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
385
386
387
388
389
390
391
392
393
use super::{CompileTimeEvalContext, CompileTimeInterpreter, ConstEvalErr};
use crate::interpret::eval_nullary_intrinsic;
use crate::interpret::{
    intern_const_alloc_recursive, Allocation, ConstAlloc, ConstValue, CtfeValidationMode, GlobalId,
    Immediate, InternKind, InterpCx, InterpResult, MPlaceTy, MemoryKind, OpTy, RefTracking,
    ScalarMaybeUninit, StackPopCleanup,
};

use rustc_hir::def::DefKind;
use rustc_middle::mir;
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::mir::pretty::display_allocation;
use rustc_middle::traits::Reveal;
use rustc_middle::ty::layout::LayoutOf;
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::{self, subst::Subst, TyCtxt};
use rustc_span::source_map::Span;
use rustc_target::abi::{self, Abi};
use std::borrow::Cow;
use std::convert::TryInto;

const NOTE_ON_UNDEFINED_BEHAVIOR_ERROR: &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;
    assert!(
        cid.promoted.is_some()
            || matches!(
                ecx.tcx.def_kind(cid.instance.def_id()),
                DefKind::Const
                    | DefKind::Static(_)
                    | DefKind::ConstParam
                    | DefKind::AnonConst
                    | DefKind::InlineConst
                    | DefKind::AssocConst
            ),
        "Unexpected DefKind: {:?}",
        ecx.tcx.def_kind(cid.instance.def_id())
    );
    let layout = ecx.layout_of(body.bound_return_ty().subst(tcx, cid.instance.substs))?;
    assert!(!layout.is_unsized());
    let ret = ecx.allocate(layout, MemoryKind::Stack)?;

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

    ecx.push_stack_frame(
        cid.instance,
        body,
        &ret.into(),
        StackPopCleanup::Root { cleanup: false },
    )?;

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

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

    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>,
    root_span: Span,
    param_env: ty::ParamEnv<'tcx>,
    can_access_statics: bool,
) -> CompileTimeEvalContext<'mir, 'tcx> {
    debug!("mk_eval_cx: {:?}", param_env);
    InterpCx::new(
        tcx,
        root_span,
        param_env,
        CompileTimeInterpreter::new(tcx.const_eval_limit(), can_access_statics),
    )
}

/// This function converts an interpreter value into a constant that is meant for use in the
/// type system.
#[instrument(skip(ecx), level = "debug")]
pub(super) fn op_to_const<'tcx>(
    ecx: &CompileTimeEvalContext<'_, 'tcx>,
    op: &OpTy<'tcx>,
) -> ConstValue<'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 uninitialized 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 `ScalarMaybeUninit`, 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 {
        Abi::Scalar(abi::Scalar::Initialized { .. }) => true,
        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 destructure_mir_constant, 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()
    };

    debug!(?immediate);

    // We know `offset` is relative to the allocation, so we can use `into_parts`.
    let to_const_value = |mplace: &MPlaceTy<'_>| {
        debug!("to_const_value(mplace: {:?})", mplace);
        match mplace.ptr.into_parts() {
            (Some(alloc_id), offset) => {
                let alloc = ecx.tcx.global_alloc(alloc_id).unwrap_memory();
                ConstValue::ByRef { alloc, offset }
            }
            (None, offset) => {
                assert!(mplace.layout.is_zst());
                assert_eq!(
                    offset.bytes() % mplace.layout.align.abi.bytes(),
                    0,
                    "this MPlaceTy must come from a validated constant, thus we can assume the \
                alignment is correct",
                );
                ConstValue::ZeroSized
            }
        }
    };
    match immediate {
        Ok(ref mplace) => to_const_value(mplace),
        // see comment on `let try_as_immediate` above
        Err(imm) => match *imm {
            _ if imm.layout.is_zst() => ConstValue::ZeroSized,
            Immediate::Scalar(x) => match x {
                ScalarMaybeUninit::Scalar(s) => ConstValue::Scalar(s),
                ScalarMaybeUninit::Uninit => to_const_value(&op.assert_mem_place()),
            },
            Immediate::ScalarPair(a, b) => {
                debug!("ScalarPair(a: {:?}, b: {:?})", a, b);
                // We know `offset` is relative to the allocation, so we can use `into_parts`.
                let (data, start) = match a.to_pointer(ecx).unwrap().into_parts() {
                    (Some(alloc_id), offset) => {
                        (ecx.tcx.global_alloc(alloc_id).unwrap_memory(), offset.bytes())
                    }
                    (None, _offset) => (
                        ecx.tcx.intern_const_alloc(Allocation::from_bytes_byte_aligned_immutable(
                            b"" as &[u8],
                        )),
                        0,
                    ),
                };
                let len = b.to_machine_usize(ecx).unwrap();
                let start = start.try_into().unwrap();
                let len: usize = len.try_into().unwrap();
                ConstValue::Slice { data, start, end: start + len }
            }
            Immediate::Uninit => to_const_value(&op.assert_mem_place()),
        },
    }
}

#[instrument(skip(tcx), level = "debug")]
pub(crate) fn turn_into_const_value<'tcx>(
    tcx: TyCtxt<'tcx>,
    constant: ConstAlloc<'tcx>,
    key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
) -> ConstValue<'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 mplace = ecx.raw_const_to_mplace(constant).expect(
        "can only fail if layout computation failed, \
        which should have given a good error before ever invoking this function",
    );
    assert!(
        !is_static || cid.promoted.is_some(),
        "the `eval_to_const_value_raw` query should not be used for statics, use `eval_to_allocation` instead"
    );

