| <!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/mbe/src/expander.rs`."><title>expander.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="mbe" 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">mbe/</div>expander.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 module takes a (parsed) definition of `macro_rules` invocation, a |
| <a href=#2 id=2 data-nosnippet>2</a>//! `tt::TokenTree` representing an argument of macro invocation, and produces a |
| <a href=#3 id=3 data-nosnippet>3</a>//! `tt::TokenTree` for the result of the expansion. |
| <a href=#4 id=4 data-nosnippet>4</a> |
| <a href=#5 id=5 data-nosnippet>5</a></span><span class="kw">mod </span>matcher; |
| <a href=#6 id=6 data-nosnippet>6</a><span class="kw">mod </span>transcriber; |
| <a href=#7 id=7 data-nosnippet>7</a> |
| <a href=#8 id=8 data-nosnippet>8</a><span class="kw">use </span>intern::Symbol; |
| <a href=#9 id=9 data-nosnippet>9</a><span class="kw">use </span>rustc_hash::FxHashMap; |
| <a href=#10 id=10 data-nosnippet>10</a><span class="kw">use </span>span::{Edition, Span}; |
| <a href=#11 id=11 data-nosnippet>11</a> |
| <a href=#12 id=12 data-nosnippet>12</a><span class="kw">use crate</span>::{ExpandError, ExpandErrorKind, ExpandResult, MatchedArmIndex, parser::MetaVarKind}; |
| <a href=#13 id=13 data-nosnippet>13</a> |
| <a href=#14 id=14 data-nosnippet>14</a><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>expand_rules( |
| <a href=#15 id=15 data-nosnippet>15</a> rules: <span class="kw-2">&</span>[<span class="kw">crate</span>::Rule], |
| <a href=#16 id=16 data-nosnippet>16</a> input: <span class="kw-2">&</span>tt::TopSubtree<Span>, |
| <a href=#17 id=17 data-nosnippet>17</a> marker: <span class="kw">impl </span>Fn(<span class="kw-2">&mut </span>Span) + Copy, |
| <a href=#18 id=18 data-nosnippet>18</a> call_site: Span, |
| <a href=#19 id=19 data-nosnippet>19</a> def_site_edition: Edition, |
| <a href=#20 id=20 data-nosnippet>20</a>) -> ExpandResult<(tt::TopSubtree<Span>, MatchedArmIndex)> { |
| <a href=#21 id=21 data-nosnippet>21</a> <span class="kw">let </span><span class="kw-2">mut </span>match_: <span class="prelude-ty">Option</span><(matcher::Match<<span class="lifetime">'_</span>>, <span class="kw-2">&</span><span class="kw">crate</span>::Rule, usize)> = <span class="prelude-val">None</span>; |
| <a href=#22 id=22 data-nosnippet>22</a> <span class="kw">for </span>(idx, rule) <span class="kw">in </span>rules.iter().enumerate() { |
| <a href=#23 id=23 data-nosnippet>23</a> <span class="kw">let </span>new_match = matcher::match_(<span class="kw-2">&</span>rule.lhs, input, def_site_edition); |
| <a href=#24 id=24 data-nosnippet>24</a> |
| <a href=#25 id=25 data-nosnippet>25</a> <span class="kw">if </span>new_match.err.is_none() { |
| <a href=#26 id=26 data-nosnippet>26</a> <span class="comment">// If we find a rule that applies without errors, we're done. |
| <a href=#27 id=27 data-nosnippet>27</a> // Unconditionally returning the transcription here makes the |
| <a href=#28 id=28 data-nosnippet>28</a> // `test_repeat_bad_var` test fail. |
| <a href=#29 id=29 data-nosnippet>29</a> </span><span class="kw">let </span>ExpandResult { value, err: transcribe_err } = |
| <a href=#30 id=30 data-nosnippet>30</a> transcriber::transcribe(<span class="kw-2">&</span>rule.rhs, <span class="kw-2">&</span>new_match.bindings, marker, call_site); |
| <a href=#31 id=31 data-nosnippet>31</a> <span class="kw">if </span>transcribe_err.is_none() { |
| <a href=#32 id=32 data-nosnippet>32</a> <span class="kw">return </span>ExpandResult::ok((value, <span class="prelude-val">Some</span>(idx <span class="kw">as </span>u32))); |
| <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="comment">// Use the rule if we matched more tokens, or bound variables count |
