rustc_middle/ty/
list.rs

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
use std::alloc::Layout;
use std::cmp::Ordering;
use std::hash::{Hash, Hasher};
use std::ops::Deref;
use std::{fmt, iter, mem, ptr, slice};

use rustc_data_structures::aligned::{Aligned, align_of};
#[cfg(parallel_compiler)]
use rustc_data_structures::sync::DynSync;
use rustc_serialize::{Encodable, Encoder};

use super::flags::FlagComputation;
use super::{DebruijnIndex, TypeFlags};
use crate::arena::Arena;

/// `List<T>` is a bit like `&[T]`, but with some critical differences.
/// - IMPORTANT: Every `List<T>` is *required* to have unique contents. The
///   type's correctness relies on this, *but it does not enforce it*.
///   Therefore, any code that creates a `List<T>` must ensure uniqueness
///   itself. In practice this is achieved by interning.
/// - The length is stored within the `List<T>`, so `&List<Ty>` is a thin
///   pointer.
/// - Because of this, you cannot get a `List<T>` that is a sub-list of another
///   `List<T>`. You can get a sub-slice `&[T]`, however.
/// - `List<T>` can be used with `CopyTaggedPtr`, which is useful within
///   structs whose size must be minimized.
/// - Because of the uniqueness assumption, we can use the address of a
///   `List<T>` for faster equality comparisons and hashing.
/// - `T` must be `Copy`. This lets `List<T>` be stored in a dropless arena and
///   iterators return a `T` rather than a `&T`.
/// - `T` must not be zero-sized.
pub type List<T> = RawList<(), T>;

/// A generic type that can be used to prepend a [`List`] with some header.
///
/// The header will be ignored for value-based operations like [`PartialEq`],
/// [`Hash`] and [`Encodable`].
#[repr(C)]
pub struct RawList<H, T> {
    skel: ListSkeleton<H, T>,
    opaque: OpaqueListContents,
}

/// A [`RawList`] without the unsized tail. This type is used for layout computation
/// and constructing empty lists.
#[repr(C)]
struct ListSkeleton<H, T> {
    header: H,
    len: usize,
    /// Although this claims to be a zero-length array, in practice `len`
    /// elements are actually present.
    data: [T; 0],
}

impl<T> Default for &List<T> {
    fn default() -> Self {
        List::empty()
    }
}

unsafe extern "C" {
    /// A dummy type used to force `List` to be unsized while not requiring
    /// references to it be wide pointers.
    type OpaqueListContents;
}

impl<H, T> RawList<H, T> {
    #[inline(always)]
    pub fn len(&self) -> usize {
        self.skel.len
    }

    #[inline(always)]
    pub fn as_slice(&self) -> &[T] {
        self
    }

    /// Allocates a list from `arena` and copies the contents of `slice` into it.
    ///
    /// WARNING: the contents *must be unique*, such that no list with these
    /// contents has been previously created. If not, operations such as `eq`
    /// and `hash` might give incorrect results.
    ///
    /// Panics if `T` is `Drop`, or `T` is zero-sized, or the slice is empty
    /// (because the empty list exists statically, and is available via
    /// `empty()`).
    #[inline]
    pub(super) fn from_arena<'tcx>(
        arena: &'tcx Arena<'tcx>,
        header: H,
        slice: &[T],
    ) -> &'tcx RawList<H, T>
    where
        T: Copy,
    {
        assert!(!mem::needs_drop::<T>());
        assert!(mem::size_of::<T>() != 0);
        assert!(!slice.is_empty());

        let (layout, _offset) =
            Layout::new::<ListSkeleton<H, T>>().extend(Layout::for_value::<[T]>(slice)).unwrap();

        let mem = arena.dropless.alloc_raw(layout) as *mut RawList<H, T>;
        unsafe {
            // Write the header
            (&raw mut (*mem).skel.header).write(header);

            // Write the length
            (&raw mut (*mem).skel.len).write(slice.len());

            // Write the elements
            (&raw mut (*mem).skel.data)
                .cast::<T>()
                .copy_from_nonoverlapping(slice.as_ptr(), slice.len());

            &*mem
        }
    }

    // If this method didn't exist, we would use `slice.iter` due to
    // deref coercion.
    //
    // This would be weird, as `self.into_iter` iterates over `T` directly.
    #[inline(always)]
    pub fn iter(&self) -> <&'_ RawList<H, T> as IntoIterator>::IntoIter
    where
        T: Copy,
    {
        self.into_iter()
    }
}

impl<'a, H, T: Copy> rustc_type_ir::inherent::SliceLike for &'a RawList<H, T> {
    type Item = T;

    type IntoIter = iter::Copied<<&'a [T] as IntoIterator>::IntoIter>;

    fn iter(self) -> Self::IntoIter {
        (*self).iter()
    }

    fn as_slice(&self) -> &[Self::Item] {
        (*self).as_slice()
    }
}

macro_rules! impl_list_empty {
    ($header_ty:ty, $header_init:expr) => {
        impl<T> RawList<$header_ty, T> {
            /// Returns a reference to the (per header unique, static) empty list.
            #[inline(always)]
            pub fn empty<'a>() -> &'a RawList<$header_ty, T> {
                #[repr(align(64))]
                struct MaxAlign;

                static EMPTY: ListSkeleton<$header_ty, MaxAlign> =
                    ListSkeleton { header: $header_init, len: 0, data: [] };

                assert!(mem::align_of::<T>() <= mem::align_of::<MaxAlign>());

