summaryrefslogtreecommitdiffstats
path: root/src/librustc_mir/const_eval/eval_queries.rs
blob: 4fdabed54b852a683dc079e50d269c5ddcd99a7b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
use super::{error_to_const_error, CompileTimeEvalContext, CompileTimeInterpreter, MemoryExtra};
use crate::interpret::eval_nullary_intrinsic;
use crate::interpret::{
    intern_const_alloc_recursive, Allocation, ConstValue, GlobalId, ImmTy, Immediate, InternKind,
    InterpCx, InterpResult, MPlaceTy, MemoryKind, OpTy, RawConst, RefTracking, Scalar,
    ScalarMaybeUndef, StackPopCleanup,
};
use rustc::mir;
use rustc::mir::interpret::{ConstEvalErr, ErrorHandled};
use rustc::traits::Reveal;
use rustc::ty::{self, layout, layout::LayoutOf, subst::Subst, TyCtxt};
use rustc_hir::def::DefKind;
use rustc_span::source_map::Span;
use std::convert::TryInto;

pub fn note_on_undefined_behavior_error() -> &'static str {
    "The rules on what exactly is undefined behavior aren't clear, \
     so this check might be overzealous. Please open an issue on the rustc \
     repository if you believe it should not be considered undefined behavior."
}

// Returns a pointer to where the result lives
fn eval_body_using_ecx<'mir, 'tcx>(
    ecx: &mut CompileTimeEvalContext<'mir, 'tcx>,
    cid: GlobalId<'tcx>,
    body: &'mir mir::Body<'tcx>,
) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
    debug!("eval_body_using_ecx: {:?}, {:?}", cid, ecx.param_env);
    let tcx = ecx.tcx.tcx;
    let layout = ecx.layout_of(body.return_ty().subst(tcx, cid.instance.substs))?;
    assert!(!layout.is_unsized());
    let ret = ecx.allocate(layout, MemoryKind::Stack);

    let name = ty::tls::with(|tcx| tcx.def_path_str(cid.instance.def_id()));
    let prom = cid.promoted.map_or(String::new(), |p| format!("::promoted[{:?}]", p));
    trace!("eval_body_using_ecx: pushing stack frame for global: {}{}", name, prom);

    // Assert all args (if any) are zero-sized types; `eval_body_using_ecx` doesn't
    // make sense if the body is expecting nontrivial arguments.
    // (The alternative would be to use `eval_fn_call` with an args slice.)
    for arg in body.args_iter() {
        let decl = body.local_decls.get(arg).expect("arg missing from local_decls");
        let layout = ecx.layout_of(decl.ty.subst(tcx, cid.instance.substs))?;
        assert!(layout.is_zst())
    }

    ecx.push_stack_frame(
        cid.instance,
        body.span,
        body,
        Some(ret.into()),
        StackPopCleanup::None { cleanup: false },
    )?;

    // The main interpreter loop.
    ecx.run()?;

    // Intern the result
    let intern_kind = match tcx.static_mutability(cid.instance.def_id()) {
        Some(m) => InternKind::Static(m),
        None if cid.promoted.is_some() => InternKind::Promoted,
        _ => InternKind::Constant,
    };
    intern_const_alloc_recursive(
        ecx,
        intern_kind,
        ret,
        body.ignore_interior_mut_in_const_validation,
    )?;

    debug!("eval_body_using_ecx done: {:?}", *ret);
    Ok(ret)
}

/// The `InterpCx` is only meant to be used to do field and index projections into constants for
/// `simd_shuffle` and const patterns in match arms.
///
/// The function containing the `match` that is currently being analyzed may have generic bounds
/// that inform us about the generic bounds of the constant. E.g., using an associated constant
/// of a function's generic parameter will require knowledge about the bounds on the generic
/// parameter. These bounds are passed to `mk_eval_cx` via the `ParamEnv` argument.
pub(super) fn mk_eval_cx<'mir, 'tcx>(
    tcx: TyCtxt<'tcx>,
    span: Span,
    param_env: ty::ParamEnv<'tcx>,
    can_access_statics: bool,
) -> CompileTimeEvalContext<'mir, 'tcx> {
    debug!("mk_eval_cx: {:?}", param_env);
    InterpCx::new(
        tcx.at(span),
        param_env,
        CompileTimeInterpreter::new(),
        MemoryExtra { can_access_statics },
    )
}

