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//! This module specifies the type based interner for constants.
//!
//! After a const evaluation has computed a value, before we destroy the const evaluator's session
//! memory, we need to extract all memory allocations to the global memory pool so they stay around.
//!
//! In principle, this is not very complicated: we recursively walk the final value, follow all the
//! pointers, and move all reachable allocations to the global `tcx` memory. The only complication
//! is picking the right mutability: the outermost allocation generally has a clear mutability, but
//! what about the other allocations it points to that have also been created with this value? We
//! don't want to do guesswork here. The rules are: `static`, `const`, and promoted can only create
//! immutable allocations that way. `static mut` can be initialized with expressions like `&mut 42`,
//! so all inner allocations are marked mutable. Some of them could potentially be made immutable,
//! but that would require relying on type information, and given how many ways Rust has to lie
//! about type information, we want to avoid doing that.
use rustc_ast::Mutability;
use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
use rustc_errors::ErrorGuaranteed;
use rustc_hir as hir;
use rustc_middle::mir::interpret::{CtfeProvenance, InterpResult};
use rustc_middle::ty::layout::TyAndLayout;
use rustc_session::lint;
use super::{AllocId, Allocation, InterpCx, MPlaceTy, Machine, MemoryKind, PlaceTy};
use crate::const_eval;
use crate::errors::{DanglingPtrInFinal, MutablePtrInFinal};
pub trait CompileTimeMachine<'mir, 'tcx: 'mir, T> = Machine<
'mir,
'tcx,
MemoryKind = T,
Provenance = CtfeProvenance,
ExtraFnVal = !,
FrameExtra = (),
AllocExtra = (),
MemoryMap = FxIndexMap<AllocId, (MemoryKind<T>, Allocation)>,
>;
/// Intern an allocation. Returns `Err` if the allocation does not exist in the local memory.
///
/// `mutability` can be used to force immutable interning: if it is `Mutability::Not`, the
/// allocation is interned immutably; if it is `Mutability::Mut`, then the allocation *must be*
/// already mutable (as a sanity check).
///
/// `recursive_alloc` is called for all recursively encountered allocations.
fn intern_shallow<'rt, 'mir, 'tcx, T, M: CompileTimeMachine<'mir, 'tcx, T>>(
ecx: &'rt mut InterpCx<'mir, 'tcx, M>,
alloc_id: AllocId,
mutability: Mutability,
mut recursive_alloc: impl FnMut(&InterpCx<'mir, 'tcx, M>, CtfeProvenance),
) -> Result<(), ()> {
trace!("intern_shallow {:?}", alloc_id);
// remove allocation
let Some((_kind, mut alloc)) = ecx.memory.alloc_map.remove(&alloc_id) else {
return Err(());
};
// Set allocation mutability as appropriate. This is used by LLVM to put things into
// read-only memory, and also by Miri when evaluating other globals that
// access this one.
match mutability {
Mutability::Not => {
alloc.mutability = Mutability::Not;
}
Mutability::Mut => {
// This must be already mutable, we won't "un-freeze" allocations ever.
assert_eq!(alloc.mutability, Mutability::Mut);
}
}
// record child allocations
for &(_, prov) in alloc.provenance().ptrs().iter() {
recursive_alloc(ecx, prov);
}
// link the alloc id to the actual allocation
let alloc = ecx.tcx.mk_const_alloc(alloc);
ecx.tcx.set_alloc_id_memory(alloc_id, alloc);
Ok(())
}
/// How a constant value should be interned.
#[derive(Copy, Clone, Debug, PartialEq, Hash, Eq)]
pub enum InternKind {
/// The `mutability` of the static, ignoring the type which may have interior mutability.
Static(hir::Mutability),
/// A `const` item
Constant,
Promoted,
}
/// Intern `ret` and everything it references.
///
/// This *cannot raise an interpreter error*. Doing so is left to validation, which
/// tracks where in the value we are and thus can show much better error messages.