| <a href=#36 id=36 data-nosnippet>36</a> </span><span class="kw">if let </span><span class="prelude-val">Some</span>((prev_match, <span class="kw">_</span>, <span class="kw">_</span>)) = <span class="kw-2">&</span>match_ { |
| <a href=#37 id=37 data-nosnippet>37</a> <span class="kw">if </span>(new_match.unmatched_tts, -(new_match.bound_count <span class="kw">as </span>i32)) |
| <a href=#38 id=38 data-nosnippet>38</a> < (prev_match.unmatched_tts, -(prev_match.bound_count <span class="kw">as </span>i32)) |
| <a href=#39 id=39 data-nosnippet>39</a> { |
| <a href=#40 id=40 data-nosnippet>40</a> match_ = <span class="prelude-val">Some</span>((new_match, rule, idx)); |
| <a href=#41 id=41 data-nosnippet>41</a> } |
| <a href=#42 id=42 data-nosnippet>42</a> } <span class="kw">else </span>{ |
| <a href=#43 id=43 data-nosnippet>43</a> match_ = <span class="prelude-val">Some</span>((new_match, rule, idx)); |
| <a href=#44 id=44 data-nosnippet>44</a> } |
| <a href=#45 id=45 data-nosnippet>45</a> } |
| <a href=#46 id=46 data-nosnippet>46</a> <span class="kw">if let </span><span class="prelude-val">Some</span>((match_, rule, idx)) = match_ { |
| <a href=#47 id=47 data-nosnippet>47</a> <span class="comment">// if we got here, there was no match without errors |
| <a href=#48 id=48 data-nosnippet>48</a> </span><span class="kw">let </span>ExpandResult { value, err: transcribe_err } = |
| <a href=#49 id=49 data-nosnippet>49</a> transcriber::transcribe(<span class="kw-2">&</span>rule.rhs, <span class="kw-2">&</span>match_.bindings, marker, call_site); |
| <a href=#50 id=50 data-nosnippet>50</a> ExpandResult { value: (value, idx.try_into().ok()), err: match_.err.or(transcribe_err) } |
| <a href=#51 id=51 data-nosnippet>51</a> } <span class="kw">else </span>{ |
| <a href=#52 id=52 data-nosnippet>52</a> ExpandResult::new( |
| <a href=#53 id=53 data-nosnippet>53</a> (tt::TopSubtree::empty(tt::DelimSpan::from_single(call_site)), <span class="prelude-val">None</span>), |
| <a href=#54 id=54 data-nosnippet>54</a> ExpandError::new(call_site, ExpandErrorKind::NoMatchingRule), |
| <a href=#55 id=55 data-nosnippet>55</a> ) |
| <a href=#56 id=56 data-nosnippet>56</a> } |
| <a href=#57 id=57 data-nosnippet>57</a>} |
| <a href=#58 id=58 data-nosnippet>58</a> |
| <a href=#59 id=59 data-nosnippet>59</a><span class="doccomment">/// The actual algorithm for expansion is not too hard, but is pretty tricky. |
| <a href=#60 id=60 data-nosnippet>60</a>/// `Bindings` structure is the key to understanding what we are doing here. |
| <a href=#61 id=61 data-nosnippet>61</a>/// |
| <a href=#62 id=62 data-nosnippet>62</a>/// On the high level, it stores mapping from meta variables to the bits of |
| <a href=#63 id=63 data-nosnippet>63</a>/// syntax it should be substituted with. For example, if `$e:expr` is matched |
| <a href=#64 id=64 data-nosnippet>64</a>/// with `1 + 1` by macro_rules, the `Binding` will store `$e -> 1 + 1`. |
| <a href=#65 id=65 data-nosnippet>65</a>/// |
| <a href=#66 id=66 data-nosnippet>66</a>/// The tricky bit is dealing with repetitions (`$()*`). Consider this example: |
| <a href=#67 id=67 data-nosnippet>67</a>/// |
| <a href=#68 id=68 data-nosnippet>68</a>/// ```not_rust |
| <a href=#69 id=69 data-nosnippet>69</a>/// macro_rules! foo { |
| <a href=#70 id=70 data-nosnippet>70</a>/// ($($ i:ident $($ e:expr),*);*) => { |
| <a href=#71 id=71 data-nosnippet>71</a>/// $(fn $ i() { $($ e);*; })* |
| <a href=#72 id=72 data-nosnippet>72</a>/// } |
| <a href=#73 id=73 data-nosnippet>73</a>/// } |
| <a href=#74 id=74 data-nosnippet>74</a>/// foo! { foo 1,2,3; bar 4,5,6 } |
| <a href=#75 id=75 data-nosnippet>75</a>/// ``` |
| <a href=#76 id=76 data-nosnippet>76</a>/// |
