compiler/rustc_const_eval/src/const_eval/eval_queries.rs

   1 use super::{CompileTimeEvalContext, CompileTimeInterpreter, ConstEvalErr};
   2 use crate::interpret::eval_nullary_intrinsic;
   3 use crate::interpret::{
   4     intern_const_alloc_recursive, Allocation, ConstAlloc, ConstValue, CtfeValidationMode, GlobalId,
   5     Immediate, InternKind, InterpCx, InterpResult, MPlaceTy, MemoryKind, OpTy, RefTracking, Scalar,
   6     ScalarMaybeUninit, StackPopCleanup,
   7 };
   8
   9 use rustc_hir::def::DefKind;
  10 use rustc_middle::mir;
  11 use rustc_middle::mir::interpret::ErrorHandled;
  12 use rustc_middle::mir::pretty::display_allocation;
  13 use rustc_middle::traits::Reveal;
  14 use rustc_middle::ty::layout::LayoutOf;
  15 use rustc_middle::ty::print::with_no_trimmed_paths;
  16 use rustc_middle::ty::{self, subst::Subst, EarlyBinder, TyCtxt};
  17 use rustc_span::source_map::Span;
  18 use rustc_target::abi::{self, Abi};
  19 use std::borrow::Cow;
  20 use std::convert::TryInto;
  21
  22 pub fn note_on_undefined_behavior_error() -> &'static str {
  23     "The rules on what exactly is undefined behavior aren't clear, \
  24      so this check might be overzealous. Please open an issue on the rustc \
  25      repository if you believe it should not be considered undefined behavior."
  26 }
  27
  28 // Returns a pointer to where the result lives
  29 fn eval_body_using_ecx<'mir, 'tcx>(
  30     ecx: &mut CompileTimeEvalContext<'mir, 'tcx>,
  31     cid: GlobalId<'tcx>,
  32     body: &'mir mir::Body<'tcx>,
  33 ) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
  34     debug!("eval_body_using_ecx: {:?}, {:?}", cid, ecx.param_env);
  35     let tcx = *ecx.tcx;
  36     assert!(
  37         cid.promoted.is_some()
  38             || matches!(
  39                 ecx.tcx.def_kind(cid.instance.def_id()),
  40                 DefKind::Const
  41                     | DefKind::Static(_)
  42                     | DefKind::ConstParam
  43                     | DefKind::AnonConst
  44                     | DefKind::InlineConst
  45                     | DefKind::AssocConst
  46             ),
  47         "Unexpected DefKind: {:?}",
  48         ecx.tcx.def_kind(cid.instance.def_id())
  49     );
  50     let layout = ecx.layout_of(EarlyBinder(body.return_ty()).subst(tcx, cid.instance.substs))?;
  51     assert!(!layout.is_unsized());
  52     let ret = ecx.allocate(layout, MemoryKind::Stack)?;
  53
  54     trace!(
  55         "eval_body_using_ecx: pushing stack frame for global: {}{}",
  56         with_no_trimmed_paths!(ty::tls::with(|tcx| tcx.def_path_str(cid.instance.def_id()))),
  57         cid.promoted.map_or_else(String::new, |p| format!("::promoted[{:?}]", p))
  58     );
  59
  60     ecx.push_stack_frame(
  61         cid.instance,
  62         body,
  63         &ret.into(),
  64         StackPopCleanup::Root { cleanup: false },
  65     )?;
  66
  67     // The main interpreter loop.
  68     ecx.run()?;
  69
  70     // Intern the result
  71     let intern_kind = if cid.promoted.is_some() {
  72         InternKind::Promoted
  73     } else {
  74         match tcx.static_mutability(cid.instance.def_id()) {
  75             Some(m) => InternKind::Static(m),
  76             None => InternKind::Constant,
  77         }
  78     };
  79     intern_const_alloc_recursive(ecx, intern_kind, &ret)?;
  80
  81     debug!("eval_body_using_ecx done: {:?}", *ret);
  82     Ok(ret)
  83 }
  84
  85 /// The `InterpCx` is only meant to be used to do field and index projections into constants for
  86 /// `simd_shuffle` and const patterns in match arms.
  87 ///
  88 /// The function containing the `match` that is currently being analyzed may have generic bounds
  89 /// that inform us about the generic bounds of the constant. E.g., using an associated constant
  90 /// of a function's generic parameter will require knowledge about the bounds on the generic
  91 /// parameter. These bounds are passed to `mk_eval_cx` via the `ParamEnv` argument.
