rustc_middle/mir/consts.rs
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use std::fmt::{self, Debug, Display, Formatter};
use rustc_abi::{HasDataLayout, Size};
use rustc_hir::def_id::DefId;
use rustc_macros::{HashStable, Lift, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
use rustc_session::RemapFileNameExt;
use rustc_session::config::RemapPathScopeComponents;
use rustc_span::{DUMMY_SP, Span};
use rustc_type_ir::visit::TypeVisitableExt;
use super::interpret::ReportedErrorInfo;
use crate::mir::interpret::{AllocId, ConstAllocation, ErrorHandled, Scalar, alloc_range};
use crate::mir::{Promoted, pretty_print_const_value};
use crate::ty::print::{pretty_print_const, with_no_trimmed_paths};
use crate::ty::{self, ConstKind, GenericArgsRef, ScalarInt, Ty, TyCtxt};
///////////////////////////////////////////////////////////////////////////
/// Evaluated Constants
/// Represents the result of const evaluation via the `eval_to_allocation` query.
/// Not to be confused with `ConstAllocation`, which directly refers to the underlying data!
/// Here we indirect via an `AllocId`.
#[derive(Copy, Clone, HashStable, TyEncodable, TyDecodable, Debug, Hash, Eq, PartialEq)]
pub struct ConstAlloc<'tcx> {
/// The value lives here, at offset 0, and that allocation definitely is an `AllocKind::Memory`
/// (so you can use `AllocMap::unwrap_memory`).
pub alloc_id: AllocId,
pub ty: Ty<'tcx>,
}
/// Represents a constant value in Rust. `Scalar` and `Slice` are optimizations for
/// array length computations, enum discriminants and the pattern matching logic.
#[derive(Copy, Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, Hash)]
#[derive(HashStable, Lift)]
pub enum ConstValue<'tcx> {
/// Used for types with `layout::abi::Scalar` ABI.
///
/// Not using the enum `Value` to encode that this must not be `Uninit`.
Scalar(Scalar),
/// Only for ZSTs.
ZeroSized,
/// Used for references to unsized types with slice tail.
///
/// This is worth an optimized representation since Rust has literals of type `&str` and
/// `&[u8]`. Not having to indirect those through an `AllocId` (or two, if we used `Indirect`)
/// has shown measurable performance improvements on stress tests. We then reuse this
/// optimization for slice-tail types more generally during valtree-to-constval conversion.
Slice {
/// The allocation storing the slice contents.
/// This always points to the beginning of the allocation.
data: ConstAllocation<'tcx>,
/// The metadata field of the reference.
/// This is a "target usize", so we use `u64` as in the interpreter.
meta: u64,
},
/// A value not representable by the other variants; needs to be stored in-memory.
///
/// Must *not* be used for scalars or ZST, but having `&str` or other slices in this variant is fine.
Indirect {
/// The backing memory of the value. May contain more memory than needed for just the value
/// if this points into some other larger ConstValue.
///
/// We use an `AllocId` here instead of a `ConstAllocation<'tcx>` to make sure that when a
/// raw constant (which is basically just an `AllocId`) is turned into a `ConstValue` and
/// back, we can preserve the original `AllocId`.
alloc_id: AllocId,
/// Offset into `alloc`
offset: Size,
},
}
#[cfg(target_pointer_width = "64")]
rustc_data_structures::static_assert_size!(ConstValue<'_>, 24);
impl<'tcx> ConstValue<'tcx> {
#[inline]
pub fn try_to_scalar(&self) -> Option<Scalar> {
match *self {
ConstValue::Indirect { .. } | ConstValue::Slice { .. } | ConstValue::ZeroSized => None,
ConstValue::Scalar(val) => Some(val),
}
}
pub fn try_to_scalar_int(&self) -> Option<ScalarInt> {
self.try_to_scalar()?.try_to_scalar_int().ok()
}
pub fn try_to_bits(&self, size: Size) -> Option<u128> {
Some(self.try_to_scalar_int()?.to_bits(size))
}
pub fn try_to_bool(&self) -> Option<bool> {
self.try_to_scalar_int()?.try_into().ok()
}
pub fn try_to_target_usize(&self, tcx: TyCtxt<'tcx>) -> Option<u64> {
Some(self.try_to_scalar_int()?.to_target_usize(tcx))
}
pub fn try_to_bits_for_ty(
&self,
tcx: TyCtxt<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
ty: Ty<'tcx>,
) -> Option<u128> {
let size = tcx
.layout_of(typing_env.with_post_analysis_normalized(tcx).as_query_input(ty))
.ok()?
