));
+                fields[field_idx].unwrap_leaf() == ScalarInt::TRUE
+            };
+
+            Self {
+                alignment: get_field(sym::alignment),
+                lifetimes: get_field(sym::lifetimes),
+                safety: get_field(sym::safety),
+                validity: get_field(sym::validity),
+            }
+        }
+    }
 }
 
 #[cfg(feature = "rustc")]

compiler/rustc_transmute/src/maybe_transmutable/mod.rs

@@ -105,7 +105,7 @@ where
     #[inline(always)]
     #[instrument(level = "debug", skip(self), fields(src = ?self.src, dst = ?self.dst))]
     pub(crate) fn answer(self) -> Answer<<C as QueryContext>::Ref> {
-        let assume_visibility = self.assume.visibility;
+        let assume_visibility = self.assume.safety;
         let query_or_answer = self.map_layouts(|src, dst, scope, context| {
             // Remove all `Def` nodes from `src`, without checking their visibility.
             let src = src.prune(&|def| true);

compiler/rustc_transmute/src/maybe_transmutable/tests.rs

@@ -13,7 +13,7 @@ mod bool {
             layout::Tree::<Def, !>::bool(),
             layout::Tree::<Def, !>::bool(),
             (),
-            crate::Assume { alignment: false, lifetimes: false, validity: true, visibility: false },
+            crate::Assume { alignment: false, lifetimes: false, validity: true, safety: false },
             UltraMinimal,
         )
         .answer();
@@ -26,7 +26,7 @@ mod bool {
             layout::Dfa::<!>::bool(),
             layout::Dfa::<!>::bool(),
             (),
-            crate::Assume { alignment: false, lifetimes: false, validity: true, visibility: false },
+            crate::Assume { alignment: false, lifetimes: false, validity: true, safety: false },
             UltraMinimal,
         )
         .answer();