  92 pub(super) fn mk_eval_cx<'mir, 'tcx>(
  93     tcx: TyCtxt<'tcx>,
  94     root_span: Span,
  95     param_env: ty::ParamEnv<'tcx>,
  96     can_access_statics: bool,
  97 ) -> CompileTimeEvalContext<'mir, 'tcx> {
  98     debug!("mk_eval_cx: {:?}", param_env);
  99     InterpCx::new(
 100         tcx,
 101         root_span,
 102         param_env,
 103         CompileTimeInterpreter::new(tcx.const_eval_limit(), can_access_statics),
 104     )
 105 }
 106
 107 /// This function converts an interpreter value into a constant that is meant for use in the
 108 /// type system.
 109 #[instrument(skip(ecx), level = "debug")]
 110 pub(super) fn op_to_const<'tcx>(
 111     ecx: &CompileTimeEvalContext<'_, 'tcx>,
 112     op: &OpTy<'tcx>,
 113 ) -> ConstValue<'tcx> {
 114     // We do not have value optimizations for everything.
 115     // Only scalars and slices, since they are very common.
 116     // Note that further down we turn scalars of uninitialized bits back to `ByRef`. These can result
 117     // from scalar unions that are initialized with one of their zero sized variants. We could
 118     // instead allow `ConstValue::Scalar` to store `ScalarMaybeUninit`, but that would affect all
 119     // the usual cases of extracting e.g. a `usize`, without there being a real use case for the
 120     // `Undef` situation.
 121     let try_as_immediate = match op.layout.abi {
 122         Abi::Scalar(abi::Scalar::Initialized { .. }) => true,
 123         Abi::ScalarPair(..) => match op.layout.ty.kind() {
 124             ty::Ref(_, inner, _) => match *inner.kind() {
 125                 ty::Slice(elem) => elem == ecx.tcx.types.u8,
 126                 ty::Str => true,
 127                 _ => false,
 128             },
 129             _ => false,
 130         },
 131         _ => false,
 132     };
 133     let immediate = if try_as_immediate {
 134         Err(ecx.read_immediate(op).expect("normalization works on validated constants"))
 135     } else {
 136         // It is guaranteed that any non-slice scalar pair is actually ByRef here.
 137         // When we come back from raw const eval, we are always by-ref. The only way our op here is
 138         // by-val is if we are in destructure_mir_constant, i.e., if this is (a field of) something that we
 139         // "tried to make immediate" before. We wouldn't do that for non-slice scalar pairs or
 140         // structs containing such.
 141         op.try_as_mplace()
 142     };
 143
 144     debug!(?immediate);
 145
 146     // We know `offset` is relative to the allocation, so we can use `into_parts`.
 147     let to_const_value = |mplace: &MPlaceTy<'_>| {
 148         debug!("to_const_value(mplace: {:?})", mplace);
 149         match mplace.ptr.into_parts() {
 150             (Some(alloc_id), offset) => {
 151                 let alloc = ecx.tcx.global_alloc(alloc_id).unwrap_memory();
 152                 ConstValue::ByRef { alloc, offset }
 153             }
 154             (None, offset) => {
 155                 assert!(mplace.layout.is_zst());
 156                 assert_eq!(
 157                     offset.bytes() % mplace.layout.align.abi.bytes(),
 158                     0,
 159                     "this MPlaceTy must come from a validated constant, thus we can assume the \
 160                 alignment is correct",
 161                 );
 162                 ConstValue::Scalar(Scalar::ZST)
 163             }
 164         }
 165     };
 166     match immediate {
 167         Ok(ref mplace) => to_const_value(mplace),
 168         // see comment on `let try_as_immediate` above
 169         Err(imm) => match *imm {
 170             Immediate::Scalar(x) => match x {
 171                 ScalarMaybeUninit::Scalar(s) => ConstValue::Scalar(s),
 172                 ScalarMaybeUninit::Uninit => to_const_value(&op.assert_mem_place()),
 173             },
 174             Immediate::ScalarPair(a, b) => {
 175                 debug!("ScalarPair(a: {:?}, b: {:?})", a, b);
 176                 // We know `offset` is relative to the allocation, so we can use `into_parts`.