.size;
self.try_to_bits(size)
}
pub fn from_bool(b: bool) -> Self {
ConstValue::Scalar(Scalar::from_bool(b))
}
pub fn from_u64(i: u64) -> Self {
ConstValue::Scalar(Scalar::from_u64(i))
}
pub fn from_u128(i: u128) -> Self {
ConstValue::Scalar(Scalar::from_u128(i))
}
pub fn from_target_usize(i: u64, cx: &impl HasDataLayout) -> Self {
ConstValue::Scalar(Scalar::from_target_usize(i, cx))
}
/// Must only be called on constants of type `&str` or `&[u8]`!
pub fn try_get_slice_bytes_for_diagnostics(&self, tcx: TyCtxt<'tcx>) -> Option<&'tcx [u8]> {
let (data, start, end) = match self {
ConstValue::Scalar(_) | ConstValue::ZeroSized => {
bug!("`try_get_slice_bytes` on non-slice constant")
}
&ConstValue::Slice { data, meta } => (data, 0, meta),
&ConstValue::Indirect { alloc_id, offset } => {
// The reference itself is stored behind an indirection.
// Load the reference, and then load the actual slice contents.
let a = tcx.global_alloc(alloc_id).unwrap_memory().inner();
let ptr_size = tcx.data_layout.pointer_size;
if a.size() < offset + 2 * ptr_size {
// (partially) dangling reference
return None;
}
// Read the wide pointer components.
let ptr = a
.read_scalar(
&tcx,
alloc_range(offset, ptr_size),
/* read_provenance */ true,
)
.ok()?;
let ptr = ptr.to_pointer(&tcx).discard_err()?;
let len = a
.read_scalar(
&tcx,
alloc_range(offset + ptr_size, ptr_size),
/* read_provenance */ false,
)
.ok()?;
let len = len.to_target_usize(&tcx).discard_err()?;
if len == 0 {
return Some(&[]);
}
// Non-empty slice, must have memory. We know this is a relative pointer.
let (inner_prov, offset) = ptr.into_parts();
let data = tcx.global_alloc(inner_prov?.alloc_id()).unwrap_memory();
(data, offset.bytes(), offset.bytes() + len)
}
};
// This is for diagnostics only, so we are okay to use `inspect_with_uninit_and_ptr_outside_interpreter`.
let start = start.try_into().unwrap();
let end = end.try_into().unwrap();
Some(data.inner().inspect_with_uninit_and_ptr_outside_interpreter(start..end))
}
/// Check if a constant may contain provenance information. This is used by MIR opts.
/// Can return `true` even if there is no provenance.
pub fn may_have_provenance(&self, tcx: TyCtxt<'tcx>, size: Size) -> bool {
match *self {
ConstValue::ZeroSized | ConstValue::Scalar(Scalar::Int(_)) => return false,
ConstValue::Scalar(Scalar::Ptr(..)) => return true,
// It's hard to find out the part of the allocation we point to;
// just conservatively check everything.
ConstValue::Slice { data, meta: _ } => !data.inner().provenance().ptrs().is_empty(),
ConstValue::Indirect { alloc_id, offset } => !tcx
.global_alloc(alloc_id)
.unwrap_memory()
.inner()
.provenance()
.range_empty(super::AllocRange::from(offset..offset + size), &tcx),
}
}
}
///////////////////////////////////////////////////////////////////////////
/// Constants
#[derive(Clone, Copy, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable, Debug)]
#[derive(TypeFoldable, TypeVisitable, Lift)]
pub enum Const<'tcx> {
/// This constant came from the type system.
///
/// Any way of turning `ty::Const` into `ConstValue` should go through `valtree_to_const_val`;
/// this ensures that we consistently produce "clean" values without data in the padding or
/// anything like that.
///
/// FIXME(BoxyUwU): We should remove this `Ty` and look up the type for params via `ParamEnv`
Ty(Ty<'tcx>, ty::Const<'tcx>),
/// An unevaluated mir constant which is not part of the type system.