pub(super) fn op_to_const<'tcx>(
    ecx: &CompileTimeEvalContext<'_, 'tcx>,
    op: OpTy<'tcx>,
) -> &'tcx ty::Const<'tcx> {
    // We do not have value optimizations for everything.
    // Only scalars and slices, since they are very common.
    // Note that further down we turn scalars of undefined bits back to `ByRef`. These can result
    // from scalar unions that are initialized with one of their zero sized variants. We could
    // instead allow `ConstValue::Scalar` to store `ScalarMaybeUndef`, but that would affect all
    // the usual cases of extracting e.g. a `usize`, without there being a real use case for the
    // `Undef` situation.
    let try_as_immediate = match op.layout.abi {
        layout::Abi::Scalar(..) => true,
        layout::Abi::ScalarPair(..) => match op.layout.ty.kind {
            ty::Ref(_, inner, _) => match inner.kind {
                ty::Slice(elem) => elem == ecx.tcx.types.u8,
                ty::Str => true,
                _ => false,
            },
            _ => false,
        },
        _ => false,
    };
    let immediate = if try_as_immediate {
        Err(ecx.read_immediate(op).expect("normalization works on validated constants"))
    } else {
        // It is guaranteed that any non-slice scalar pair is actually ByRef here.
        // When we come back from raw const eval, we are always by-ref. The only way our op here is
        // by-val is if we are in const_field, i.e., if this is (a field of) something that we
        // "tried to make immediate" before. We wouldn't do that for non-slice scalar pairs or
        // structs containing such.
        op.try_as_mplace(ecx)
    };

    let to_const_value = |mplace: MPlaceTy<'_>| match mplace.ptr {
        Scalar::Ptr(ptr) => {
            let alloc = ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
            ConstValue::ByRef { alloc, offset: ptr.offset }
        }
        Scalar::Raw { data, .. } => {
            assert!(mplace.layout.is_zst());
            assert_eq!(
                data,
                mplace.layout.align.abi.bytes().into(),
                "this MPlaceTy must come from `try_as_mplace` being used on a zst, so we know what
                 value this integer address must have",
            );
            ConstValue::Scalar(Scalar::zst())
        }
    };
    let val = match immediate {
        Ok(mplace) => to_const_value(mplace),
        // see comment on `let try_as_immediate` above
        Err(ImmTy { imm: Immediate::Scalar(x), .. }) => match x {
            ScalarMaybeUndef::Scalar(s) => ConstValue::Scalar(s),
            ScalarMaybeUndef::Undef => to_const_value(op.assert_mem_place(ecx)),
        },
        Err(ImmTy { imm: Immediate::ScalarPair(a, b), .. }) => {
            let (data, start) = match a.not_undef().unwrap() {
                Scalar::Ptr(ptr) => {
                    (ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id), ptr.offset.bytes())
                }
                Scalar::Raw { .. } => (
                    ecx.tcx.intern_const_alloc(Allocation::from_byte_aligned_bytes(b"" as &[u8])),
                    0,
                ),
            };
            let len = b.to_machine_usize(&ecx.tcx.tcx).unwrap();
            let start = start.try_into().unwrap();
            let len: usize = len.try_into().unwrap();
            ConstValue::Slice { data, start, end: start + len }
        }
    };
    ecx.tcx.mk_const(ty::Const { val: ty::ConstKind::Value(val), ty: op.layout.ty })
}

fn validate_and_turn_into_const<'tcx>(
    tcx: TyCtxt<'tcx>,
    constant: RawConst<'tcx>,
    key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
    let cid = key.value;
    let def_id = cid.instance.def.def_id();
    let is_static = tcx.is_static(def_id);
    let ecx = mk_eval_cx(tcx, tcx.def_span(key.value.instance.def_id()), key.param_env, is_static);
    let val = (|| {
        let mplace = ecx.raw_const_to_mplace(constant)?;