 177                 let (data, start) =
 178                     match ecx.scalar_to_ptr(a.check_init().unwrap()).unwrap().into_parts() {
 179                         (Some(alloc_id), offset) => {
 180                             (ecx.tcx.global_alloc(alloc_id).unwrap_memory(), offset.bytes())
 181                         }
 182                         (None, _offset) => (
 183                             ecx.tcx.intern_const_alloc(
 184                                 Allocation::from_bytes_byte_aligned_immutable(b"" as &[u8]),
 185                             ),
 186                             0,
 187                         ),
 188                     };
 189                 let len = b.to_machine_usize(ecx).unwrap();
 190                 let start = start.try_into().unwrap();
 191                 let len: usize = len.try_into().unwrap();
 192                 ConstValue::Slice { data, start, end: start + len }
 193             }
 194         },
 195     }
 196 }
 197
 198 #[instrument(skip(tcx), level = "debug")]
 199 pub(crate) fn turn_into_const_value<'tcx>(
 200     tcx: TyCtxt<'tcx>,
 201     constant: ConstAlloc<'tcx>,
 202     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 203 ) -> ConstValue<'tcx> {
 204     let cid = key.value;
 205     let def_id = cid.instance.def.def_id();
 206     let is_static = tcx.is_static(def_id);
 207     let ecx = mk_eval_cx(tcx, tcx.def_span(key.value.instance.def_id()), key.param_env, is_static);
 208
 209     let mplace = ecx.raw_const_to_mplace(constant).expect(
 210         "can only fail if layout computation failed, \
 211         which should have given a good error before ever invoking this function",
 212     );
 213     assert!(
 214         !is_static || cid.promoted.is_some(),
 215         "the `eval_to_const_value_raw` query should not be used for statics, use `eval_to_allocation` instead"
 216     );
 217
 218     // Turn this into a proper constant.
 219     let const_val = op_to_const(&ecx, &mplace.into());
 220     debug!(?const_val);
 221
 222     const_val
 223 }
 224
 225 #[instrument(skip(tcx), level = "debug")]
 226 pub fn eval_to_const_value_raw_provider<'tcx>(
 227     tcx: TyCtxt<'tcx>,
 228     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 229 ) -> ::rustc_middle::mir::interpret::EvalToConstValueResult<'tcx> {
 230     assert!(key.param_env.is_const());
 231     // see comment in eval_to_allocation_raw_provider for what we're doing here
 232     if key.param_env.reveal() == Reveal::All {
 233         let mut key = key;
 234         key.param_env = key.param_env.with_user_facing();
 235         match tcx.eval_to_const_value_raw(key) {
 236             // try again with reveal all as requested
 237             Err(ErrorHandled::TooGeneric) => {}
 238             // deduplicate calls
 239             other => return other,
 240         }
 241     }
 242
 243     // We call `const_eval` for zero arg intrinsics, too, in order to cache their value.
 244     // Catch such calls and evaluate them instead of trying to load a constant's MIR.
 245     if let ty::InstanceDef::Intrinsic(def_id) = key.value.instance.def {
 246         let ty = key.value.instance.ty(tcx, key.param_env);
 247         let ty::FnDef(_, substs) = ty.kind() else {
 248             bug!("intrinsic with type {:?}", ty);
 249         };
 250         return eval_nullary_intrinsic(tcx, key.param_env, def_id, substs).map_err(|error| {
 251             let span = tcx.def_span(def_id);
 252             let error = ConstEvalErr { error: error.into_kind(), stacktrace: vec![], span };
 253             error.report_as_error(tcx.at(span), "could not evaluate nullary intrinsic")
 254         });
 255     }
 256
 257     tcx.eval_to_allocation_raw(key).map(|val| turn_into_const_value(tcx, val, key))
 258 }
 259
 260 #[instrument(skip(tcx), level = "debug")]
 261 pub fn eval_to_allocation_raw_provider<'tcx>(
 262     tcx: TyCtxt<'tcx>,
 263     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 264 ) -> ::rustc_middle::mir::interpret::EvalToAllocationRawResult<'tcx> {
 265     assert!(key.param_env.is_const());
 266     // Because the constant is computed twice (once per value of `Reveal`), we are at risk of
 267     // reporting the same error twice here. To resolve this, we check whether we can evaluate the
 268     // constant in the more restrictive `Reveal::UserFacing`, which most likely already was
 269     // computed. For a large percentage of constants that will already have succeeded. Only
 270     // associated constants of generic functions will fail due to not enough monomorphization
 271     // information being available.
 272
 273     // In case we fail in the `UserFacing` variant, we just do the real computation.
 274     if key.param_env.reveal() == Reveal::All {
 275         let mut key = key;
 276         key.param_env = key.param_env.with_user_facing();
 277         match tcx.eval_to_allocation_raw(key) {
 278             // try again with reveal all as requested
 279             Err(ErrorHandled::TooGeneric) => {}
 280             // deduplicate calls
 281             other => return other,
 282         }
 283     }
 284     if cfg!(debug_assertions) {
 285         // Make sure we format the instance even if we do not print it.
 286         // This serves as a regression test against an ICE on printing.