///
/// Note that `Ty(ty::ConstKind::Unevaluated)` and this variant are *not* identical! `Ty` will
/// always flow through a valtree, so all data not captured in the valtree is lost. This variant
/// directly uses the evaluated result of the given constant, including e.g. data stored in
/// padding.
Unevaluated(UnevaluatedConst<'tcx>, Ty<'tcx>),
/// This constant cannot go back into the type system, as it represents
/// something the type system cannot handle (e.g. pointers).
Val(ConstValue<'tcx>, Ty<'tcx>),
}
impl<'tcx> Const<'tcx> {
/// Creates an unevaluated const from a `DefId` for a const item.
/// The binders of the const item still need to be instantiated.
pub fn from_unevaluated(
tcx: TyCtxt<'tcx>,
def_id: DefId,
) -> ty::EarlyBinder<'tcx, Const<'tcx>> {
ty::EarlyBinder::bind(Const::Unevaluated(
UnevaluatedConst {
def: def_id,
args: ty::GenericArgs::identity_for_item(tcx, def_id),
promoted: None,
},
tcx.type_of(def_id).skip_binder(),
))
}
#[inline(always)]
pub fn ty(&self) -> Ty<'tcx> {
match self {
Const::Ty(ty, ct) => {
match ct.kind() {
// Dont use the outer ty as on invalid code we can wind up with them not being the same.
// this then results in allowing const eval to add `1_i64 + 1_usize` in cases where the mir
// was originally `({N: usize} + 1_usize)` under `generic_const_exprs`.
ty::ConstKind::Value(ty, _) => ty,
_ => *ty,
}
}
Const::Val(_, ty) | Const::Unevaluated(_, ty) => *ty,
}
}
/// Determines whether we need to add this const to `required_consts`. This is the case if and
/// only if evaluating it may error.
#[inline]
pub fn is_required_const(&self) -> bool {
match self {
Const::Ty(_, c) => match c.kind() {
ty::ConstKind::Value(_, _) => false, // already a value, cannot error
_ => true,
},
Const::Val(..) => false, // already a value, cannot error
Const::Unevaluated(..) => true,
}
}
#[inline]
pub fn try_to_scalar(self) -> Option<Scalar> {
match self {
Const::Ty(_, c) => match c.kind() {
ty::ConstKind::Value(ty, valtree) if ty.is_primitive() => {
// A valtree of a type where leaves directly represent the scalar const value.
// Just checking whether it is a leaf is insufficient as e.g. references are leafs
// but the leaf value is the value they point to, not the reference itself!
Some(valtree.unwrap_leaf().into())
}
_ => None,
},
Const::Val(val, _) => val.try_to_scalar(),
Const::Unevaluated(..) => None,
}
}
#[inline]
pub fn try_to_scalar_int(self) -> Option<ScalarInt> {
// This is equivalent to `self.try_to_scalar()?.try_to_int().ok()`, but measurably faster.
match self {
Const::Val(ConstValue::Scalar(Scalar::Int(x)), _) => Some(x),
Const::Ty(_, c) => match c.kind() {
ty::ConstKind::Value(ty, valtree) if ty.is_primitive() => {
Some(valtree.unwrap_leaf())
}
_ => None,
},
_ => None,
}
}
#[inline]
pub fn try_to_bits(self, size: Size) -> Option<u128> {
Some(self.try_to_scalar_int()?.to_bits(size))
}
#[inline]
pub fn try_to_bool(self) -> Option<bool> {
self.try_to_scalar_int()?.try_into().ok()
}
#[inline]
pub fn eval(
self,
tcx: TyCtxt<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
span: Span,
) -> Result<ConstValue<'tcx>, ErrorHandled> {
match self {
Const::Ty(_, c) => {
if c.has_non_region_param() {
return Err(ErrorHandled::TooGeneric(span));
}
match c.kind() {
ConstKind::Value(ty, val) => Ok(tcx.valtree_to_const_val((ty, val))),
ConstKind::Expr(_) => {
bug!("Normalization of `ty::ConstKind::Expr` is unimplemented")
}
_ => Err(ReportedErrorInfo::non_const_eval_error(
tcx.dcx().delayed_bug("Unevaluated `ty::Const` in MIR body"),
)
.into()),
}
}
Const::Unevaluated(uneval, _) => {
// FIXME: We might want to have a `try_eval`-like function on `Unevaluated`
tcx.const_eval_resolve(typing_env, uneval, span)
}
Const::Val(val, _) => Ok(val),
}
}
#[inline]
pub fn try_eval_scalar(
self,
tcx: TyCtxt<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
) -> Option<Scalar> {
if let Const::Ty(_, c) = self
&& let ty::ConstKind::Value(ty, val) = c.kind()
&& ty.is_primitive()
{
// Avoid the `valtree_to_const_val` query. Can only be done on primitive types that
// are valtree leaves, and *not* on references. (References should return the
// pointer here, which valtrees don't represent.)
