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::CopyForDeref(place) => check_place(tcx, *place, span, body),
 128         Rvalue::Repeat(operand, _)
 129         | Rvalue::Use(operand)
 130         | Rvalue::Cast(
 131             CastKind::PointerFromExposedAddress
 132             | CastKind::Misc
 133             | CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer),
 134             operand,
 135             _,
 136         ) => check_operand(tcx, operand, span, body),
 137         Rvalue::Cast(
 138             CastKind::Pointer(
 139                 PointerCast::UnsafeFnPointer | PointerCast::ClosureFnPointer(_) | PointerCast::ReifyFnPointer,
 140             ),
 141             _,
 142             _,
 143         ) => Err((span, "function pointer casts are not allowed in const fn".into())),
 144         Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), op, cast_ty) => {
 145             let pointee_ty = if let Some(deref_ty) = cast_ty.builtin_deref(true) {
 146                 deref_ty.ty
 147             } else {
 148                 // We cannot allow this for now.
 149                 return Err((span, "unsizing casts are only allowed for references right now".into()));
 150             };
 151             let unsized_ty = tcx.struct_tail_erasing_lifetimes(pointee_ty, tcx.param_env(def_id));
 152             if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
 153                 check_operand(tcx, op, span, body)?;
 154                 // Casting/coercing things to slices is fine.
 155                 Ok(())
 156             } else {
 157                 // We just can't allow trait objects until we have figured out trait method calls.
 158                 Err((span, "unsizing casts are not allowed in const fn".into()))
 159             }
 160         },
 161         Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => {
 162             Err((span, "casting pointers to ints is unstable in const fn".into()))
 163         },
 164         // binops are fine on integers
 165         Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
 166             check_operand(tcx, lhs, span, body)?;
 167             check_operand(tcx, rhs, span, body)?;
 168             let ty = lhs.ty(body, tcx);
 169             if ty.is_integral() || ty.is_bool() || ty.is_char() {
 170                 Ok(())
 171             } else {
 172                 Err((
 173                     span,
 174                     "only int, `bool` and `char` operations are stable in const fn".into(),
 175                 ))
 176             }
 177         },
 178         Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) | Rvalue::ShallowInitBox(_, _) => Ok(()),
 179         Rvalue::UnaryOp(_, operand) => {
 180             let ty = operand.ty(body, tcx);
 181             if ty.is_integral() || ty.is_bool() {
 182                 check_operand(tcx, operand, span, body)
 183             } else {
 184                 Err((span, "only int and `bool` operations are stable in const fn".into()))
 185             }
 186         },
 187         Rvalue::Aggregate(_, operands) => {
 188             for operand in operands {
 189                 check_operand(tcx, operand, span, body)?;
 190             }
 191             Ok(())
 192         },
 193     }
 194 }
 195
 196 fn check_statement<'tcx>(
 197     tcx: TyCtxt<'tcx>,
 198     body: &Body<'tcx>,
 199     def_id: DefId,
 200     statement: &Statement<'tcx>,
 201 ) -> McfResult {
 202     let span = statement.source_info.span;
 203     match &statement.kind {
 204         StatementKind::Assign(box (place, rval)) => {
 205             check_place(tcx, *place, span, body)?;
 206             check_rvalue(tcx, body, def_id, rval, span)
 207         },
 208
 209         StatementKind::FakeRead(box (_, place)) => check_place(tcx, *place, span, body),
 210         // just an assignment
 211         StatementKind::SetDiscriminant { place, .. } | StatementKind::Deinit(place) => {
 212             check_place(tcx, **place, span, body)
 213         },
 214
 215         StatementKind::CopyNonOverlapping(box rustc_middle::mir::CopyNonOverlapping { dst, src, count }) => {
 216             check_operand(tcx, dst, span, body)?;
 217             check_operand(tcx, src, span, body)?;
 218             check_operand(tcx, count, span, body)
 219         },
 220         // These are all NOPs
 221         StatementKind::StorageLive(_)
 222         | StatementKind::StorageDead(_)
 223         | StatementKind::Retag { .. }
 224         | StatementKind::AscribeUserType(..)
 225         | StatementKind::Coverage(..)
 226         | StatementKind::Nop => Ok(()),
 227     }
 228 }
 229
 230 fn check_operand<'tcx>(tcx: TyCtxt<'tcx>, operand: &Operand<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
 231     match operand {
 232         Operand::Move(place) | Operand::Copy(place) => check_place(tcx, *place, span, body),
 233         Operand::Constant(c) => match c.check_static_ptr(tcx) {
 234             Some(_) => Err((span, "cannot access `static` items in const fn".into())),
 235             None => Ok(()),
 236         },
 237     }
 238 }
 239
 240 fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
 241     let mut cursor = place.projection.as_ref();
 242     while let [ref proj_base @ .., elem] = *cursor {
 243         cursor = proj_base;
 244         match elem {
 245             ProjectionElem::Field(..) => {
 246                 let base_ty = Place::ty_from(place.local, proj_base, body, tcx).ty;
 247                 if let Some(def) = base_ty.ty_adt_def() {
 248                     // No union field accesses in `const fn`
 249                     if def.is_union() {
 250                         return Err((span, "accessing union fields is unstable".into()));
 251                     }
 252                 }
 253             },
 254             ProjectionElem::ConstantIndex { .. }
 255             | ProjectionElem::Downcast(..)