                // SAFETY: `EMPTY` is sufficiently aligned to be an empty list for all
                // types with `align_of(T) <= align_of(MaxAlign)`, which we checked above.
                unsafe { &*((&raw const EMPTY) as *const RawList<$header_ty, T>) }
            }
        }
    };
}

impl_list_empty!((), ());

impl<H, T: fmt::Debug> fmt::Debug for RawList<H, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

impl<H, S: Encoder, T: Encodable<S>> Encodable<S> for RawList<H, T> {
    #[inline]
    fn encode(&self, s: &mut S) {
        (**self).encode(s);
    }
}

impl<H, T: PartialEq> PartialEq for RawList<H, T> {
    #[inline]
    fn eq(&self, other: &RawList<H, T>) -> bool {
        // Pointer equality implies list equality (due to the unique contents
        // assumption).
        ptr::eq(self, other)
    }
}

impl<H, T: Eq> Eq for RawList<H, T> {}

impl<H, T> Ord for RawList<H, T>
where
    T: Ord,
{
    fn cmp(&self, other: &RawList<H, T>) -> Ordering {
        // Pointer equality implies list equality (due to the unique contents
        // assumption), but the contents must be compared otherwise.
        if self == other { Ordering::Equal } else { <[T] as Ord>::cmp(&**self, &**other) }
    }
}

impl<H, T> PartialOrd for RawList<H, T>
where
    T: PartialOrd,
{
    fn partial_cmp(&self, other: &RawList<H, T>) -> Option<Ordering> {
        // Pointer equality implies list equality (due to the unique contents
        // assumption), but the contents must be compared otherwise.
        if self == other {
            Some(Ordering::Equal)
        } else {
            <[T] as PartialOrd>::partial_cmp(&**self, &**other)
        }
    }
}

impl<Hdr, T> Hash for RawList<Hdr, T> {
    #[inline]
    fn hash<H: Hasher>(&self, s: &mut H) {
        // Pointer hashing is sufficient (due to the unique contents
        // assumption).
        ptr::from_ref(self).hash(s)
    }
}

impl<H, T> Deref for RawList<H, T> {
    type Target = [T];
    #[inline(always)]
    fn deref(&self) -> &[T] {
        self.as_ref()
    }
}

impl<H, T> AsRef<[T]> for RawList<H, T> {
    #[inline(always)]
    fn as_ref(&self) -> &[T] {
        let data_ptr = (&raw const self.skel.data).cast::<T>();
        // SAFETY: `data_ptr` has the same provenance as `self` and can therefore
        // access the `self.skel.len` elements stored at `self.skel.data`.
        // Note that we specifically don't reborrow `&self.skel.data`, because that
        // would give us a pointer with provenance over 0 bytes.
        unsafe { slice::from_raw_parts(data_ptr, self.skel.len) }
    }
}

impl<'a, H, T: Copy> IntoIterator for &'a RawList<H, T> {
    type Item = T;
    type IntoIter = iter::Copied<<&'a [T] as IntoIterator>::IntoIter>;
    #[inline(always)]
    fn into_iter(self) -> Self::IntoIter {
        self[..].iter().copied()
    }
}

unsafe impl<H: Sync, T: Sync> Sync for RawList<H, T> {}

// We need this since `List` uses extern type `OpaqueListContents`.
#[cfg(parallel_compiler)]
unsafe impl<H: DynSync, T: DynSync> DynSync for RawList<H, T> {}

// Safety:
// Layouts of `ListSkeleton<H, T>` and `RawList<H, T>` are the same, modulo opaque tail,
// thus aligns of `ListSkeleton<H, T>` and `RawList<H, T>` must be the same.
unsafe impl<H, T> Aligned for RawList<H, T> {
    const ALIGN: ptr::Alignment = align_of::<ListSkeleton<H, T>>();
}

/// A [`List`] that additionally stores type information inline to speed up
/// [`TypeVisitableExt`](super::TypeVisitableExt) operations.
pub type ListWithCachedTypeInfo<T> = RawList<TypeInfo, T>;

impl<T> ListWithCachedTypeInfo<T> {
    #[inline(always)]
    pub fn flags(&self) -> TypeFlags {
        self.skel.header.flags
    }

    #[inline(always)]
    pub fn outer_exclusive_binder(&self) -> DebruijnIndex {
        self.skel.header.outer_exclusive_binder
    }
}

impl_list_empty!(TypeInfo, TypeInfo::empty());

/// The additional info that is stored in [`ListWithCachedTypeInfo`].
#[repr(C)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct TypeInfo {
    flags: TypeFlags,
    outer_exclusive_binder: DebruijnIndex,
}

impl TypeInfo {
    const fn empty() -> Self {
        Self { flags: TypeFlags::empty(), outer_exclusive_binder: super::INNERMOST }
    }
}

impl From<FlagComputation> for TypeInfo {
    fn from(computation: FlagComputation) -> TypeInfo {
        TypeInfo {
            flags: computation.flags,
            outer_exclusive_binder: computation.outer_exclusive_binder,
        }
    }
}