Some(val.unwrap_leaf().into())
} else {
self.eval(tcx, typing_env, DUMMY_SP).ok()?.try_to_scalar()
}
}
#[inline]
pub fn try_eval_scalar_int(
self,
tcx: TyCtxt<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
) -> Option<ScalarInt> {
self.try_eval_scalar(tcx, typing_env)?.try_to_scalar_int().ok()
}
#[inline]
pub fn try_eval_bits(
&self,
tcx: TyCtxt<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
) -> Option<u128> {
let int = self.try_eval_scalar_int(tcx, typing_env)?;
let size = tcx
.layout_of(typing_env.with_post_analysis_normalized(tcx).as_query_input(self.ty()))
.ok()?
.size;
Some(int.to_bits(size))
}
/// Panics if the value cannot be evaluated or doesn't contain a valid integer of the given type.
#[inline]
pub fn eval_bits(self, tcx: TyCtxt<'tcx>, typing_env: ty::TypingEnv<'tcx>) -> u128 {
self.try_eval_bits(tcx, typing_env)
.unwrap_or_else(|| bug!("expected bits of {:#?}, got {:#?}", self.ty(), self))
}
#[inline]
pub fn try_eval_target_usize(
self,
tcx: TyCtxt<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
) -> Option<u64> {
Some(self.try_eval_scalar_int(tcx, typing_env)?.to_target_usize(tcx))
}
#[inline]
/// Panics if the value cannot be evaluated or doesn't contain a valid `usize`.
pub fn eval_target_usize(self, tcx: TyCtxt<'tcx>, typing_env: ty::TypingEnv<'tcx>) -> u64 {
self.try_eval_target_usize(tcx, typing_env)
.unwrap_or_else(|| bug!("expected usize, got {:#?}", self))
}
#[inline]
pub fn try_eval_bool(self, tcx: TyCtxt<'tcx>, typing_env: ty::TypingEnv<'tcx>) -> Option<bool> {
self.try_eval_scalar_int(tcx, typing_env)?.try_into().ok()
}
#[inline]
pub fn from_value(val: ConstValue<'tcx>, ty: Ty<'tcx>) -> Self {
Self::Val(val, ty)
}
pub fn from_bits(
tcx: TyCtxt<'tcx>,
bits: u128,
typing_env: ty::TypingEnv<'tcx>,
ty: Ty<'tcx>,
) -> Self {
let size = tcx
.layout_of(typing_env.as_query_input(ty))
.unwrap_or_else(|e| bug!("could not compute layout for {ty:?}: {e:?}"))
.size;
let cv = ConstValue::Scalar(Scalar::from_uint(bits, size));
Self::Val(cv, ty)
}
#[inline]
pub fn from_bool(tcx: TyCtxt<'tcx>, v: bool) -> Self {
let cv = ConstValue::from_bool(v);
Self::Val(cv, tcx.types.bool)
}
#[inline]
pub fn zero_sized(ty: Ty<'tcx>) -> Self {
let cv = ConstValue::ZeroSized;
Self::Val(cv, ty)
}
pub fn from_usize(tcx: TyCtxt<'tcx>, n: u64) -> Self {
let ty = tcx.types.usize;
let typing_env = ty::TypingEnv::fully_monomorphized();
Self::from_bits(tcx, n as u128, typing_env, ty)
}
#[inline]
pub fn from_scalar(_tcx: TyCtxt<'tcx>, s: Scalar, ty: Ty<'tcx>) -> Self {
let val = ConstValue::Scalar(s);
Self::Val(val, ty)
}
pub fn from_ty_const(c: ty::Const<'tcx>, ty: Ty<'tcx>, tcx: TyCtxt<'tcx>) -> Self {
match c.kind() {
ty::ConstKind::Value(ty, valtree) => {
// Make sure that if `c` is normalized, then the return value is normalized.
