rustc_const_eval/interpret/
intern.rs

1//! This module specifies the type based interner for constants.
2//!
3//! After a const evaluation has computed a value, before we destroy the const evaluator's session
4//! memory, we need to extract all memory allocations to the global memory pool so they stay around.
5//!
6//! In principle, this is not very complicated: we recursively walk the final value, follow all the
7//! pointers, and move all reachable allocations to the global `tcx` memory. The only complication
8//! is picking the right mutability: the outermost allocation generally has a clear mutability, but
9//! what about the other allocations it points to that have also been created with this value? We
10//! don't want to do guesswork here. The rules are: `static`, `const`, and promoted can only create
11//! immutable allocations that way. `static mut` can be initialized with expressions like `&mut 42`,
12//! so all inner allocations are marked mutable. Some of them could potentially be made immutable,
13//! but that would require relying on type information, and given how many ways Rust has to lie
14//! about type information, we want to avoid doing that.
15
16use hir::def::DefKind;
17use rustc_ast::Mutability;
18use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
19use rustc_hir as hir;
20use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
21use rustc_middle::mir::interpret::{ConstAllocation, CtfeProvenance, InterpResult};
22use rustc_middle::query::TyCtxtAt;
23use rustc_middle::span_bug;
24use rustc_middle::ty::layout::TyAndLayout;
25use rustc_span::def_id::LocalDefId;
26use rustc_span::sym;
27use tracing::{instrument, trace};
28
29use super::{
30    AllocId, Allocation, InterpCx, MPlaceTy, Machine, MemoryKind, PlaceTy, err_ub, interp_ok,
31};
32use crate::const_eval;
33use crate::const_eval::DummyMachine;
34use crate::errors::NestedStaticInThreadLocal;
35
36pub trait CompileTimeMachine<'tcx, T> = Machine<
37        'tcx,
38        MemoryKind = T,
39        Provenance = CtfeProvenance,
40        ExtraFnVal = !,
41        FrameExtra = (),
42        AllocExtra = (),
43        MemoryMap = FxIndexMap<AllocId, (MemoryKind<T>, Allocation)>,
44    > + HasStaticRootDefId;
45
46pub trait HasStaticRootDefId {
47    /// Returns the `DefId` of the static item that is currently being evaluated.
48    /// Used for interning to be able to handle nested allocations.
49    fn static_def_id(&self) -> Option<LocalDefId>;
50}
51
52impl HasStaticRootDefId for const_eval::CompileTimeMachine<'_> {
53    fn static_def_id(&self) -> Option<LocalDefId> {
54        Some(self.static_root_ids?.1)
55    }
56}
57
58/// Intern an allocation. Returns `Err` if the allocation does not exist in the local memory.
59///
60/// `mutability` can be used to force immutable interning: if it is `Mutability::Not`, the
61/// allocation is interned immutably; if it is `Mutability::Mut`, then the allocation *must be*
62/// already mutable (as a sanity check).
63///
64/// Returns an iterator over all relocations referred to by this allocation.
65fn intern_shallow<'tcx, T, M: CompileTimeMachine<'tcx, T>>(
66    ecx: &mut InterpCx<'tcx, M>,
67    alloc_id: AllocId,
68    mutability: Mutability,
69) -> Result<impl Iterator<Item = CtfeProvenance> + 'tcx, ()> {
70    trace!("intern_shallow {:?}", alloc_id);
71    // remove allocation
72    // FIXME(#120456) - is `swap_remove` correct?
73    let Some((_kind, mut alloc)) = ecx.memory.alloc_map.swap_remove(&alloc_id) else {
74        return Err(());
75    };
76    // Set allocation mutability as appropriate. This is used by LLVM to put things into
77    // read-only memory, and also by Miri when evaluating other globals that
78    // access this one.
79    match mutability {
80        Mutability::Not => {
81            alloc.mutability = Mutability::Not;
82        }
83        Mutability::Mut => {
84            // This must be already mutable, we won't "un-freeze" allocations ever.
