src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

   1 // This code used to be a part of `rustc` but moved to Clippy as a result of
   2 // https://github.com/rust-lang/rust/issues/76618. Because of that, it contains unused code and some
   3 // of terminologies might not be relevant in the context of Clippy. Note that its behavior might
   4 // differ from the time of `rustc` even if the name stays the same.
   5
   6 use rustc_hir as hir;
   7 use rustc_hir::def_id::DefId;
   8 use rustc_middle::mir::{
   9     Body, CastKind, NullOp, Operand, Place, ProjectionElem, Rvalue, Statement, StatementKind, Terminator,
  10     TerminatorKind,
  11 };
  12 use rustc_middle::ty::subst::GenericArgKind;
  13 use rustc_middle::ty::{self, adjustment::PointerCast, Ty, TyCtxt};
  14 use rustc_semver::RustcVersion;
  15 use rustc_span::symbol::sym;
  16 use rustc_span::Span;
  17 use std::borrow::Cow;
  18
  19 type McfResult = Result<(), (Span, Cow<'static, str>)>;
  20
  21 pub fn is_min_const_fn<'a, 'tcx>(tcx: TyCtxt<'tcx>, body: &'a Body<'tcx>, msrv: Option<RustcVersion>) -> McfResult {
  22     let def_id = body.source.def_id();
  23     let mut current = def_id;
  24     loop {
  25         let predicates = tcx.predicates_of(current);
  26         for (predicate, _) in predicates.predicates {
  27             match predicate.kind().skip_binder() {
  28                 ty::PredicateKind::RegionOutlives(_)
  29                 | ty::PredicateKind::TypeOutlives(_)
  30                 | ty::PredicateKind::WellFormed(_)
  31                 | ty::PredicateKind::Projection(_)
  32                 | ty::PredicateKind::ConstEvaluatable(..)
  33                 | ty::PredicateKind::ConstEquate(..)
  34                 | ty::PredicateKind::Trait(..)
  35                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => continue,
  36                 ty::PredicateKind::ObjectSafe(_) => panic!("object safe predicate on function: {:#?}", predicate),
  37                 ty::PredicateKind::ClosureKind(..) => panic!("closure kind predicate on function: {:#?}", predicate),
  38                 ty::PredicateKind::Subtype(_) => panic!("subtype predicate on function: {:#?}", predicate),
  39                 ty::PredicateKind::Coerce(_) => panic!("coerce predicate on function: {:#?}", predicate),
  40             }
  41         }
  42         match predicates.parent {
  43             Some(parent) => current = parent,
  44             None => break,
  45         }
  46     }
  47
  48     for local in &body.local_decls {
  49         check_ty(tcx, local.ty, local.source_info.span)?;
  50     }
  51     // impl trait is gone in MIR, so check the return type manually
  52     check_ty(
  53         tcx,
  54         tcx.fn_sig(def_id).output().skip_binder(),
  55         body.local_decls.iter().next().unwrap().source_info.span,
  56     )?;
  57
  58     for bb in body.basic_blocks() {
  59         check_terminator(tcx, body, bb.terminator(), msrv)?;
  60         for stmt in &bb.statements {
  61             check_statement(tcx, body, def_id, stmt)?;
  62         }
  63     }
  64     Ok(())
  65 }
  66
  67 fn check_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span) -> McfResult {
  68     for arg in ty.walk() {
  69         let ty = match arg.unpack() {
  70             GenericArgKind::Type(ty) => ty,
  71
  72             // No constraints on lifetimes or constants, except potentially
  73             // constants' types, but `walk` will get to them as well.
  74             GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
  75         };
  76
  77         match ty.kind() {
  78             ty::Ref(_, _, hir::Mutability::Mut) => {
  79                 return Err((span, "mutable references in const fn are unstable".into()));
  80             },
  81             ty::Opaque(..) => return Err((span, "`impl Trait` in const fn is unstable".into())),
  82             ty::FnPtr(..) => {
  83                 return Err((span, "function pointers in const fn are unstable".into()));
  84             },
  85             ty::Dynamic(preds, _) => {
  86                 for pred in preds.iter() {
  87                     match pred.skip_binder() {
  88                         ty::ExistentialPredicate::AutoTrait(_) | ty::ExistentialPredicate::Projection(_) => {
  89                             return Err((
  90                                 span,
  91                                 "trait bounds other than `Sized` \
  92                                  on const fn parameters are unstable"
  93                                     .into(),
  94                             ));
  95                         },
  96                         ty::ExistentialPredicate::Trait(trait_ref) => {
  97                             if Some(trait_ref.def_id) != tcx.lang_items().sized_trait() {
  98                                 return Err((
  99                                     span,
 100                                     "trait bounds other than `Sized` \
 101                                      on const fn parameters are unstable"
 102                                         .into(),
 103                                 ));
 104                             }
 105                         },
 106                     }
 107                 }
 108             },
 109             _ => {},
 110         }
 111     }
 112     Ok(())
 113 }
 114
 115 fn check_rvalue<'tcx>(
 116     tcx: TyCtxt<'tcx>,
 117     body: &Body<'tcx>,
 118     def_id: DefId,
 119     rvalue: &Rvalue<'tcx>,
 120     span: Span,
 121 ) -> McfResult {
 122     match rvalue {
 123         Rvalue::ThreadLocalRef(_) => Err((span, "cannot access thread local storage in const fn".into())),
 124         Rvalue::Len(place) | Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
 125             check_place(tcx, *place, span, body)
 126         },
 127         Rvalue::Repeat(operand, _)
 128         | Rvalue::Use(operand)
 129         | Rvalue::Cast(
 130             CastKind::PointerFromExposedAddress
 131             | CastKind::Misc
 132             | CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer),
 133             operand,
 134             _,
 135         ) => check_operand(tcx, operand, span, body),
 136         Rvalue::Cast(
 137             CastKind::Pointer(
 138                 PointerCast::UnsafeFnPointer | PointerCast::ClosureFnPointer(_) | PointerCast::ReifyFnPointer,
 139             ),
 140             _,
 141             _,
 142         ) => Err((span, "function pointer casts are not allowed in const fn".into())),
 143         Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), op, cast_ty) => {
 144             let pointee_ty = if let Some(deref_ty) = cast_ty.builtin_deref(true) {
 145                 deref_ty.ty
 146             } else {
 147                 // We cannot allow this for now.