#[instrument(level = "debug", skip(ecx))]
pub fn intern_const_alloc_recursive<
'mir,
'tcx: 'mir,
M: CompileTimeMachine<'mir, 'tcx, const_eval::MemoryKind>,
>(
ecx: &mut InterpCx<'mir, 'tcx, M>,
intern_kind: InternKind,
ret: &MPlaceTy<'tcx>,
) -> Result<(), ErrorGuaranteed> {
// We are interning recursively, and for mutability we are distinguishing the "root" allocation
// that we are starting in, and all other allocations that we are encountering recursively.
let (base_mutability, inner_mutability) = match intern_kind {
InternKind::Constant | InternKind::Promoted => {
// Completely immutable. Interning anything mutably here can only lead to unsoundness,
// since all consts are conceptually independent values but share the same underlying
// memory.
(Mutability::Not, Mutability::Not)
}
InternKind::Static(Mutability::Not) => {
(
// Outermost allocation is mutable if `!Freeze`.
if ret.layout.ty.is_freeze(*ecx.tcx, ecx.param_env) {
Mutability::Not
} else {
Mutability::Mut
},
// Inner allocations are never mutable. They can only arise via the "tail
// expression" / "outer scope" rule, and we treat them consistently with `const`.
Mutability::Not,
)
}
InternKind::Static(Mutability::Mut) => {
// Just make everything mutable. We accept code like
// `static mut X = &mut [42]`, so even inner allocations need to be mutable.
(Mutability::Mut, Mutability::Mut)
}
};
// Initialize recursive interning.
let base_alloc_id = ret.ptr().provenance.unwrap().alloc_id();
let mut todo = vec![(base_alloc_id, base_mutability)];
// We need to distinguish "has just been interned" from "was already in `tcx`",
// so we track this in a separate set.
let mut just_interned = FxHashSet::default();
// Whether we encountered a bad mutable pointer.
// We want to first report "dangling" and then "mutable", so we need to delay reporting these
// errors.
let mut found_bad_mutable_pointer = false;
// Keep interning as long as there are things to intern.
// We show errors if there are dangling pointers, or mutable pointers in immutable contexts
// (i.e., everything except for `static mut`). When these errors affect references, it is
// unfortunate that we show these errors here and not during validation, since validation can
// show much nicer errors. However, we do need these checks to be run on all pointers, including
// raw pointers, so we cannot rely on validation to catch them -- and since interning runs
// before validation, and interning doesn't know the type of anything, this means we can't show
// better errors. Maybe we should consider doing validation before interning in the future.
while let Some((alloc_id, mutability)) = todo.pop() {
if ecx.tcx.try_get_global_alloc(alloc_id).is_some() {
// Already interned.
debug_assert!(!ecx.memory.alloc_map.contains_key(&alloc_id));
continue;
}
just_interned.insert(alloc_id);
intern_shallow(ecx, alloc_id, mutability, |ecx, prov| {
let alloc_id = prov.alloc_id();
if intern_kind != InternKind::Promoted
&& inner_mutability == Mutability::Not
&& !prov.immutable()
{
if ecx.tcx.try_get_global_alloc(alloc_id).is_some()
&& !just_interned.contains(&alloc_id)
{
// This is a pointer to some memory from another constant. We encounter mutable
// pointers to such memory since we do not always track immutability through
// these "global" pointers. Allowing them is harmless; the point of these checks
// during interning is to justify why we intern the *new* allocations immutably,
// so we can completely ignore existing allocations. We also don't need to add
// this to the todo list, since after all it is already interned.
return;
}
// Found a mutable pointer inside a const where inner allocations should be
// immutable. We exclude promoteds from this, since things like `&mut []` and
// `&None::<Cell<i32>>` lead to promotion that can produce mutable pointers. We rely
// on the promotion analysis not screwing up to ensure that it is sound to intern
// promoteds as immutable.