85            assert_eq!(alloc.mutability, Mutability::Mut);
86        }
87    }
88    // link the alloc id to the actual allocation
89    let alloc = ecx.tcx.mk_const_alloc(alloc);
90    if let Some(static_id) = ecx.machine.static_def_id() {
91        intern_as_new_static(ecx.tcx, static_id, alloc_id, alloc);
92    } else {
93        ecx.tcx.set_alloc_id_memory(alloc_id, alloc);
94    }
95    Ok(alloc.0.0.provenance().ptrs().iter().map(|&(_, prov)| prov))
96}
97
98/// Creates a new `DefId` and feeds all the right queries to make this `DefId`
99/// appear as if it were a user-written `static` (though it has no HIR).
100fn intern_as_new_static<'tcx>(
101    tcx: TyCtxtAt<'tcx>,
102    static_id: LocalDefId,
103    alloc_id: AllocId,
104    alloc: ConstAllocation<'tcx>,
105) {
106    let feed = tcx.create_def(
107        static_id,
108        Some(sym::nested),
109        DefKind::Static { safety: hir::Safety::Safe, mutability: alloc.0.mutability, nested: true },
110    );
111    tcx.set_nested_alloc_id_static(alloc_id, feed.def_id());
112
113    if tcx.is_thread_local_static(static_id.into()) {
114        tcx.dcx().emit_err(NestedStaticInThreadLocal { span: tcx.def_span(static_id) });
115    }
116
117    // These do not inherit the codegen attrs of the parent static allocation, since
118    // it doesn't make sense for them to inherit their `#[no_mangle]` and `#[link_name = ..]`
119    // and the like.
120    feed.codegen_fn_attrs(CodegenFnAttrs::new());
121
122    feed.eval_static_initializer(Ok(alloc));
123    feed.generics_of(tcx.generics_of(static_id).clone());
124    feed.def_ident_span(tcx.def_ident_span(static_id));
125    feed.explicit_predicates_of(tcx.explicit_predicates_of(static_id));
126    feed.feed_hir();
127}
128
129/// How a constant value should be interned.
130#[derive(Copy, Clone, Debug, PartialEq, Hash, Eq)]
131pub enum InternKind {
132    /// The `mutability` of the static, ignoring the type which may have interior mutability.
133    Static(hir::Mutability),
134    /// A `const` item
135    Constant,
136    Promoted,
137}
138
139#[derive(Debug)]
140pub enum InternResult {
141    FoundBadMutablePointer,
142    FoundDanglingPointer,
143}
144
145/// Intern `ret` and everything it references.
146///
147/// This *cannot raise an interpreter error*. Doing so is left to validation, which
148/// tracks where in the value we are and thus can show much better error messages.
149///
150/// For `InternKind::Static` the root allocation will not be interned, but must be handled by the caller.
151#[instrument(level = "debug", skip(ecx))]
152pub fn intern_const_alloc_recursive<'tcx, M: CompileTimeMachine<'tcx, const_eval::MemoryKind>>(
153    ecx: &mut InterpCx<'tcx, M>,
154    intern_kind: InternKind,
155    ret: &MPlaceTy<'tcx>,
156) -> Result<(), InternResult> {
157    // We are interning recursively, and for mutability we are distinguishing the "root" allocation
158    // that we are starting in, and all other allocations that we are encountering recursively.
159    let (base_mutability, inner_mutability, is_static) = match intern_kind {
160        InternKind::Constant | InternKind::Promoted => {
161            // Completely immutable. Interning anything mutably here can only lead to unsoundness,
162            // since all consts are conceptually independent values but share the same underlying
163            // memory.
164            (Mutability::Not, Mutability::Not, false)
165        }
166        InternKind::Static(Mutability::Not) => {
167            (
168                // Outermost allocation is mutable if `!Freeze`.
169                if ret.layout.ty.is_freeze(*ecx.tcx, ecx.typing_env) {
170                    Mutability::Not
171                } else {
172                    Mutability::Mut
173                },
174                // Inner allocations are never mutable. They can only arise via the "tail
175                // expression" / "outer scope" rule, and we treat them consistently with `const`.
176                Mutability::Not,
177                true,
178            )
179        }
180        InternKind::Static(Mutability::Mut) => {
181            // Just make everything mutable. We accept code like
182            // `static mut X = &mut [42]`, so even inner allocations need to be mutable.
183            (Mutability::Mut, Mutability::Mut, true)
184        }
185    };
186
187    // Intern the base allocation, and initialize todo list for recursive interning.
188    let base_alloc_id = ret.ptr().provenance.unwrap().alloc_id();
189    trace!(?base_alloc_id, ?base_mutability);
190    // First we intern the base allocation, as it requires a different mutability.