    // Turn this into a proper constant.
    let const_val = op_to_const(&ecx, &mplace.into());
    debug!(?const_val);

    const_val
}

#[instrument(skip(tcx), level = "debug")]
pub fn eval_to_const_value_raw_provider<'tcx>(
    tcx: TyCtxt<'tcx>,
    key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
) -> ::rustc_middle::mir::interpret::EvalToConstValueResult<'tcx> {
    assert!(key.param_env.is_const());
    // see comment in eval_to_allocation_raw_provider for what we're doing here
    if key.param_env.reveal() == Reveal::All {
        let mut key = key;
        key.param_env = key.param_env.with_user_facing();
        match tcx.eval_to_const_value_raw(key) {
            // try again with reveal all as requested
            Err(ErrorHandled::TooGeneric) => {}
            // deduplicate 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(tcx, key.param_env);
        let ty::FnDef(_, substs) = ty.kind() else {
            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.into_kind(), stacktrace: vec![], span };
            error.report_as_error(tcx.at(span), "could not evaluate nullary intrinsic")
        });
    }

    tcx.eval_to_allocation_raw(key).map(|val| turn_into_const_value(tcx, val, key))
}

#[instrument(skip(tcx), level = "debug")]
pub fn eval_to_allocation_raw_provider<'tcx>(
    tcx: TyCtxt<'tcx>,
    key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
) -> ::rustc_middle::mir::interpret::EvalToAllocationRawResult<'tcx> {
    assert!(key.param_env.is_const());
    // 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 = key.param_env.with_user_facing();
        match tcx.eval_to_allocation_raw(key) {
            // try again with reveal all as requested
            Err(ErrorHandled::TooGeneric) => {}
            // deduplicate 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 = with_no_trimmed_paths!(key.value.instance.to_string());
        trace!("const eval: {:?} ({})", key, instance);
    }

    let cid = key.value;
    let def = cid.instance.def.with_opt_param();
    let is_static = tcx.is_static(def.did);

    let mut ecx = InterpCx::new(
        tcx,
        tcx.def_span(def.did),
        key.param_env,
        // Statics (and promoteds inside statics) may access other statics, because unlike consts
        // they do not have to behave "as if" they were evaluated at runtime.
        CompileTimeInterpreter::new(tcx.const_eval_limit(), /*can_access_statics:*/ is_static),
    );

    let res = ecx.load_mir(cid.instance.def, cid.promoted);
    match res.and_then(|body| eval_body_using_ecx(&mut ecx, cid, &body)) {
        Err(error) => {
            let err = ConstEvalErr::new(&ecx, error, None);
            // Some CTFE errors raise just a lint, not a hard error; see
            // <https://github.com/rust-lang/rust/issues/71800>.
            let is_hard_err = if let Some(def) = def.as_local() {
                // (Associated) consts only emit a lint, since they might be unused.
                !matches!(tcx.def_kind(def.did.to_def_id()), DefKind::Const | DefKind::AssocConst)
                    // check if the inner InterpError is hard
                    || err.error.is_hard_err()
            } else {
                // use of broken constant from other crate: always an error
                true
            };

            if is_hard_err {
                let msg = if is_static {
                    Cow::from("could not evaluate static initializer")
                } else {
                    // If the current item has generics, we'd like to enrich the message with the
                    // instance and its substs: to show the actual compile-time values, in addition to
                    // the expression, leading to the const eval error.
                    let instance = &key.value.instance;
                    if !instance.substs.is_empty() {
                        let instance = with_no_trimmed_paths!(instance.to_string());
                        let msg = format!("evaluation of `{}` failed", instance);
                        Cow::from(msg)
                    } else {
                        Cow::from("evaluation of constant value failed")
                    }
                };

                Err(err.report_as_error(ecx.tcx.at(err.span), &msg))
            } else {
                let hir_id = tcx.hir().local_def_id_to_hir_id(def.as_local().unwrap().did);
                Err(err.report_as_lint(
                    tcx.at(tcx.def_span(def.did)),
                    "any use of this value will cause an error",
                    hir_id,
                    Some(err.span),
                ))
            }
        }
        Ok(mplace) => {
            // Since evaluation had no errors, validate the resulting constant.
            // This is a separate `try` block to provide more targeted error reporting.
            let validation = try {
                let mut ref_tracking = RefTracking::new(mplace);
                let mut inner = false;
                while let Some((mplace, path)) = ref_tracking.todo.pop() {
                    let mode = match tcx.static_mutability(cid.instance.def_id()) {
                        Some(_) if cid.promoted.is_some() => {
                            // Promoteds in statics are allowed to point to statics.
                            CtfeValidationMode::Const { inner, allow_static_ptrs: true }
                        }
                        Some(_) => CtfeValidationMode::Regular, // a `static`
                        None => CtfeValidationMode::Const { inner, allow_static_ptrs: false },
                    };
                    ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)?;
                    inner = true;
                }
            };
            let alloc_id = mplace.ptr.provenance.unwrap();
            if let Err(error) = validation {
                // Validation failed, report an error. This is always a hard error.
                let err = ConstEvalErr::new(&ecx, error, None);
                Err(err.struct_error(
                    ecx.tcx,
                    "it is undefined behavior to use this value",
                    |diag| {
                        diag.note(NOTE_ON_UNDEFINED_BEHAVIOR_ERROR);
                        diag.note(&format!(
                            "the raw bytes of the constant ({}",
                            display_allocation(
                                *ecx.tcx,
                                ecx.tcx.global_alloc(alloc_id).unwrap_memory().inner()
                            )
                        ));
                    },
                ))
            } else {
                // Convert to raw constant
                Ok(ConstAlloc { alloc_id, ty: mplace.layout.ty })
            }
        }
    }
}