 148                 return Err((span, "unsizing casts are only allowed for references right now".into()));
 149             };
 150             let unsized_ty = tcx.struct_tail_erasing_lifetimes(pointee_ty, tcx.param_env(def_id));
 151             if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
 152                 check_operand(tcx, op, span, body)?;
 153                 // Casting/coercing things to slices is fine.
 154                 Ok(())
 155             } else {
 156                 // We just can't allow trait objects until we have figured out trait method calls.
 157                 Err((span, "unsizing casts are not allowed in const fn".into()))
 158             }
 159         },
 160         Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => {
 161             Err((span, "casting pointers to ints is unstable in const fn".into()))
 162         },
 163         // binops are fine on integers
 164         Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
 165             check_operand(tcx, lhs, span, body)?;
 166             check_operand(tcx, rhs, span, body)?;
 167             let ty = lhs.ty(body, tcx);
 168             if ty.is_integral() || ty.is_bool() || ty.is_char() {
 169                 Ok(())
 170             } else {
 171                 Err((
 172                     span,
 173                     "only int, `bool` and `char` operations are stable in const fn".into(),
 174                 ))
 175             }
 176         },
 177         Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) | Rvalue::ShallowInitBox(_, _) => Ok(()),
 178         Rvalue::UnaryOp(_, operand) => {
 179             let ty = operand.ty(body, tcx);
 180             if ty.is_integral() || ty.is_bool() {
 181                 check_operand(tcx, operand, span, body)
 182             } else {
 183                 Err((span, "only int and `bool` operations are stable in const fn".into()))
 184             }
 185         },
 186         Rvalue::Aggregate(_, operands) => {
 187             for operand in operands {
 188                 check_operand(tcx, operand, span, body)?;
 189             }
 190             Ok(())
 191         },
 192     }
 193 }
 194
 195 fn check_statement<'tcx>(
 196     tcx: TyCtxt<'tcx>,
 197     body: &Body<'tcx>,
 198     def_id: DefId,
 199     statement: &Statement<'tcx>,
 200 ) -> McfResult {
 201     let span = statement.source_info.span;
 202     match &statement.kind {
 203         StatementKind::Assign(box (place, rval)) => {
 204             check_place(tcx, *place, span, body)?;
 205             check_rvalue(tcx, body, def_id, rval, span)
 206         },
 207
 208         StatementKind::FakeRead(box (_, place)) => check_place(tcx, *place, span, body),
 209         // just an assignment
 210         StatementKind::SetDiscriminant { place, .. } | StatementKind::Deinit(place) => {
 211             check_place(tcx, **place, span, body)
 212         },
 213
 214         StatementKind::CopyNonOverlapping(box rustc_middle::mir::CopyNonOverlapping { dst, src, count }) => {
 215             check_operand(tcx, dst, span, body)?;
 216             check_operand(tcx, src, span, body)?;
 217             check_operand(tcx, count, span, body)
 218         },
 219         // These are all NOPs
 220         StatementKind::StorageLive(_)
 221         | StatementKind::StorageDead(_)
 222         | StatementKind::Retag { .. }
 223         | StatementKind::AscribeUserType(..)
 224         | StatementKind::Coverage(..)