        // FIXME do not validate promoteds until a decision on
        // https://github.com/rust-lang/rust/issues/67465 is made
        if cid.promoted.is_none() {
            let mut ref_tracking = RefTracking::new(mplace);
            while let Some((mplace, path)) = ref_tracking.todo.pop() {
                ecx.validate_operand(mplace.into(), path, Some(&mut ref_tracking))?;
            }
        }
        // Now that we validated, turn this into a proper constant.
        // Statics/promoteds are always `ByRef`, for the rest `op_to_const` decides
        // whether they become immediates.
        if is_static || cid.promoted.is_some() {
            let ptr = mplace.ptr.assert_ptr();
            Ok(tcx.mk_const(ty::Const {
                val: ty::ConstKind::Value(ConstValue::ByRef {
                    alloc: ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id),
                    offset: ptr.offset,
                }),
                ty: mplace.layout.ty,
            }))
        } else {
            Ok(op_to_const(&ecx, mplace.into()))
        }
    })();

    val.map_err(|error| {
        let err = error_to_const_error(&ecx, error);
        match err.struct_error(ecx.tcx, "it is undefined behavior to use this value", |mut diag| {
            diag.note(note_on_undefined_behavior_error());
            diag.emit();
        }) {
            Ok(_) => ErrorHandled::Reported,
            Err(err) => err,
        }
    })
}

pub fn const_eval_validated_provider<'tcx>(
    tcx: TyCtxt<'tcx>,
    key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
    // see comment in const_eval_raw_provider for what we're doing here
    if key.param_env.reveal == Reveal::All {
        let mut key = key;
        key.param_env.reveal = Reveal::UserFacing;
        match tcx.const_eval_validated(key) {
            // try again with reveal all as requested
            Err(ErrorHandled::TooGeneric) => {}
            // dedupliate calls
            other => return other,
        }
    }

    // We call `const_eval` for zero arg intrinsics, too, in order to cache their value.
    // Catch such calls and evaluate them instead of trying to load a constant's MIR.
    if let ty::InstanceDef::Intrinsic(def_id) = key.value.instance.def {
        let ty = key.value.instance.ty_env(tcx, key.param_env);
        let substs = match ty.kind {
            ty::FnDef(_, substs) => substs,
            _ => bug!("intrinsic with type {:?}", ty),
        };
        return eval_nullary_intrinsic(tcx, key.param_env, def_id, substs).map_err(|error| {
            let span = tcx.def_span(def_id);
            let error = ConstEvalErr { error: error.kind, stacktrace: vec![], span };
            error.report_as_error(tcx.at(span), "could not evaluate nullary intrinsic")
        });
    }

    tcx.const_eval_raw(key).and_then(|val| validate_and_turn_into_const(tcx, val, key))
}

pub fn const_eval_raw_provider<'tcx>(
    tcx: TyCtxt<'tcx>,
    key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
) -> ::rustc::mir::interpret::ConstEvalRawResult<'tcx> {
    // Because the constant is computed twice (once per value of `Reveal`), we are at risk of
    // reporting the same error twice here. To resolve this, we check whether we can evaluate the
    // constant in the more restrictive `Reveal::UserFacing`, which most likely already was
    // computed. For a large percentage of constants that will already have succeeded. Only
    // associated constants of generic functions will fail due to not enough monomorphization
    // information being available.

