| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `crates/stdx/src/variance.rs`."><title>variance.rs - source</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../static.files/rustdoc-84e720fa.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="stdx" data-themes="" data-resource-suffix="" data-rustdoc-version="1.89.0 (29483883e 2025-08-04)" data-channel="1.89.0" data-search-js="search-92309212.js" data-settings-js="settings-5514c975.js" ><script src="../../static.files/storage-4e99c027.js"></script><script defer src="../../static.files/src-script-813739b1.js"></script><script defer src="../../src-files.js"></script><script defer src="../../static.files/main-fd3af306.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-32bb7600.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-044be391.svg"></head><body class="rustdoc src"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="src-sidebar-title"><h2>Files</h2></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><h1><div class="sub-heading">stdx/</div>variance.rs</h1><rustdoc-toolbar></rustdoc-toolbar></div><div class="example-wrap digits-3"><pre class="rust"><code><a href=#1 id=1 data-nosnippet>1</a><span class="doccomment">//! This is a copy of [`std::marker::variance`]. |
| <a href=#2 id=2 data-nosnippet>2</a> |
| <a href=#3 id=3 data-nosnippet>3</a></span><span class="kw">use </span>std::any::type_name; |
| <a href=#4 id=4 data-nosnippet>4</a><span class="kw">use </span>std::cmp::Ordering; |
| <a href=#5 id=5 data-nosnippet>5</a><span class="kw">use </span>std::fmt; |
| <a href=#6 id=6 data-nosnippet>6</a><span class="kw">use </span>std::hash::{Hash, Hasher}; |
| <a href=#7 id=7 data-nosnippet>7</a><span class="kw">use </span>std::marker::PhantomData; |
| <a href=#8 id=8 data-nosnippet>8</a> |
| <a href=#9 id=9 data-nosnippet>9</a><span class="macro">macro_rules!</span> first_token { |
| <a href=#10 id=10 data-nosnippet>10</a> (<span class="macro-nonterminal">$first</span>:tt $(<span class="macro-nonterminal">$rest</span>:tt)<span class="kw-2">*</span>) => { |
| <a href=#11 id=11 data-nosnippet>11</a> <span class="macro-nonterminal">$first |
| <a href=#12 id=12 data-nosnippet>12</a> </span>}; |
| <a href=#13 id=13 data-nosnippet>13</a>} |
| <a href=#14 id=14 data-nosnippet>14</a><span class="macro">macro_rules!</span> phantom_type { |
| <a href=#15 id=15 data-nosnippet>15</a> ($( |
| <a href=#16 id=16 data-nosnippet>16</a> $(<span class="attr">#[<span class="macro-nonterminal">$attr</span>:meta]</span>)* |
| <a href=#17 id=17 data-nosnippet>17</a> <span class="kw">pub struct </span><span class="macro-nonterminal">$name</span>:ident <<span class="macro-nonterminal">$t</span>:ident> ($(<span class="macro-nonterminal">$inner</span>:tt)<span class="kw-2">*</span>); |
| <a href=#18 id=18 data-nosnippet>18</a> )<span class="kw-2">*</span>) => {$( |
| <a href=#19 id=19 data-nosnippet>19</a> $(<span class="attr">#[<span class="macro-nonterminal">$attr</span>]</span>)* |
| <a href=#20 id=20 data-nosnippet>20</a> <span class="kw">pub struct </span><span class="macro-nonterminal">$name</span><<span class="macro-nonterminal">$t</span>>($(<span class="macro-nonterminal">$inner</span>)<span class="kw-2">*</span>) <span class="kw">where </span>T: <span class="question-mark">?</span>Sized; |
| <a href=#21 id=21 data-nosnippet>21</a> |
| <a href=#22 id=22 data-nosnippet>22</a> <span class="kw">impl</span><T> <span class="macro-nonterminal">$name</span><T> |
| <a href=#23 id=23 data-nosnippet>23</a> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized |
| <a href=#24 id=24 data-nosnippet>24</a> { |
| <a href=#25 id=25 data-nosnippet>25</a> <span class="doccomment">/// Constructs a new instance of the variance marker. |