| <a href=#77 id=77 data-nosnippet>77</a>/// Here, the `$i` meta variable is matched first with `foo` and then with |
| <a href=#78 id=78 data-nosnippet>78</a>/// `bar`, and `$e` is matched in turn with `1`, `2`, `3`, `4`, `5`, `6`. |
| <a href=#79 id=79 data-nosnippet>79</a>/// |
| <a href=#80 id=80 data-nosnippet>80</a>/// To represent such "multi-mappings", we use a recursive structures: we map |
| <a href=#81 id=81 data-nosnippet>81</a>/// variables not to values, but to *lists* of values or other lists (that is, |
| <a href=#82 id=82 data-nosnippet>82</a>/// to the trees). |
| <a href=#83 id=83 data-nosnippet>83</a>/// |
| <a href=#84 id=84 data-nosnippet>84</a>/// For the above example, the bindings would store |
| <a href=#85 id=85 data-nosnippet>85</a>/// |
| <a href=#86 id=86 data-nosnippet>86</a>/// ```not_rust |
| <a href=#87 id=87 data-nosnippet>87</a>/// i -> [foo, bar] |
| <a href=#88 id=88 data-nosnippet>88</a>/// e -> [[1, 2, 3], [4, 5, 6]] |
| <a href=#89 id=89 data-nosnippet>89</a>/// ``` |
| <a href=#90 id=90 data-nosnippet>90</a>/// |
| <a href=#91 id=91 data-nosnippet>91</a>/// We construct `Bindings` in the `match_lhs`. The interesting case is |
| <a href=#92 id=92 data-nosnippet>92</a>/// `TokenTree::Repeat`, where we use `push_nested` to create the desired |
| <a href=#93 id=93 data-nosnippet>93</a>/// nesting structure. |
| <a href=#94 id=94 data-nosnippet>94</a>/// |
| <a href=#95 id=95 data-nosnippet>95</a>/// The other side of the puzzle is `expand_subtree`, where we use the bindings |
| <a href=#96 id=96 data-nosnippet>96</a>/// to substitute meta variables in the output template. When expanding, we |
| <a href=#97 id=97 data-nosnippet>97</a>/// maintain a `nesting` stack of indices which tells us which occurrence from |
| <a href=#98 id=98 data-nosnippet>98</a>/// the `Bindings` we should take. We push to the stack when we enter a |
| <a href=#99 id=99 data-nosnippet>99</a>/// repetition. |
| <a href=#100 id=100 data-nosnippet>100</a>/// |
| <a href=#101 id=101 data-nosnippet>101</a>/// In other words, `Bindings` is a *multi* mapping from `Symbol` to |
| <a href=#102 id=102 data-nosnippet>102</a>/// `tt::TokenTree`, where the index to select a particular `TokenTree` among |
| <a href=#103 id=103 data-nosnippet>103</a>/// many is not a plain `usize`, but a `&[usize]`. |
| <a href=#104 id=104 data-nosnippet>104</a></span><span class="attr">#[derive(Debug, Default, Clone)] |
| <a href=#105 id=105 data-nosnippet>105</a></span><span class="kw">struct </span>Bindings<<span class="lifetime">'a</span>> { |
| <a href=#106 id=106 data-nosnippet>106</a> inner: FxHashMap<Symbol, Binding<<span class="lifetime">'a</span>>>, |
| <a href=#107 id=107 data-nosnippet>107</a>} |
| <a href=#108 id=108 data-nosnippet>108</a> |
| <a href=#109 id=109 data-nosnippet>109</a><span class="attr">#[derive(Debug, Clone)] |
| <a href=#110 id=110 data-nosnippet>110</a></span><span class="kw">enum </span>Binding<<span class="lifetime">'a</span>> { |
| <a href=#111 id=111 data-nosnippet>111</a> Fragment(Fragment<<span class="lifetime">'a</span>>), |
| <a href=#112 id=112 data-nosnippet>112</a> Nested(Vec<Binding<<span class="lifetime">'a</span>>>), |
| <a href=#113 id=113 data-nosnippet>113</a> Empty, |
| <a href=#114 id=114 data-nosnippet>114</a> Missing(MetaVarKind), |
| <a href=#115 id=115 data-nosnippet>115</a>} |
| <a href=#116 id=116 data-nosnippet>116</a> |
| <a href=#117 id=117 data-nosnippet>117</a><span class="attr">#[derive(Debug, Default, Clone)] |
| <a href=#118 id=118 data-nosnippet>118</a></span><span class="kw">enum </span>Fragment<<span class="lifetime">'a</span>> { |
| <a href=#119 id=119 data-nosnippet>119</a> <span class="attr">#[default] |
| <a href=#120 id=120 data-nosnippet>120</a> </span>Empty, |