found_bad_mutable_pointer = true;
}
// We always intern with `inner_mutability`, and furthermore we ensured above that if
// that is "immutable", then there are *no* mutable pointers anywhere in the newly
// interned memory -- justifying that we can indeed intern immutably. However this also
// means we can *not* easily intern immutably here if `prov.immutable()` is true and
// `inner_mutability` is `Mut`: there might be other pointers to that allocation, and
// we'd have to somehow check that they are *all* immutable before deciding that this
// allocation can be made immutable. In the future we could consider analyzing all
// pointers before deciding which allocations can be made immutable; but for now we are
// okay with losing some potential for immutability here. This can anyway only affect
// `static mut`.
todo.push((alloc_id, inner_mutability));
})
.map_err(|()| {
ecx.tcx.dcx().emit_err(DanglingPtrInFinal { span: ecx.tcx.span, kind: intern_kind })
})?;
}
if found_bad_mutable_pointer {
let err_diag = MutablePtrInFinal { span: ecx.tcx.span, kind: intern_kind };
ecx.tcx.emit_node_span_lint(
lint::builtin::CONST_EVAL_MUTABLE_PTR_IN_FINAL_VALUE,
ecx.best_lint_scope(),
err_diag.span,
err_diag,
)
}
Ok(())
}
/// Intern `ret`. This function assumes that `ret` references no other allocation.
#[instrument(level = "debug", skip(ecx))]
pub fn intern_const_alloc_for_constprop<
'mir,
'tcx: 'mir,
T,
M: CompileTimeMachine<'mir, 'tcx, T>,
>(
ecx: &mut InterpCx<'mir, 'tcx, M>,
alloc_id: AllocId,
) -> InterpResult<'tcx, ()> {
if ecx.tcx.try_get_global_alloc(alloc_id).is_some() {
// The constant is already in global memory. Do nothing.
return Ok(());
}
// Move allocation to `tcx`.
intern_shallow(ecx, alloc_id, Mutability::Not, |_ecx, _| {
// We are not doing recursive interning, so we don't currently support provenance.
// (If this assertion ever triggers, we should just implement a
// proper recursive interning loop -- or just call `intern_const_alloc_recursive`.
panic!("`intern_const_alloc_for_constprop` called on allocation with nested provenance")
})
.map_err(|()| err_ub!(DeadLocal).into())
}
impl<'mir, 'tcx: 'mir, M: super::intern::CompileTimeMachine<'mir, 'tcx, !>>
InterpCx<'mir, 'tcx, M>
{
/// A helper function that allocates memory for the layout given and gives you access to mutate
/// it. Once your own mutation code is done, the backing `Allocation` is removed from the
/// current `Memory` and interned as read-only into the global memory.
pub fn intern_with_temp_alloc(
&mut self,
layout: TyAndLayout<'tcx>,
f: impl FnOnce(
&mut InterpCx<'mir, 'tcx, M>,
&PlaceTy<'tcx, M::Provenance>,
) -> InterpResult<'tcx, ()>,
) -> InterpResult<'tcx, AllocId> {
// `allocate` picks a fresh AllocId that we will associate with its data below.
let dest = self.allocate(layout, MemoryKind::Stack)?;
f(self, &dest.clone().into())?;
let alloc_id = dest.ptr().provenance.unwrap().alloc_id(); // this was just allocated, it must have provenance
intern_shallow(self, alloc_id, Mutability::Not, |ecx, prov| {
// We are not doing recursive interning, so we don't currently support provenance.
// (If this assertion ever triggers, we should just implement a
// proper recursive interning loop -- or just call `intern_const_alloc_recursive`.
if !ecx.tcx.try_get_global_alloc(prov.alloc_id()).is_some() {
panic!("`intern_with_temp_alloc` with nested allocations");
}
})
.unwrap();
Ok(alloc_id)
}
}