| <a href=#26 id=26 data-nosnippet>26</a> </span><span class="kw">pub const fn </span>new() -> <span class="self">Self </span>{ |
| <a href=#27 id=27 data-nosnippet>27</a> <span class="self">Self</span>(PhantomData) |
| <a href=#28 id=28 data-nosnippet>28</a> } |
| <a href=#29 id=29 data-nosnippet>29</a> } |
| <a href=#30 id=30 data-nosnippet>30</a> |
| <a href=#31 id=31 data-nosnippet>31</a> <span class="kw">impl</span><T> <span class="self">self</span>::sealed::Sealed <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized { |
| <a href=#32 id=32 data-nosnippet>32</a> <span class="kw">const </span>VALUE: <span class="self">Self </span>= <span class="self">Self</span>::new(); |
| <a href=#33 id=33 data-nosnippet>33</a> } |
| <a href=#34 id=34 data-nosnippet>34</a> |
| <a href=#35 id=35 data-nosnippet>35</a> <span class="kw">impl</span><T> Variance <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized {} |
| <a href=#36 id=36 data-nosnippet>36</a> |
| <a href=#37 id=37 data-nosnippet>37</a> <span class="kw">impl</span><T> Default <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> |
| <a href=#38 id=38 data-nosnippet>38</a> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized |
| <a href=#39 id=39 data-nosnippet>39</a> { |
| <a href=#40 id=40 data-nosnippet>40</a> <span class="kw">fn </span>default() -> <span class="self">Self </span>{ |
| <a href=#41 id=41 data-nosnippet>41</a> <span class="self">Self</span>(PhantomData) |
| <a href=#42 id=42 data-nosnippet>42</a> } |
| <a href=#43 id=43 data-nosnippet>43</a> } |
| <a href=#44 id=44 data-nosnippet>44</a> |
| <a href=#45 id=45 data-nosnippet>45</a> <span class="kw">impl</span><T> fmt::Debug <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> |
| <a href=#46 id=46 data-nosnippet>46</a> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized |
| <a href=#47 id=47 data-nosnippet>47</a> { |
| <a href=#48 id=48 data-nosnippet>48</a> <span class="kw">fn </span>fmt(<span class="kw-2">&</span><span class="self">self</span>, f: <span class="kw-2">&mut </span>fmt::Formatter<<span class="lifetime">'_</span>>) -> fmt::Result { |
| <a href=#49 id=49 data-nosnippet>49</a> <span class="macro">write!</span>(f, <span class="string">"{}<{}>"</span>, <span class="macro">stringify!</span>(<span class="macro-nonterminal">$name</span>), type_name::<T>()) |
| <a href=#50 id=50 data-nosnippet>50</a> } |
| <a href=#51 id=51 data-nosnippet>51</a> } |
| <a href=#52 id=52 data-nosnippet>52</a> |
| <a href=#53 id=53 data-nosnippet>53</a> <span class="kw">impl</span><T> Clone <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> |
| <a href=#54 id=54 data-nosnippet>54</a> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized |
| <a href=#55 id=55 data-nosnippet>55</a> { |
| <a href=#56 id=56 data-nosnippet>56</a> <span class="kw">fn </span>clone(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="self">Self </span>{ |
| <a href=#57 id=57 data-nosnippet>57</a> <span class="kw-2">*</span><span class="self">self |
| <a href=#58 id=58 data-nosnippet>58</a> </span>} |
| <a href=#59 id=59 data-nosnippet>59</a> } |
| <a href=#60 id=60 data-nosnippet>60</a> |
| <a href=#61 id=61 data-nosnippet>61</a> <span class="kw">impl</span><T> Copy <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized {} |
| <a href=#62 id=62 data-nosnippet>62</a> |
| <a href=#63 id=63 data-nosnippet>63</a> <span class="kw">impl</span><T> PartialEq <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> |
| <a href=#64 id=64 data-nosnippet>64</a> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized |
| <a href=#65 id=65 data-nosnippet>65</a> { |