 256             | ProjectionElem::Subslice { .. }
 257             | ProjectionElem::Deref
 258             | ProjectionElem::Index(_) => {},
 259         }
 260     }
 261
 262     Ok(())
 263 }
 264
 265 fn check_terminator<'a, 'tcx>(
 266     tcx: TyCtxt<'tcx>,
 267     body: &'a Body<'tcx>,
 268     terminator: &Terminator<'tcx>,
 269     msrv: Option<RustcVersion>,
 270 ) -> McfResult {
 271     let span = terminator.source_info.span;
 272     match &terminator.kind {
 273         TerminatorKind::FalseEdge { .. }
 274         | TerminatorKind::FalseUnwind { .. }
 275         | TerminatorKind::Goto { .. }
 276         | TerminatorKind::Return
 277         | TerminatorKind::Resume
 278         | TerminatorKind::Unreachable => Ok(()),
 279
 280         TerminatorKind::Drop { place, .. } => check_place(tcx, *place, span, body),
 281         TerminatorKind::DropAndReplace { place, value, .. } => {
 282             check_place(tcx, *place, span, body)?;
 283             check_operand(tcx, value, span, body)
 284         },
 285
 286         TerminatorKind::SwitchInt {
 287             discr,
 288             switch_ty: _,
 289             targets: _,
 290         } => check_operand(tcx, discr, span, body),
 291
 292         TerminatorKind::Abort => Err((span, "abort is not stable in const fn".into())),
 293         TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
 294             Err((span, "const fn generators are unstable".into()))
 295         },
 296
 297         TerminatorKind::Call {
 298             func,
 299             args,
 300             from_hir_call: _,
 301             destination: _,
 302             target: _,
 303             cleanup: _,
 304             fn_span: _,
 305         } => {
 306             let fn_ty = func.ty(body, tcx);
 307             if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
 308                 if !is_const_fn(tcx, fn_def_id, msrv) {
 309                     return Err((
 310                         span,
 311                         format!(
 312                             "can only call other `const fn` within a `const fn`, \
 313                              but `{:?}` is not stable as `const fn`",
 314                             func,
 315                         )
 316                         .into(),
 317                     ));
 318                 }
 319
 320                 // HACK: This is to "unstabilize" the `transmute` intrinsic
 321                 // within const fns. `transmute` is allowed in all other const contexts.
 322                 // This won't really scale to more intrinsics or functions. Let's allow const
 323                 // transmutes in const fn before we add more hacks to this.
 324                 if tcx.is_intrinsic(fn_def_id) && tcx.item_name(fn_def_id) == sym::transmute {
 325                     return Err((
 326                         span,
 327                         "can only call `transmute` from const items, not `const fn`".into(),
 328                     ));
 329                 }
 330
 331                 check_operand(tcx, func, span, body)?;
 332
 333                 for arg in args {
 334                     check_operand(tcx, arg, span, body)?;
 335                 }
 336                 Ok(())
 337             } else {
 338                 Err((span, "can only call other const fns within const fn".into()))
 339             }
 340         },
 341
 342         TerminatorKind::Assert {
 343             cond,
 344             expected: _,
 345             msg: _,
 346             target: _,
 347             cleanup: _,
 348         } => check_operand(tcx, cond, span, body),
 349
 350         TerminatorKind::InlineAsm { .. } => Err((span, "cannot use inline assembly in const fn".into())),
 351     }
 352 }
 353
 354 fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: Option<RustcVersion>) -> bool {
 355     tcx.is_const_fn(def_id)
 356         && tcx.lookup_const_stability(def_id).map_or(true, |const_stab| {
 357             if let rustc_attr::StabilityLevel::Stable { since, .. } = const_stab.level {
 358                 // Checking MSRV is manually necessary because `rustc` has no such concept. This entire
 359                 // function could be removed if `rustc` provided a MSRV-aware version of `is_const_fn`.
 360                 // as a part of an unimplemented MSRV check https://github.com/rust-lang/rust/issues/65262.
 361                 crate::meets_msrv(
 362                     msrv,
 363                     RustcVersion::parse(since.as_str())
 364                         .expect("`rustc_attr::StabilityLevel::Stable::since` is ill-formatted"),
 365                 )
 366             } else {
 367                 // Unstable const fn with the feature enabled.
 368                 msrv.is_none()
 369             }
 370         })
 371 }