let const_val = tcx.valtree_to_const_val((ty, valtree));
Self::Val(const_val, ty)
}
_ => Self::Ty(ty, c),
}
}
/// Return true if any evaluation of this constant always returns the same value,
/// taking into account even pointer identity tests.
pub fn is_deterministic(&self) -> bool {
// Some constants may generate fresh allocations for pointers they contain,
// so using the same constant twice can yield two different results:
// - valtrees purposefully generate new allocations
// - ConstValue::Slice also generate new allocations
match self {
Const::Ty(_, c) => match c.kind() {
ty::ConstKind::Param(..) => true,
// A valtree may be a reference. Valtree references correspond to a
// different allocation each time they are evaluated. Valtrees for primitive
// types are fine though.
ty::ConstKind::Value(ty, _) => ty.is_primitive(),
ty::ConstKind::Unevaluated(..) | ty::ConstKind::Expr(..) => false,
// This can happen if evaluation of a constant failed. The result does not matter
// much since compilation is doomed.
ty::ConstKind::Error(..) => false,
// Should not appear in runtime MIR.
ty::ConstKind::Infer(..)
| ty::ConstKind::Bound(..)
| ty::ConstKind::Placeholder(..) => bug!(),
},
Const::Unevaluated(..) => false,
// If the same slice appears twice in the MIR, we cannot guarantee that we will
// give the same `AllocId` to the data.
Const::Val(ConstValue::Slice { .. }, _) => false,
Const::Val(
ConstValue::ZeroSized | ConstValue::Scalar(_) | ConstValue::Indirect { .. },
_,
) => true,
}
}
}
/// An unevaluated (potentially generic) constant used in MIR.
#[derive(Copy, Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable)]
#[derive(Hash, HashStable, TypeFoldable, TypeVisitable, Lift)]
pub struct UnevaluatedConst<'tcx> {
pub def: DefId,
pub args: GenericArgsRef<'tcx>,
pub promoted: Option<Promoted>,
}
impl<'tcx> UnevaluatedConst<'tcx> {
#[inline]
pub fn shrink(self) -> ty::UnevaluatedConst<'tcx> {
assert_eq!(self.promoted, None);
ty::UnevaluatedConst { def: self.def, args: self.args }
}
}
impl<'tcx> UnevaluatedConst<'tcx> {
#[inline]
pub fn new(def: DefId, args: GenericArgsRef<'tcx>) -> UnevaluatedConst<'tcx> {
UnevaluatedConst { def, args, promoted: Default::default() }
}
#[inline]
pub fn from_instance(instance: ty::Instance<'tcx>) -> Self {
UnevaluatedConst::new(instance.def_id(), instance.args)
}
}
impl<'tcx> Display for Const<'tcx> {
fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
match *self {
Const::Ty(_, c) => pretty_print_const(c, fmt, true),
Const::Val(val, ty) => pretty_print_const_value(val, ty, fmt),
// FIXME(valtrees): Correctly print mir constants.
Const::Unevaluated(c, _ty) => {
ty::tls::with(move |tcx| {
let c = tcx.lift(c).unwrap();
// Matches `GlobalId` printing.
let instance =
with_no_trimmed_paths!(tcx.def_path_str_with_args(c.def, c.args));
write!(fmt, "{instance}")?;
if let Some(promoted) = c.promoted {
write!(fmt, "::{promoted:?}")?;
}
Ok(())
})
}
}
}
}
///////////////////////////////////////////////////////////////////////////
/// Const-related utilities
impl<'tcx> TyCtxt<'tcx> {
pub fn span_as_caller_location(self, span: Span) -> ConstValue<'tcx> {
let topmost = span.ctxt().outer_expn().expansion_cause().unwrap_or(span);
let caller = self.sess.source_map().lookup_char_pos(topmost.lo());
self.const_caller_location(
rustc_span::symbol::Symbol::intern(
&caller
.file
.name
.for_scope(self.sess, RemapPathScopeComponents::MACRO)
.to_string_lossy(),
),
caller.line as u32,
caller.col_display as u32 + 1,
)
}
}