| <a href=#66 id=66 data-nosnippet>66</a> <span class="kw">fn </span>eq(<span class="kw-2">&</span><span class="self">self</span>, <span class="kw">_</span>: <span class="kw-2">&</span><span class="self">Self</span>) -> bool { |
| <a href=#67 id=67 data-nosnippet>67</a> <span class="bool-val">true |
| <a href=#68 id=68 data-nosnippet>68</a> </span>} |
| <a href=#69 id=69 data-nosnippet>69</a> } |
| <a href=#70 id=70 data-nosnippet>70</a> |
| <a href=#71 id=71 data-nosnippet>71</a> <span class="kw">impl</span><T> Eq <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized {} |
| <a href=#72 id=72 data-nosnippet>72</a> |
| <a href=#73 id=73 data-nosnippet>73</a> <span class="attr">#[allow(clippy::non_canonical_partial_ord_impl)] |
| <a href=#74 id=74 data-nosnippet>74</a> </span><span class="kw">impl</span><T> PartialOrd <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> |
| <a href=#75 id=75 data-nosnippet>75</a> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized |
| <a href=#76 id=76 data-nosnippet>76</a> { |
| <a href=#77 id=77 data-nosnippet>77</a> <span class="kw">fn </span>partial_cmp(<span class="kw-2">&</span><span class="self">self</span>, <span class="kw">_</span>: <span class="kw-2">&</span><span class="self">Self</span>) -> <span class="prelude-ty">Option</span><Ordering> { |
| <a href=#78 id=78 data-nosnippet>78</a> <span class="prelude-val">Some</span>(Ordering::Equal) |
| <a href=#79 id=79 data-nosnippet>79</a> } |
| <a href=#80 id=80 data-nosnippet>80</a> } |
| <a href=#81 id=81 data-nosnippet>81</a> |
| <a href=#82 id=82 data-nosnippet>82</a> <span class="kw">impl</span><T> Ord <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> |
| <a href=#83 id=83 data-nosnippet>83</a> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized |
| <a href=#84 id=84 data-nosnippet>84</a> { |
| <a href=#85 id=85 data-nosnippet>85</a> <span class="kw">fn </span>cmp(<span class="kw-2">&</span><span class="self">self</span>, <span class="kw">_</span>: <span class="kw-2">&</span><span class="self">Self</span>) -> Ordering { |
| <a href=#86 id=86 data-nosnippet>86</a> Ordering::Equal |
| <a href=#87 id=87 data-nosnippet>87</a> } |
| <a href=#88 id=88 data-nosnippet>88</a> } |
| <a href=#89 id=89 data-nosnippet>89</a> |
| <a href=#90 id=90 data-nosnippet>90</a> <span class="kw">impl</span><T> Hash <span class="kw">for </span><span class="macro-nonterminal">$name</span><T> |
| <a href=#91 id=91 data-nosnippet>91</a> <span class="kw">where </span>T: <span class="question-mark">?</span>Sized |
| <a href=#92 id=92 data-nosnippet>92</a> { |
| <a href=#93 id=93 data-nosnippet>93</a> <span class="kw">fn </span>hash<H: Hasher>(<span class="kw-2">&</span><span class="self">self</span>, <span class="kw">_</span>: <span class="kw-2">&mut </span>H) {} |
| <a href=#94 id=94 data-nosnippet>94</a> } |
| <a href=#95 id=95 data-nosnippet>95</a> )<span class="kw-2">*</span>}; |
| <a href=#96 id=96 data-nosnippet>96</a>} |
| <a href=#97 id=97 data-nosnippet>97</a> |
| <a href=#98 id=98 data-nosnippet>98</a><span class="macro">macro_rules!</span> phantom_lifetime { |
| <a href=#99 id=99 data-nosnippet>99</a> ($( |
| <a href=#100 id=100 data-nosnippet>100</a> $(<span class="attr">#[<span class="macro-nonterminal">$attr</span>:meta]</span>)* |
| <a href=#101 id=101 data-nosnippet>101</a> <span class="kw">pub struct </span><span class="macro-nonterminal">$name</span>:ident <<span class="macro-nonterminal">$lt</span>:lifetime> ($(<span class="macro-nonterminal">$inner</span>:tt)<span class="kw-2">*</span>); |
| <a href=#102 id=102 data-nosnippet>102</a> )<span class="kw-2">*</span>) => {$( |
| <a href=#103 id=103 data-nosnippet>103</a> $(<span class="attr">#[<span class="macro-nonterminal">$attr</span>]</span>)* |