 225         | StatementKind::Nop => Ok(()),
 226     }
 227 }
 228
 229 fn check_operand<'tcx>(tcx: TyCtxt<'tcx>, operand: &Operand<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
 230     match operand {
 231         Operand::Move(place) | Operand::Copy(place) => check_place(tcx, *place, span, body),
 232         Operand::Constant(c) => match c.check_static_ptr(tcx) {
 233             Some(_) => Err((span, "cannot access `static` items in const fn".into())),
 234             None => Ok(()),
 235         },
 236     }
 237 }
 238
 239 fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
 240     let mut cursor = place.projection.as_ref();
 241     while let [ref proj_base @ .., elem] = *cursor {
 242         cursor = proj_base;
 243         match elem {
 244             ProjectionElem::Field(..) => {
 245                 let base_ty = Place::ty_from(place.local, proj_base, body, tcx).ty;
 246                 if let Some(def) = base_ty.ty_adt_def() {
 247                     // No union field accesses in `const fn`
 248                     if def.is_union() {
 249                         return Err((span, "accessing union fields is unstable".into()));
 250                     }
 251                 }
 252             },
 253             ProjectionElem::ConstantIndex { .. }
 254             | ProjectionElem::Downcast(..)
 255             | ProjectionElem::Subslice { .. }
 256             | ProjectionElem::Deref
 257             | ProjectionElem::Index(_) => {},
 258         }
 259     }
 260
 261     Ok(())
 262 }
 263
 264 fn check_terminator<'a, 'tcx>(
 265     tcx: TyCtxt<'tcx>,
 266     body: &'a Body<'tcx>,
 267     terminator: &Terminator<'tcx>,
 268     msrv: Option<RustcVersion>,
 269 ) -> McfResult {
 270     let span = terminator.source_info.span;
 271     match &terminator.kind {
 272         TerminatorKind::FalseEdge { .. }
 273         | TerminatorKind::FalseUnwind { .. }
 274         | TerminatorKind::Goto { .. }
 275         | TerminatorKind::Return
 276         | TerminatorKind::Resume
 277         | TerminatorKind::Unreachable => Ok(()),
 278
 279         TerminatorKind::Drop { place, .. } => check_place(tcx, *place, span, body),
 280         TerminatorKind::DropAndReplace { place, value, .. } => {
 281             check_place(tcx, *place, span, body)?;
 282             check_operand(tcx, value, span, body)
 283         },
 284
 285         TerminatorKind::SwitchInt {
 286             discr,
 287             switch_ty: _,
 288             targets: _,
 289         } => check_operand(tcx, discr, span, body),
 290
 291         TerminatorKind::Abort => Err((span, "abort is not stable in const fn".into())),
 292         TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
 293             Err((span, "const fn generators are unstable".into()))
 294         },
 295
 296         TerminatorKind::Call {
 297             func,
 298             args,
 299             from_hir_call: _,
 300             destination: _,
 301             target: _,
 302             cleanup: _,
 303             fn_span: _,
 304         } => {
 305             let fn_ty = func.ty(body, tcx);
 306             if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
 307                 if !is_const_fn(tcx, fn_def_id, msrv) {
 308                     return Err((
 309                         span,
 310                         format!(
 311                             "can only call other `const fn` within a `const fn`, \
 312                              but `{:?}` is not stable as `const fn`",
 313                             func,
 314                         )
 315                         .into(),
 316                     ));
 317                 }
 318
 319                 // HACK: This is to "unstabilize" the `transmute` intrinsic
 320                 // within const fns. `transmute` is allowed in all other const contexts.
 321                 // This won't really scale to more intrinsics or functions. Let's allow const
 322                 // transmutes in const fn before we add more hacks to this.
 323                 if tcx.is_intrinsic(fn_def_id) && tcx.item_name(fn_def_id) == sym::transmute {
 324                     return Err((
 325                         span,
 326                         "can only call `transmute` from const items, not `const fn`".into(),
 327                     ));
 328                 }
 329
 330                 check_operand(tcx, func, span, body)?;
 331
 332                 for arg in args {
 333                     check_operand(tcx, arg, span, body)?;
 334                 }
 335                 Ok(())
 336             } else {
 337                 Err((span, "can only call other const fns within const fn".into()))
 338             }
 339         },
 340
 341         TerminatorKind::Assert {
 342             cond,
 343             expected: _,
 344             msg: _,
 345             target: _,
 346             cleanup: _,
 347         } => check_operand(tcx, cond, span, body),
 348
 349         TerminatorKind::InlineAsm { .. } => Err((span, "cannot use inline assembly in const fn".into())),
 350     }
 351 }
 352
 353 fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: Option<RustcVersion>) -> bool {
 354     tcx.is_const_fn(def_id)
 355         && tcx.lookup_const_stability(def_id).map_or(true, |const_stab| {
 356             if let rustc_attr::StabilityLevel::Stable { since } = const_stab.level {
 357                 // Checking MSRV is manually necessary because `rustc` has no such concept. This entire
 358                 // function could be removed if `rustc` provided a MSRV-aware version of `is_const_fn`.
 359                 // as a part of an unimplemented MSRV check https://github.com/rust-lang/rust/issues/65262.
 360                 crate::meets_msrv(
 361                     msrv,
 362                     RustcVersion::parse(since.as_str())
 363                         .expect("`rustc_attr::StabilityLevel::Stable::since` is ill-formatted"),
 364                 )
 365             } else {
 366                 // Unstable const fn with the feature enabled.
 367                 msrv.is_none()
 368             }
 369         })
 370 }