    // In case we fail in the `UserFacing` variant, we just do the real computation.
    if key.param_env.reveal == Reveal::All {
        let mut key = key;
        key.param_env.reveal = Reveal::UserFacing;
        match tcx.const_eval_raw(key) {
            // try again with reveal all as requested
            Err(ErrorHandled::TooGeneric) => {}
            // dedupliate calls
            other => return other,
        }
    }
    if cfg!(debug_assertions) {
        // Make sure we format the instance even if we do not print it.
        // This serves as a regression test against an ICE on printing.
        // The next two lines concatenated contain some discussion:
        // https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/
        // subject/anon_const_instance_printing/near/135980032
        let instance = key.value.instance.to_string();
        trace!("const eval: {:?} ({})", key, instance);
    }

    let cid = key.value;
    let def_id = cid.instance.def.def_id();

    if def_id.is_local()
        && tcx.has_typeck_tables(def_id)
        && tcx.typeck_tables_of(def_id).tainted_by_errors
    {
        return Err(ErrorHandled::Reported);
    }

    let is_static = tcx.is_static(def_id);

    let span = tcx.def_span(cid.instance.def_id());
    let mut ecx = InterpCx::new(
        tcx.at(span),
        key.param_env,
        CompileTimeInterpreter::new(),
        MemoryExtra { can_access_statics: is_static },
    );

    let res = ecx.load_mir(cid.instance.def, cid.promoted);
    res.and_then(|body| eval_body_using_ecx(&mut ecx, cid, *body))
        .and_then(|place| {
            Ok(RawConst { alloc_id: place.ptr.assert_ptr().alloc_id, ty: place.layout.ty })
        })
        .map_err(|error| {
            let err = error_to_const_error(&ecx, error);
            // errors in statics are always emitted as fatal errors
            if is_static {
                // Ensure that if the above error was either `TooGeneric` or `Reported`
                // an error must be reported.
                let v = err.report_as_error(ecx.tcx, "could not evaluate static initializer");

                // If this is `Reveal:All`, then we need to make sure an error is reported but if
                // this is `Reveal::UserFacing`, then it's expected that we could get a
                // `TooGeneric` error. When we fall back to `Reveal::All`, then it will either
                // succeed or we'll report this error then.
                if key.param_env.reveal == Reveal::All {
                    tcx.sess.delay_span_bug(
                        err.span,
                        &format!("static eval failure did not emit an error: {:#?}", v),
                    );
                }

                v
            } else if def_id.is_local() {
                // constant defined in this crate, we can figure out a lint level!
                match tcx.def_kind(def_id) {
                    // constants never produce a hard error at the definition site. Anything else is
                    // a backwards compatibility hazard (and will break old versions of winapi for
                    // sure)
                    //
                    // note that validation may still cause a hard error on this very same constant,
                    // because any code that existed before validation could not have failed
                    // validation thus preventing such a hard error from being a backwards
                    // compatibility hazard
                    Some(DefKind::Const) | Some(DefKind::AssocConst) => {
                        let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
                        err.report_as_lint(
                            tcx.at(tcx.def_span(def_id)),
                            "any use of this value will cause an error",
                            hir_id,
                            Some(err.span),
                        )
                    }
                    // promoting runtime code is only allowed to error if it references broken
                    // constants any other kind of error will be reported to the user as a
                    // deny-by-default lint
                    _ => {
                        if let Some(p) = cid.promoted {
                            let span = tcx.promoted_mir(def_id)[p].span;
                            if let err_inval!(ReferencedConstant) = err.error {
                                err.report_as_error(
                                    tcx.at(span),
                                    "evaluation of constant expression failed",
                                )
                            } else {
                                err.report_as_lint(
                                    tcx.at(span),
                                    "reaching this expression at runtime will panic or abort",
                                    tcx.hir().as_local_hir_id(def_id).unwrap(),
                                    Some(err.span),
                                )
                            }
                        // anything else (array lengths, enum initializers, constant patterns) are
                        // reported as hard errors
                        } else {
                            err.report_as_error(ecx.tcx, "evaluation of constant value failed")
                        }
                    }
                }
            } else {
                // use of broken constant from other crate
                err.report_as_error(ecx.tcx, "could not evaluate constant")
            }
        })
}