| <a href=#104 id=104 data-nosnippet>104</a> |
| <a href=#105 id=105 data-nosnippet>105</a> <span class="attr">#[derive(Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)] |
| <a href=#106 id=106 data-nosnippet>106</a> </span><span class="kw">pub struct </span><span class="macro-nonterminal">$name</span><<span class="macro-nonterminal">$lt</span>>($(<span class="macro-nonterminal">$inner</span>)<span class="kw-2">*</span>); |
| <a href=#107 id=107 data-nosnippet>107</a> |
| <a href=#108 id=108 data-nosnippet>108</a> <span class="kw">impl </span><span class="macro-nonterminal">$name</span><<span class="lifetime">'_</span>> { |
| <a href=#109 id=109 data-nosnippet>109</a> <span class="doccomment">/// Constructs a new instance of the variance marker. |
| <a href=#110 id=110 data-nosnippet>110</a> </span><span class="kw">pub const fn </span>new() -> <span class="self">Self </span>{ |
| <a href=#111 id=111 data-nosnippet>111</a> <span class="self">Self</span>(<span class="macro">first_token!</span>($(<span class="macro-nonterminal">$inner</span>)<span class="kw-2">*</span>)(PhantomData)) |
| <a href=#112 id=112 data-nosnippet>112</a> } |
| <a href=#113 id=113 data-nosnippet>113</a> } |
| <a href=#114 id=114 data-nosnippet>114</a> |
| <a href=#115 id=115 data-nosnippet>115</a> <span class="kw">impl </span><span class="self">self</span>::sealed::Sealed <span class="kw">for </span><span class="macro-nonterminal">$name</span><<span class="lifetime">'_</span>> { |
| <a href=#116 id=116 data-nosnippet>116</a> <span class="kw">const </span>VALUE: <span class="self">Self </span>= <span class="self">Self</span>::new(); |
| <a href=#117 id=117 data-nosnippet>117</a> } |
| <a href=#118 id=118 data-nosnippet>118</a> |
| <a href=#119 id=119 data-nosnippet>119</a> <span class="kw">impl </span>Variance <span class="kw">for </span><span class="macro-nonterminal">$name</span><<span class="lifetime">'_</span>> {} |
| <a href=#120 id=120 data-nosnippet>120</a> |
| <a href=#121 id=121 data-nosnippet>121</a> <span class="kw">impl </span>fmt::Debug <span class="kw">for </span><span class="macro-nonterminal">$name</span><<span class="lifetime">'_</span>> { |
| <a href=#122 id=122 data-nosnippet>122</a> <span class="kw">fn </span>fmt(<span class="kw-2">&</span><span class="self">self</span>, f: <span class="kw-2">&mut </span>fmt::Formatter<<span class="lifetime">'_</span>>) -> fmt::Result { |
| <a href=#123 id=123 data-nosnippet>123</a> <span class="macro">write!</span>(f, <span class="string">"{}"</span>, <span class="macro">stringify!</span>(<span class="macro-nonterminal">$name</span>)) |
| <a href=#124 id=124 data-nosnippet>124</a> } |
| <a href=#125 id=125 data-nosnippet>125</a> } |
| <a href=#126 id=126 data-nosnippet>126</a> )<span class="kw-2">*</span>}; |
| <a href=#127 id=127 data-nosnippet>127</a>} |
| <a href=#128 id=128 data-nosnippet>128</a> |
| <a href=#129 id=129 data-nosnippet>129</a><span class="macro">phantom_lifetime!</span> { |
| <a href=#130 id=130 data-nosnippet>130</a> <span class="doccomment">/// Zero-sized type used to mark a lifetime as covariant. |
| <a href=#131 id=131 data-nosnippet>131</a> /// |
| <a href=#132 id=132 data-nosnippet>132</a> /// Covariant lifetimes must live at least as long as declared. See [the reference][1] for more |
| <a href=#133 id=133 data-nosnippet>133</a> /// information. |
| <a href=#134 id=134 data-nosnippet>134</a> /// |
| <a href=#135 id=135 data-nosnippet>135</a> /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance |
| <a href=#136 id=136 data-nosnippet>136</a> /// |
| <a href=#137 id=137 data-nosnippet>137</a> /// ## Layout |