| <a href=#121 id=121 data-nosnippet>121</a> <span class="doccomment">/// token fragments are just copy-pasted into the output |
| <a href=#122 id=122 data-nosnippet>122</a> </span>Tokens(tt::TokenTreesView<<span class="lifetime">'a</span>, Span>), |
| <a href=#123 id=123 data-nosnippet>123</a> <span class="doccomment">/// Expr ast fragments are surrounded with `()` on transcription to preserve precedence. |
| <a href=#124 id=124 data-nosnippet>124</a> /// Note that this impl is different from the one currently in `rustc` -- |
| <a href=#125 id=125 data-nosnippet>125</a> /// `rustc` doesn't translate fragments into token trees at all. |
| <a href=#126 id=126 data-nosnippet>126</a> /// |
| <a href=#127 id=127 data-nosnippet>127</a> /// At one point in time, we tried to use "fake" delimiters here à la |
| <a href=#128 id=128 data-nosnippet>128</a> /// proc-macro delimiter=none. As we later discovered, "none" delimiters are |
| <a href=#129 id=129 data-nosnippet>129</a> /// tricky to handle in the parser, and rustc doesn't handle those either. |
| <a href=#130 id=130 data-nosnippet>130</a> /// |
| <a href=#131 id=131 data-nosnippet>131</a> /// The span of the outer delimiters is marked on transcription. |
| <a href=#132 id=132 data-nosnippet>132</a> </span>Expr(tt::TokenTreesView<<span class="lifetime">'a</span>, Span>), |
| <a href=#133 id=133 data-nosnippet>133</a> <span class="doccomment">/// There are roughly two types of paths: paths in expression context, where a |
| <a href=#134 id=134 data-nosnippet>134</a> /// separator `::` between an identifier and its following generic argument list |
| <a href=#135 id=135 data-nosnippet>135</a> /// is mandatory, and paths in type context, where `::` can be omitted. |
| <a href=#136 id=136 data-nosnippet>136</a> /// |
| <a href=#137 id=137 data-nosnippet>137</a> /// Unlike rustc, we need to transform the parsed fragments back into tokens |
| <a href=#138 id=138 data-nosnippet>138</a> /// during transcription. When the matched path fragment is a type-context path |
| <a href=#139 id=139 data-nosnippet>139</a> /// and is trasncribed as an expression-context path, verbatim transcription |
| <a href=#140 id=140 data-nosnippet>140</a> /// would cause a syntax error. We need to fix it up just before transcribing; |
| <a href=#141 id=141 data-nosnippet>141</a> /// see `transcriber::fix_up_and_push_path_tt()`. |
| <a href=#142 id=142 data-nosnippet>142</a> </span>Path(tt::TokenTreesView<<span class="lifetime">'a</span>, Span>), |
| <a href=#143 id=143 data-nosnippet>143</a> TokensOwned(tt::TopSubtree<Span>), |
| <a href=#144 id=144 data-nosnippet>144</a>} |
| <a href=#145 id=145 data-nosnippet>145</a> |
| <a href=#146 id=146 data-nosnippet>146</a><span class="kw">impl </span>Fragment<<span class="lifetime">'_</span>> { |
| <a href=#147 id=147 data-nosnippet>147</a> <span class="kw">fn </span>is_empty(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| <a href=#148 id=148 data-nosnippet>148</a> <span class="kw">match </span><span class="self">self </span>{ |
| <a href=#149 id=149 data-nosnippet>149</a> Fragment::Empty => <span class="bool-val">true</span>, |
| <a href=#150 id=150 data-nosnippet>150</a> Fragment::Tokens(it) => it.len() == <span class="number">0</span>, |
| <a href=#151 id=151 data-nosnippet>151</a> Fragment::Expr(it) => it.len() == <span class="number">0</span>, |
| <a href=#152 id=152 data-nosnippet>152</a> Fragment::Path(it) => it.len() == <span class="number">0</span>, |
| <a href=#153 id=153 data-nosnippet>153</a> Fragment::TokensOwned(it) => it.<span class="number">0</span>.is_empty(), |
| <a href=#154 id=154 data-nosnippet>154</a> } |
| <a href=#155 id=155 data-nosnippet>155</a> } |
| <a href=#156 id=156 data-nosnippet>156</a>}</code></pre></div></section></main></body></html> |