191    // This gives us the initial set of nested allocations, which will then all be processed
192    // recursively in the loop below.
193    let mut todo: Vec<_> = if is_static {
194        // Do not steal the root allocation, we need it later to create the return value of `eval_static_initializer`.
195        // But still change its mutability to match the requested one.
196        let alloc = ecx.memory.alloc_map.get_mut(&base_alloc_id).unwrap();
197        alloc.1.mutability = base_mutability;
198        alloc.1.provenance().ptrs().iter().map(|&(_, prov)| prov).collect()
199    } else {
200        intern_shallow(ecx, base_alloc_id, base_mutability).unwrap().collect()
201    };
202    // We need to distinguish "has just been interned" from "was already in `tcx`",
203    // so we track this in a separate set.
204    let mut just_interned: FxHashSet<_> = std::iter::once(base_alloc_id).collect();
205    // Whether we encountered a bad mutable pointer.
206    // We want to first report "dangling" and then "mutable", so we need to delay reporting these
207    // errors.
208    let mut result = Ok(());
209
210    // Keep interning as long as there are things to intern.
211    // We show errors if there are dangling pointers, or mutable pointers in immutable contexts
212    // (i.e., everything except for `static mut`). When these errors affect references, it is
213    // unfortunate that we show these errors here and not during validation, since validation can
214    // show much nicer errors. However, we do need these checks to be run on all pointers, including
215    // raw pointers, so we cannot rely on validation to catch them -- and since interning runs
216    // before validation, and interning doesn't know the type of anything, this means we can't show
217    // better errors. Maybe we should consider doing validation before interning in the future.
218    while let Some(prov) = todo.pop() {
219        trace!(?prov);
220        let alloc_id = prov.alloc_id();
221
222        if base_alloc_id == alloc_id && is_static {
223            // This is a pointer to the static itself. It's ok for a static to refer to itself,
224            // even mutably. Whether that mutable pointer is legal at all is checked in validation.
225            // See tests/ui/statics/recursive_interior_mut.rs for how such a situation can occur.
226            // We also already collected all the nested allocations, so there's no need to do that again.
227            continue;
228        }
229
230        // Ensure that this is derived from a shared reference. Crucially, we check this *before*
231        // checking whether the `alloc_id` has already been interned. The point of this check is to
232        // ensure that when there are multiple pointers to the same allocation, they are *all*
233        // derived from a shared reference. Therefore it would be bad if we only checked the first
234        // pointer to any given allocation.
235        // (It is likely not possible to actually have multiple pointers to the same allocation,
236        // so alternatively we could also check that and ICE if there are multiple such pointers.)
237        // See <https://github.com/rust-lang/rust/pull/128543> for why we are checking for "shared
238        // reference" and not "immutable", i.e., for why we are allowing interior-mutable shared
239        // references: they can actually be created in safe code while pointing to apparently
240        // "immutable" values, via promotion or tail expression lifetime extension of
241        // `&None::<Cell<T>>`.
242        // We also exclude promoteds from this as `&mut []` can be promoted, which is a mutable
243        // reference pointing to an immutable (zero-sized) allocation. We rely on the promotion
244        // analysis not screwing up to ensure that it is sound to intern promoteds as immutable.
245        if intern_kind != InternKind::Promoted
246            && inner_mutability == Mutability::Not
247            && !prov.shared_ref()
248        {
249            let is_already_global = ecx.tcx.try_get_global_alloc(alloc_id).is_some();
250            if is_already_global && !just_interned.contains(&alloc_id) {
251                // This is a pointer to some memory from another constant. We encounter mutable
252                // pointers to such memory since we do not always track immutability through
253                // these "global" pointers. Allowing them is harmless; the point of these checks
254                // during interning is to justify why we intern the *new* allocations immutably,
255                // so we can completely ignore existing allocations.
256                // We can also skip the rest of this loop iteration, since after all it is already
257                // interned.
258                continue;
259            }
260            // If this is a dangling pointer, that's actually fine -- the problematic case is
261            // when there is memory there that someone might expect to be mutable, but we make it immutable.
262            let dangling = !is_already_global && !ecx.memory.alloc_map.contains_key(&alloc_id);
263            if !dangling {
264                // Found a mutable reference inside a const where inner allocations should be
265                // immutable.