| <a href=#138 id=138 data-nosnippet>138</a> /// |
| <a href=#139 id=139 data-nosnippet>139</a> /// For all `'a`, the following are guaranteed: |
| <a href=#140 id=140 data-nosnippet>140</a> /// * `size_of::<PhantomCovariantLifetime<'a>>() == 0` |
| <a href=#141 id=141 data-nosnippet>141</a> /// * `align_of::<PhantomCovariantLifetime<'a>>() == 1` |
| <a href=#142 id=142 data-nosnippet>142</a> |
| <a href=#143 id=143 data-nosnippet>143</a> </span><span class="kw">pub struct </span>PhantomCovariantLifetime<<span class="lifetime">'a</span>>(PhantomCovariant<<span class="kw-2">&</span><span class="lifetime">'a </span>()>); |
| <a href=#144 id=144 data-nosnippet>144</a> <span class="doccomment">/// Zero-sized type used to mark a lifetime as contravariant. |
| <a href=#145 id=145 data-nosnippet>145</a> /// |
| <a href=#146 id=146 data-nosnippet>146</a> /// Contravariant lifetimes must live at most as long as declared. See [the reference][1] for |
| <a href=#147 id=147 data-nosnippet>147</a> /// more information. |
| <a href=#148 id=148 data-nosnippet>148</a> /// |
| <a href=#149 id=149 data-nosnippet>149</a> /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance |
| <a href=#150 id=150 data-nosnippet>150</a> /// |
| <a href=#151 id=151 data-nosnippet>151</a> /// ## Layout |
| <a href=#152 id=152 data-nosnippet>152</a> /// |
| <a href=#153 id=153 data-nosnippet>153</a> /// For all `'a`, the following are guaranteed: |
| <a href=#154 id=154 data-nosnippet>154</a> /// * `size_of::<PhantomContravariantLifetime<'a>>() == 0` |
| <a href=#155 id=155 data-nosnippet>155</a> /// * `align_of::<PhantomContravariantLifetime<'a>>() == 1` |
| <a href=#156 id=156 data-nosnippet>156</a> |
| <a href=#157 id=157 data-nosnippet>157</a> </span><span class="kw">pub struct </span>PhantomContravariantLifetime<<span class="lifetime">'a</span>>(PhantomContravariant<<span class="kw-2">&</span><span class="lifetime">'a </span>()>); |
| <a href=#158 id=158 data-nosnippet>158</a> <span class="doccomment">/// Zero-sized type used to mark a lifetime as invariant. |
| <a href=#159 id=159 data-nosnippet>159</a> /// |
| <a href=#160 id=160 data-nosnippet>160</a> /// Invariant lifetimes must be live for the exact length declared, neither shorter nor longer. |
| <a href=#161 id=161 data-nosnippet>161</a> /// See [the reference][1] for more information. |
| <a href=#162 id=162 data-nosnippet>162</a> /// |
| <a href=#163 id=163 data-nosnippet>163</a> /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance |
| <a href=#164 id=164 data-nosnippet>164</a> /// |
| <a href=#165 id=165 data-nosnippet>165</a> /// ## Layout |
| <a href=#166 id=166 data-nosnippet>166</a> /// |
| <a href=#167 id=167 data-nosnippet>167</a> /// For all `'a`, the following are guaranteed: |
| <a href=#168 id=168 data-nosnippet>168</a> /// * `size_of::<PhantomInvariantLifetime<'a>>() == 0` |
| <a href=#169 id=169 data-nosnippet>169</a> /// * `align_of::<PhantomInvariantLifetime<'a>>() == 1` |
| <a href=#170 id=170 data-nosnippet>170</a> |
| <a href=#171 id=171 data-nosnippet>171</a> </span><span class="kw">pub struct </span>PhantomInvariantLifetime<<span class="lifetime">'a</span>>(PhantomInvariant<<span class="kw-2">&</span><span class="lifetime">'a </span>()>); |
| <a href=#172 id=172 data-nosnippet>172</a> |
| <a href=#173 id=173 data-nosnippet>173</a>} |
| <a href=#174 id=174 data-nosnippet>174</a> |
| <a href=#175 id=175 data-nosnippet>175</a><span class="macro">phantom_type!</span> { |
| <a href=#176 id=176 data-nosnippet>176</a> <span class="doccomment">/// Zero-sized type used to mark a type parameter as covariant. |
| <a href=#177 id=177 data-nosnippet>177</a> /// |
