| <!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 `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-2.0.18/src/discouraged.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>discouraged.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../source-script.js"></script><script defer src="../../source-files.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../syn/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../syn/index.html"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="doccomment">//! Extensions to the parsing API with niche applicability. |
| |
| </span><span class="kw">use super</span>::<span class="kw-2">*</span>; |
| <span class="kw">use </span>proc_macro2::extra::DelimSpan; |
| |
| <span class="doccomment">/// Extensions to the `ParseStream` API to support speculative parsing. |
| </span><span class="kw">pub trait </span>Speculative { |
| <span class="doccomment">/// Advance this parse stream to the position of a forked parse stream. |
| /// |
| /// This is the opposite operation to [`ParseStream::fork`]. You can fork a |
| /// parse stream, perform some speculative parsing, then join the original |
| /// stream to the fork to "commit" the parsing from the fork to the main |
| /// stream. |
| /// |
| /// If you can avoid doing this, you should, as it limits the ability to |
| /// generate useful errors. That said, it is often the only way to parse |
| /// syntax of the form `A* B*` for arbitrary syntax `A` and `B`. The problem |
| /// is that when the fork fails to parse an `A`, it's impossible to tell |
| /// whether that was because of a syntax error and the user meant to provide |
| /// an `A`, or that the `A`s are finished and it's time to start parsing |
| /// `B`s. Use with care. |
| /// |
| /// Also note that if `A` is a subset of `B`, `A* B*` can be parsed by |
| /// parsing `B*` and removing the leading members of `A` from the |
| /// repetition, bypassing the need to involve the downsides associated with |
| /// speculative parsing. |
| /// |
| /// [`ParseStream::fork`]: ParseBuffer::fork |
| /// |
| /// # Example |
| /// |
| /// There has been chatter about the possibility of making the colons in the |
| /// turbofish syntax like `path::to::<T>` no longer required by accepting |
| /// `path::to<T>` in expression position. Specifically, according to [RFC |
| /// 2544], [`PathSegment`] parsing should always try to consume a following |
| /// `<` token as the start of generic arguments, and reset to the `<` if |
| /// that fails (e.g. the token is acting as a less-than operator). |
| /// |
| /// This is the exact kind of parsing behavior which requires the "fork, |
| /// try, commit" behavior that [`ParseStream::fork`] discourages. With |
| /// `advance_to`, we can avoid having to parse the speculatively parsed |
| /// content a second time. |
| /// |
| /// This change in behavior can be implemented in syn by replacing just the |
| /// `Parse` implementation for `PathSegment`: |
| /// |
| /// ``` |
| /// # use syn::ext::IdentExt; |
| /// use syn::parse::discouraged::Speculative; |
| /// # use syn::parse::{Parse, ParseStream}; |
| /// # use syn::{Ident, PathArguments, Result, Token}; |
| /// |
| /// pub struct PathSegment { |
| /// pub ident: Ident, |
| /// pub arguments: PathArguments, |
| /// } |
| /// # |
| /// # impl<T> From<T> for PathSegment |
| /// # where |
| /// # T: Into<Ident>, |
| /// # { |
| /// # fn from(ident: T) -> Self { |
| /// # PathSegment { |
| /// # ident: ident.into(), |
| /// # arguments: PathArguments::None, |
| /// # } |
| /// # } |
| /// # } |
| /// |
| /// impl Parse for PathSegment { |
| /// fn parse(input: ParseStream) -> Result<Self> { |
| /// if input.peek(Token![super]) |
| /// || input.peek(Token![self]) |
| /// || input.peek(Token![Self]) |
| /// || input.peek(Token![crate]) |
| /// { |
| /// let ident = input.call(Ident::parse_any)?; |
| /// return Ok(PathSegment::from(ident)); |
| /// } |
| /// |
| /// let ident = input.parse()?; |
| /// if input.peek(Token![::]) && input.peek3(Token![<]) { |
| /// return Ok(PathSegment { |
| /// ident, |
| /// arguments: PathArguments::AngleBracketed(input.parse()?), |
| /// }); |
| /// } |
| /// if input.peek(Token![<]) && !input.peek(Token![<=]) { |
| /// let fork = input.fork(); |
| /// if let Ok(arguments) = fork.parse() { |
| /// input.advance_to(&fork); |
| /// return Ok(PathSegment { |
| /// ident, |
| /// arguments: PathArguments::AngleBracketed(arguments), |
| /// }); |
| /// } |
| /// } |
| /// Ok(PathSegment::from(ident)) |
| /// } |
| /// } |
| /// |
| /// # syn::parse_str::<PathSegment>("a<b,c>").unwrap(); |
| /// ``` |
| /// |
| /// # Drawbacks |
| /// |
| /// The main drawback of this style of speculative parsing is in error |
| /// presentation. Even if the lookahead is the "correct" parse, the error |
| /// that is shown is that of the "fallback" parse. To use the same example |
| /// as the turbofish above, take the following unfinished "turbofish": |
| /// |
| /// ```text |
| /// let _ = f<&'a fn(), for<'a> serde::>(); |
| /// ``` |
| /// |
| /// If this is parsed as generic arguments, we can provide the error message |
| /// |
| /// ```text |
| /// error: expected identifier |
| /// --> src.rs:L:C |
| /// | |
| /// L | let _ = f<&'a fn(), for<'a> serde::>(); |
| /// | ^ |
| /// ``` |
| /// |
| /// but if parsed using the above speculative parsing, it falls back to |
| /// assuming that the `<` is a less-than when it fails to parse the generic |