 287         // The next two lines concatenated contain some discussion:
 288         // https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/
 289         // subject/anon_const_instance_printing/near/135980032
 290         let instance = with_no_trimmed_paths!(key.value.instance.to_string());
 291         trace!("const eval: {:?} ({})", key, instance);
 292     }
 293
 294     let cid = key.value;
 295     let def = cid.instance.def.with_opt_param();
 296     let is_static = tcx.is_static(def.did);
 297
 298     let mut ecx = InterpCx::new(
 299         tcx,
 300         tcx.def_span(def.did),
 301         key.param_env,
 302         // Statics (and promoteds inside statics) may access other statics, because unlike consts
 303         // they do not have to behave "as if" they were evaluated at runtime.
 304         CompileTimeInterpreter::new(tcx.const_eval_limit(), /*can_access_statics:*/ is_static),
 305     );
 306
 307     let res = ecx.load_mir(cid.instance.def, cid.promoted);
 308     match res.and_then(|body| eval_body_using_ecx(&mut ecx, cid, &body)) {
 309         Err(error) => {
 310             let err = ConstEvalErr::new(&ecx, error, None);
 311             // Some CTFE errors raise just a lint, not a hard error; see
 312             // <https://github.com/rust-lang/rust/issues/71800>.
 313             let is_hard_err = if let Some(def) = def.as_local() {
 314                 // (Associated) consts only emit a lint, since they might be unused.
 315                 !matches!(tcx.def_kind(def.did.to_def_id()), DefKind::Const | DefKind::AssocConst)
 316                     // check if the inner InterpError is hard
 317                     || err.error.is_hard_err()
 318             } else {
 319                 // use of broken constant from other crate: always an error
 320                 true
 321             };
 322
 323             if is_hard_err {
 324                 let msg = if is_static {
 325                     Cow::from("could not evaluate static initializer")
 326                 } else {
 327                     // If the current item has generics, we'd like to enrich the message with the
 328                     // instance and its substs: to show the actual compile-time values, in addition to
 329                     // the expression, leading to the const eval error.
 330                     let instance = &key.value.instance;
 331                     if !instance.substs.is_empty() {
 332                         let instance = with_no_trimmed_paths!(instance.to_string());
 333                         let msg = format!("evaluation of `{}` failed", instance);
 334                         Cow::from(msg)
 335                     } else {
 336                         Cow::from("evaluation of constant value failed")
 337                     }
 338                 };
 339
 340                 Err(err.report_as_error(ecx.tcx.at(ecx.cur_span()), &msg))
 341             } else {
 342                 let hir_id = tcx.hir().local_def_id_to_hir_id(def.as_local().unwrap().did);
 343                 Err(err.report_as_lint(
 344                     tcx.at(tcx.def_span(def.did)),
 345                     "any use of this value will cause an error",
 346                     hir_id,
 347                     Some(err.span),
 348                 ))
 349             }
 350         }
 351         Ok(mplace) => {
 352             // Since evaluation had no errors, validate the resulting constant.
 353             // This is a separate `try` block to provide more targeted error reporting.
 354             let validation = try {
 355                 let mut ref_tracking = RefTracking::new(mplace);
 356                 let mut inner = false;
 357                 while let Some((mplace, path)) = ref_tracking.todo.pop() {
 358                     let mode = match tcx.static_mutability(cid.instance.def_id()) {
 359                         Some(_) if cid.promoted.is_some() => {
 360                             // Promoteds in statics are allowed to point to statics.
 361                             CtfeValidationMode::Const { inner, allow_static_ptrs: true }
 362                         }
 363                         Some(_) => CtfeValidationMode::Regular, // a `static`
 364                         None => CtfeValidationMode::Const { inner, allow_static_ptrs: false },
 365                     };
 366                     ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)?;
 367                     inner = true;
 368                 }
 369             };
 370             let alloc_id = mplace.ptr.provenance.unwrap();
 371             if let Err(error) = validation {
 372                 // Validation failed, report an error. This is always a hard error.
 373                 let err = ConstEvalErr::new(&ecx, error, None);
 374                 Err(err.struct_error(
 375                     ecx.tcx,
 376                     "it is undefined behavior to use this value",
 377                     |diag| {
 378                         diag.note(note_on_undefined_behavior_error());
 379                         diag.note(&format!(
 380                             "the raw bytes of the constant ({}",
 381                             display_allocation(
 382                                 *ecx.tcx,
 383                                 ecx.tcx.global_alloc(alloc_id).unwrap_memory().inner()
 384                             )
 385                         ));
 386                     },
 387                 ))
 388             } else {
 389                 // Convert to raw constant
 390                 Ok(ConstAlloc { alloc_id, ty: mplace.layout.ty })
 391             }
 392         }
 393     }
 394 }