| <a href=#178 id=178 data-nosnippet>178</a> /// Types used as part of the return value from a function are covariant. If the type is _also_ |
| <a href=#179 id=179 data-nosnippet>179</a> /// passed as a parameter then it is [invariant][PhantomInvariant]. See [the reference][1] for |
| <a href=#180 id=180 data-nosnippet>180</a> /// more information. |
| <a href=#181 id=181 data-nosnippet>181</a> /// |
| <a href=#182 id=182 data-nosnippet>182</a> /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance |
| <a href=#183 id=183 data-nosnippet>183</a> /// |
| <a href=#184 id=184 data-nosnippet>184</a> /// ## Layout |
| <a href=#185 id=185 data-nosnippet>185</a> /// |
| <a href=#186 id=186 data-nosnippet>186</a> /// For all `T`, the following are guaranteed: |
| <a href=#187 id=187 data-nosnippet>187</a> /// * `size_of::<PhantomCovariant<T>>() == 0` |
| <a href=#188 id=188 data-nosnippet>188</a> /// * `align_of::<PhantomCovariant<T>>() == 1` |
| <a href=#189 id=189 data-nosnippet>189</a> |
| <a href=#190 id=190 data-nosnippet>190</a> </span><span class="kw">pub struct </span>PhantomCovariant<T>(PhantomData<<span class="kw">fn</span>() -> T>); |
| <a href=#191 id=191 data-nosnippet>191</a> <span class="doccomment">/// Zero-sized type used to mark a type parameter as contravariant. |
| <a href=#192 id=192 data-nosnippet>192</a> /// |
| <a href=#193 id=193 data-nosnippet>193</a> /// Types passed as arguments to a function are contravariant. If the type is _also_ part of the |
| <a href=#194 id=194 data-nosnippet>194</a> /// return value from a function then it is [invariant][PhantomInvariant]. See [the |
| <a href=#195 id=195 data-nosnippet>195</a> /// reference][1] for more information. |
| <a href=#196 id=196 data-nosnippet>196</a> /// |
| <a href=#197 id=197 data-nosnippet>197</a> /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance |
| <a href=#198 id=198 data-nosnippet>198</a> /// |
| <a href=#199 id=199 data-nosnippet>199</a> /// ## Layout |
| <a href=#200 id=200 data-nosnippet>200</a> /// |
| <a href=#201 id=201 data-nosnippet>201</a> /// For all `T`, the following are guaranteed: |
| <a href=#202 id=202 data-nosnippet>202</a> /// * `size_of::<PhantomContravariant<T>>() == 0` |
| <a href=#203 id=203 data-nosnippet>203</a> /// * `align_of::<PhantomContravariant<T>>() == 1` |
| <a href=#204 id=204 data-nosnippet>204</a> |
| <a href=#205 id=205 data-nosnippet>205</a> </span><span class="kw">pub struct </span>PhantomContravariant<T>(PhantomData<<span class="kw">fn</span>(T)>); |
| <a href=#206 id=206 data-nosnippet>206</a> <span class="doccomment">/// Zero-sized type used to mark a type parameter as invariant. |
| <a href=#207 id=207 data-nosnippet>207</a> /// |
| <a href=#208 id=208 data-nosnippet>208</a> /// Types that are both passed as an argument _and_ used as part of the return value from a |
| <a href=#209 id=209 data-nosnippet>209</a> /// function are invariant. See [the reference][1] for more information. |
| <a href=#210 id=210 data-nosnippet>210</a> /// |
| <a href=#211 id=211 data-nosnippet>211</a> /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance |
| <a href=#212 id=212 data-nosnippet>212</a> /// |
| <a href=#213 id=213 data-nosnippet>213</a> /// ## Layout |
| <a href=#214 id=214 data-nosnippet>214</a> /// |
| <a href=#215 id=215 data-nosnippet>215</a> /// For all `T`, the following are guaranteed: |
| <a href=#216 id=216 data-nosnippet>216</a> /// * `size_of::<PhantomInvariant<T>>() == 0` |
| <a href=#217 id=217 data-nosnippet>217</a> /// * `align_of::<PhantomInvariant<T>>() == 1` |
| <a href=#218 id=218 data-nosnippet>218</a> |
| <a href=#219 id=219 data-nosnippet>219</a> </span><span class="kw">pub struct </span>PhantomInvariant<T>(PhantomData<<span class="kw">fn</span>(T) -> T>); |