| /// arguments, and tries to interpret the `&'a` as the start of a labelled |
| /// loop, resulting in the much less helpful error |
| /// |
| /// ```text |
| /// error: expected `:` |
| /// --> src.rs:L:C |
| /// | |
| /// L | let _ = f<&'a fn(), for<'a> serde::>(); |
| /// | ^^ |
| /// ``` |
| /// |
| /// This can be mitigated with various heuristics (two examples: show both |
| /// forks' parse errors, or show the one that consumed more tokens), but |
| /// when you can control the grammar, sticking to something that can be |
| /// parsed LL(3) and without the LL(*) speculative parsing this makes |
| /// possible, displaying reasonable errors becomes much more simple. |
| /// |
| /// [RFC 2544]: https://github.com/rust-lang/rfcs/pull/2544 |
| /// [`PathSegment`]: crate::PathSegment |
| /// |
| /// # Performance |
| /// |
| /// This method performs a cheap fixed amount of work that does not depend |
| /// on how far apart the two streams are positioned. |
| /// |
| /// # Panics |
| /// |
| /// The forked stream in the argument of `advance_to` must have been |
| /// obtained by forking `self`. Attempting to advance to any other stream |
| /// will cause a panic. |
| </span><span class="kw">fn </span>advance_to(<span class="kw-2">&</span><span class="self">self</span>, fork: <span class="kw-2">&</span><span class="self">Self</span>); |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>> Speculative <span class="kw">for </span>ParseBuffer<<span class="lifetime">'a</span>> { |
| <span class="kw">fn </span>advance_to(<span class="kw-2">&</span><span class="self">self</span>, fork: <span class="kw-2">&</span><span class="self">Self</span>) { |
| <span class="kw">if </span>!<span class="kw">crate</span>::buffer::same_scope(<span class="self">self</span>.cursor(), fork.cursor()) { |
| <span class="macro">panic!</span>(<span class="string">"Fork was not derived from the advancing parse stream"</span>); |
| } |
| |
| <span class="kw">let </span>(self_unexp, self_sp) = inner_unexpected(<span class="self">self</span>); |
| <span class="kw">let </span>(fork_unexp, fork_sp) = inner_unexpected(fork); |
| <span class="kw">if </span>!Rc::ptr_eq(<span class="kw-2">&</span>self_unexp, <span class="kw-2">&</span>fork_unexp) { |
| <span class="kw">match </span>(fork_sp, self_sp) { |
| <span class="comment">// Unexpected set on the fork, but not on `self`, copy it over. |
| </span>(<span class="prelude-val">Some</span>(span), <span class="prelude-val">None</span>) => { |
| self_unexp.set(Unexpected::Some(span)); |
| } |
| <span class="comment">// Unexpected unset. Use chain to propagate errors from fork. |
| </span>(<span class="prelude-val">None</span>, <span class="prelude-val">None</span>) => { |
| fork_unexp.set(Unexpected::Chain(self_unexp)); |
| |
| <span class="comment">// Ensure toplevel 'unexpected' tokens from the fork don't |
| // bubble up the chain by replacing the root `unexpected` |
| // pointer, only 'unexpected' tokens from existing group |
| // parsers should bubble. |
| </span>fork.unexpected |
| .set(<span class="prelude-val">Some</span>(Rc::new(Cell::new(Unexpected::None)))); |
| } |
| <span class="comment">// Unexpected has been set on `self`. No changes needed. |
| </span>(<span class="kw">_</span>, <span class="prelude-val">Some</span>(<span class="kw">_</span>)) => {} |
| } |
| } |
| |
| <span class="comment">// See comment on `cell` in the struct definition. |
| </span><span class="self">self</span>.cell |
| .set(<span class="kw">unsafe </span>{ mem::transmute::<Cursor, Cursor<<span class="lifetime">'static</span>>>(fork.cursor()) }); |
| } |
| } |
| |
| <span class="doccomment">/// Extensions to the `ParseStream` API to support manipulating invisible |
| /// delimiters the same as if they were visible. |
| </span><span class="kw">pub trait </span>AnyDelimiter { |
| <span class="doccomment">/// Returns the delimiter, the span of the delimiter token, and the nested |
| /// contents for further parsing. |
| </span><span class="kw">fn </span>parse_any_delimiter(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="prelude-ty">Result</span><(Delimiter, DelimSpan, ParseBuffer)>; |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>> AnyDelimiter <span class="kw">for </span>ParseBuffer<<span class="lifetime">'a</span>> { |
| <span class="kw">fn </span>parse_any_delimiter(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="prelude-ty">Result</span><(Delimiter, DelimSpan, ParseBuffer)> { |
| <span class="self">self</span>.step(|cursor| { |
| <span class="kw">if let </span><span class="prelude-val">Some</span>((content, delimiter, span, rest)) = cursor.any_group() { |
| <span class="kw">let </span>scope = <span class="kw">crate</span>::buffer::close_span_of_group(<span class="kw-2">*</span>cursor); |
| <span class="kw">let </span>nested = <span class="kw">crate</span>::parse::advance_step_cursor(cursor, content); |
| <span class="kw">let </span>unexpected = <span class="kw">crate</span>::parse::get_unexpected(<span class="self">self</span>); |
| <span class="kw">let </span>content = <span class="kw">crate</span>::parse::new_parse_buffer(scope, nested, unexpected); |
| <span class="prelude-val">Ok</span>(((delimiter, span, content), rest)) |
| } <span class="kw">else </span>{ |
| <span class="prelude-val">Err</span>(cursor.error(<span class="string">"expected any delimiter"</span>)) |
| } |
| }) |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="syn" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