266                if !ecx.tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you {
267                    span_bug!(
268                        ecx.tcx.span,
269                        "the static const safety checks accepted mutable references they should not have accepted"
270                    );
271                }
272                // Prefer dangling pointer errors over mutable pointer errors
273                if result.is_ok() {
274                    result = Err(InternResult::FoundBadMutablePointer);
275                }
276            }
277        }
278        if ecx.tcx.try_get_global_alloc(alloc_id).is_some() {
279            // Already interned.
280            debug_assert!(!ecx.memory.alloc_map.contains_key(&alloc_id));
281            continue;
282        }
283        // We always intern with `inner_mutability`, and furthermore we ensured above that if
284        // that is "immutable", then there are *no* mutable pointers anywhere in the newly
285        // interned memory -- justifying that we can indeed intern immutably. However this also
286        // means we can *not* easily intern immutably here if `prov.immutable()` is true and
287        // `inner_mutability` is `Mut`: there might be other pointers to that allocation, and
288        // we'd have to somehow check that they are *all* immutable before deciding that this
289        // allocation can be made immutable. In the future we could consider analyzing all
290        // pointers before deciding which allocations can be made immutable; but for now we are
291        // okay with losing some potential for immutability here. This can anyway only affect
292        // `static mut`.
293        just_interned.insert(alloc_id);
294        match intern_shallow(ecx, alloc_id, inner_mutability) {
295            Ok(nested) => todo.extend(nested),
296            Err(()) => {
297                ecx.tcx.dcx().delayed_bug("found dangling pointer during const interning");
298                result = Err(InternResult::FoundDanglingPointer);
299            }
300        }
301    }
302    result
303}
304
305/// Intern `ret`. This function assumes that `ret` references no other allocation.
306#[instrument(level = "debug", skip(ecx))]
307pub fn intern_const_alloc_for_constprop<'tcx, T, M: CompileTimeMachine<'tcx, T>>(
308    ecx: &mut InterpCx<'tcx, M>,
309    alloc_id: AllocId,
310) -> InterpResult<'tcx, ()> {
311    if ecx.tcx.try_get_global_alloc(alloc_id).is_some() {
312        // The constant is already in global memory. Do nothing.
313        return interp_ok(());
314    }
315    // Move allocation to `tcx`.
316    if let Some(_) =
317        (intern_shallow(ecx, alloc_id, Mutability::Not).map_err(|()| err_ub!(DeadLocal))?).next()
318    {
319        // We are not doing recursive interning, so we don't currently support provenance.
320        // (If this assertion ever triggers, we should just implement a
321        // proper recursive interning loop -- or just call `intern_const_alloc_recursive`.
322        panic!("`intern_const_alloc_for_constprop` called on allocation with nested provenance")
323    }
324    interp_ok(())
325}
326
327impl<'tcx> InterpCx<'tcx, DummyMachine> {
328    /// A helper function that allocates memory for the layout given and gives you access to mutate
329    /// it. Once your own mutation code is done, the backing `Allocation` is removed from the
330    /// current `Memory` and interned as read-only into the global memory.
331    pub fn intern_with_temp_alloc(
332        &mut self,
333        layout: TyAndLayout<'tcx>,
334        f: impl FnOnce(
335            &mut InterpCx<'tcx, DummyMachine>,
336            &PlaceTy<'tcx, CtfeProvenance>,
337        ) -> InterpResult<'tcx, ()>,
338    ) -> InterpResult<'tcx, AllocId> {
339        // `allocate` picks a fresh AllocId that we will associate with its data below.
340        let dest = self.allocate(layout, MemoryKind::Stack)?;
341        f(self, &dest.clone().into())?;
342        let alloc_id = dest.ptr().provenance.unwrap().alloc_id(); // this was just allocated, it must have provenance
343        for prov in intern_shallow(self, alloc_id, Mutability::Not).unwrap() {
344            // We are not doing recursive interning, so we don't currently support provenance.
345            // (If this assertion ever triggers, we should just implement a
346            // proper recursive interning loop -- or just call `intern_const_alloc_recursive`.
347            if self.tcx.try_get_global_alloc(prov.alloc_id()).is_none() {
348                panic!("`intern_with_temp_alloc` with nested allocations");
349            }
350        }
351        interp_ok(alloc_id)
352    }
353}