| <a href=#220 id=220 data-nosnippet>220</a> |
| <a href=#221 id=221 data-nosnippet>221</a>} |
| <a href=#222 id=222 data-nosnippet>222</a> |
| <a href=#223 id=223 data-nosnippet>223</a><span class="kw">mod </span>sealed { |
| <a href=#224 id=224 data-nosnippet>224</a> |
| <a href=#225 id=225 data-nosnippet>225</a> <span class="kw">pub trait </span>Sealed { |
| <a href=#226 id=226 data-nosnippet>226</a> <span class="kw">const </span>VALUE: <span class="self">Self</span>; |
| <a href=#227 id=227 data-nosnippet>227</a> } |
| <a href=#228 id=228 data-nosnippet>228</a>} |
| <a href=#229 id=229 data-nosnippet>229</a><span class="doccomment">/// A marker trait for phantom variance types. |
| <a href=#230 id=230 data-nosnippet>230</a></span><span class="kw">pub trait </span>Variance: sealed::Sealed + Default {} |
| <a href=#231 id=231 data-nosnippet>231</a><span class="doccomment">/// Construct a variance marker; equivalent to [`Default::default`]. |
| <a href=#232 id=232 data-nosnippet>232</a>/// |
| <a href=#233 id=233 data-nosnippet>233</a>/// This type can be any of the following. You generally should not need to explicitly name the |
| <a href=#234 id=234 data-nosnippet>234</a>/// type, however. |
| <a href=#235 id=235 data-nosnippet>235</a>/// |
| <a href=#236 id=236 data-nosnippet>236</a>/// - [`PhantomCovariant`] |
| <a href=#237 id=237 data-nosnippet>237</a>/// - [`PhantomContravariant`] |
| <a href=#238 id=238 data-nosnippet>238</a>/// - [`PhantomInvariant`] |
| <a href=#239 id=239 data-nosnippet>239</a>/// - [`PhantomCovariantLifetime`] |
| <a href=#240 id=240 data-nosnippet>240</a>/// - [`PhantomContravariantLifetime`] |
| <a href=#241 id=241 data-nosnippet>241</a>/// - [`PhantomInvariantLifetime`] |
| <a href=#242 id=242 data-nosnippet>242</a>/// |
| <a href=#243 id=243 data-nosnippet>243</a>/// # Example |
| <a href=#244 id=244 data-nosnippet>244</a>/// |
| <a href=#245 id=245 data-nosnippet>245</a>/// ```rust |
| <a href=#246 id=246 data-nosnippet>246</a>/// #![feature(phantom_variance_markers)] |
| <a href=#247 id=247 data-nosnippet>247</a>/// |
| <a href=#248 id=248 data-nosnippet>248</a>/// use core::marker::{PhantomCovariant, variance}; |
| <a href=#249 id=249 data-nosnippet>249</a>/// |
| <a href=#250 id=250 data-nosnippet>250</a>/// struct BoundFn<F, P, R> |
| <a href=#251 id=251 data-nosnippet>251</a>/// where |
| <a href=#252 id=252 data-nosnippet>252</a>/// F: Fn(P) -> R, |
| <a href=#253 id=253 data-nosnippet>253</a>/// { |
| <a href=#254 id=254 data-nosnippet>254</a>/// function: F, |
| <a href=#255 id=255 data-nosnippet>255</a>/// parameter: P, |
| <a href=#256 id=256 data-nosnippet>256</a>/// return_value: PhantomCovariant<R>, |
| <a href=#257 id=257 data-nosnippet>257</a>/// } |
| <a href=#258 id=258 data-nosnippet>258</a>/// |
| <a href=#259 id=259 data-nosnippet>259</a>/// let bound_fn = BoundFn { |
| <a href=#260 id=260 data-nosnippet>260</a>/// function: core::convert::identity, |
| <a href=#261 id=261 data-nosnippet>261</a>/// parameter: 5u8, |
| <a href=#262 id=262 data-nosnippet>262</a>/// return_value: variance(), |
| <a href=#263 id=263 data-nosnippet>263</a>/// }; |
| <a href=#264 id=264 data-nosnippet>264</a>/// ``` |
| <a href=#265 id=265 data-nosnippet>265</a></span><span class="kw">pub const fn </span>variance<T>() -> T |
| <a href=#266 id=266 data-nosnippet>266</a><span class="kw">where |
| <a href=#267 id=267 data-nosnippet>267</a> </span>T: Variance, |
| <a href=#268 id=268 data-nosnippet>268</a>{ |
| <a href=#269 id=269 data-nosnippet>269</a> T::VALUE |
| <a href=#270 id=270 data-nosnippet>270</a>}</code></pre></div></section></main></body></html> |
