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

// Copyright (c) Zefchain Labs, Inc.
// SPDX-License-Identifier: Apache-2.0

//! Implementations of the custom traits for types from the standard library.

use std::{borrow::Cow, ops::Add};

use frunk::{HCons, HList, HNil};

use crate::{
    GuestPointer, InstanceWithMemory, Layout, Memory, Runtime, RuntimeError, RuntimeMemory, Split,
    WitLoad, WitStore, WitType,
};

impl WitType for HNil {
    const SIZE: u32 = 0;

    type Layout = HNil;
    type Dependencies = HNil;

    fn wit_type_name() -> Cow<'static, str> {
        "hnil".into()
    }

    fn wit_type_declaration() -> Cow<'static, str> {
        "type hnil = unit".into()
    }
}

impl WitLoad for HNil {
    fn load<Instance>(
        _memory: &Memory<'_, Instance>,
        _location: GuestPointer,
    ) -> Result<Self, RuntimeError>
    where
        Instance: InstanceWithMemory,
    {
        Ok(HNil)
    }

    fn lift_from<Instance>(
        HNil: <Self::Layout as Layout>::Flat,
        _memory: &Memory<'_, Instance>,
    ) -> Result<Self, RuntimeError>
    where
        Instance: InstanceWithMemory,
    {
        Ok(HNil)
    }
}

impl WitStore for HNil {
    fn store<Instance>(
        &self,
        _memory: &mut Memory<'_, Instance>,
        _location: GuestPointer,
    ) -> Result<(), RuntimeError>
    where
        Instance: InstanceWithMemory,
        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
    {
        Ok(())
    }

    fn lower<Instance>(
        &self,
        _memory: &mut Memory<'_, Instance>,
    ) -> Result<<Self::Layout as Layout>::Flat, RuntimeError>
    where
        Instance: InstanceWithMemory,
        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
    {
        Ok(HNil)
    }
}

impl<Head, Tail> WitType for HCons<Head, Tail>
where
    Head: WitType,
    Tail: WitType + SizeCalculation,
    Head::Layout: Add<Tail::Layout>,
    <Head::Layout as Add<Tail::Layout>>::Output: Layout,
{
    const SIZE: u32 = {
        let packed_size = Self::SIZE_STARTING_AT_BYTE_BOUNDARIES[0];
        let aligned_size = GuestPointer(packed_size).after_padding_for::<Self>();

        aligned_size.0
    };

    type Layout = <Head::Layout as Add<Tail::Layout>>::Output;
    type Dependencies = HList![Head, Tail];

    fn wit_type_name() -> Cow<'static, str> {
        format!("hcons-{}-{}", Head::wit_type_name(), Tail::wit_type_name()).into()
    }

    fn wit_type_declaration() -> Cow<'static, str> {
        let head = Head::wit_type_name();
        let tail = Tail::wit_type_name();

        format!("type hcons-{head}-{tail} = tuple<{head}, {tail}>").into()
    }
}

impl<Head, Tail> WitLoad for HCons<Head, Tail>
where
    Head: WitLoad,
    Tail: WitLoad + SizeCalculation,
    Head::Layout: Add<Tail::Layout>,
    <Head::Layout as Add<Tail::Layout>>::Output: Layout,
    <Self::Layout as Layout>::Flat:
        Split<<Head::Layout as Layout>::Flat, Remainder = <Tail::Layout as Layout>::Flat>,
{
    fn load<Instance>(
        memory: &Memory<'_, Instance>,
        location: GuestPointer,
    ) -> Result<Self, RuntimeError>
    where
        Instance: InstanceWithMemory,
        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
    {
        Ok(HCons {
            head: Head::load(memory, location)?,
            tail: Tail::load(
                memory,
                location
                    .after::<Head>()
                    .after_padding_for::<Tail::FirstElement>(),
            )?,
        })
    }

    fn lift_from<Instance>(
        layout: <Self::Layout as Layout>::Flat,
        memory: &Memory<'_, Instance>,
    ) -> Result<Self, RuntimeError>
    where
        Instance: InstanceWithMemory,
        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
    {
        let (head_layout, tail_layout) = layout.split();

        Ok(HCons {
            head: Head::lift_from(head_layout, memory)?,
            tail: Tail::lift_from(tail_layout, memory)?,
        })
    }
}

impl<Head, Tail> WitStore for HCons<Head, Tail>
where
    Head: WitStore,
    Tail: WitStore + SizeCalculation,
    Head::Layout: Add<Tail::Layout>,
    <Head::Layout as Add<Tail::Layout>>::Output: Layout,
    <Head::Layout as Layout>::Flat: Add<<Tail::Layout as Layout>::Flat>,
    Self::Layout: Layout<
        Flat = <<Head::Layout as Layout>::Flat as Add<<Tail::Layout as Layout>::Flat>>::Output,
    >,
{
    fn store<Instance>(
        &self,
        memory: &mut Memory<'_, Instance>,
        location: GuestPointer,
    ) -> Result<(), RuntimeError>
    where
        Instance: InstanceWithMemory,
        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
    {
        self.head.store(memory, location)?;
        self.tail.store(
            memory,
            location
                .after::<Head>()
                .after_padding_for::<Tail::FirstElement>(),
        )?;

        Ok(())
    }

    fn lower<Instance>(
        &self,
        memory: &mut Memory<'_, Instance>,
    ) -> Result<<Self::Layout as Layout>::Flat, RuntimeError>
    where
        Instance: InstanceWithMemory,
        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
    {
        let head_layout = self.head.lower(memory)?;
        let tail_layout = self.tail.lower(memory)?;

        Ok(head_layout + tail_layout)
    }
}

/// Helper trait used to calculate the size of a heterogeneous list considering internal alignment.
///
/// Assumes the maximum alignment necessary for any type is 8 bytes, which is the alignment for the
/// largest flat types (`i64` and `f64`).
trait SizeCalculation {
    /// The size of the list considering the current size calculation starts at different offsets
    /// inside an 8-byte window.
    const SIZE_STARTING_AT_BYTE_BOUNDARIES: [u32; 8];

    /// The type of the first element of the list, used to determine the current necessary
    /// alignment.
    type FirstElement: WitType;
}

impl SizeCalculation for HNil {
    const SIZE_STARTING_AT_BYTE_BOUNDARIES: [u32; 8] = [0; 8];

    type FirstElement = ();
}

/// Unrolls a `for`-like loop so that it runs in a `const` context.
macro_rules! unroll_for {
    ($binding:ident in [ $($elements:expr),* $(,)? ] $body:tt) => {
        $(
            let $binding = $elements;
            $body
        )*
    };
}

impl<Head, Tail> SizeCalculation for HCons<Head, Tail>
where
    Head: WitType,
    Tail: SizeCalculation,
{
    const SIZE_STARTING_AT_BYTE_BOUNDARIES: [u32; 8] = {
        let mut size_at_boundaries = [0; 8];

        unroll_for!(boundary_offset in [0, 1, 2, 3, 4, 5, 6, 7] {
            let memory_location = GuestPointer(boundary_offset)
                .after_padding_for::<Head>()
                .after::<Head>();

            let tail_size = Tail::SIZE_STARTING_AT_BYTE_BOUNDARIES[memory_location.0 as usize % 8];

            size_at_boundaries[boundary_offset as usize] =
                memory_location.0 - boundary_offset + tail_size;
        });

        size_at_boundaries
    };

    type FirstElement = Head;
}