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</pre><pre class="rust"><code><span class="kw">use </span><span class="kw">crate</span>::buffer::Cursor;
<span class="kw">use </span><span class="kw">crate</span>::error::{<span class="self">self</span>, Error};
<span class="kw">use </span><span class="kw">crate</span>::sealed::lookahead::Sealed;
<span class="kw">use </span><span class="kw">crate</span>::span::IntoSpans;
<span class="kw">use </span><span class="kw">crate</span>::token::Token;
<span class="kw">use </span>proc_macro2::{Delimiter, Span};
<span class="kw">use </span>std::cell::RefCell;
<span class="doccomment">/// Support for checking the next token in a stream to decide how to parse.
///
/// An important advantage over [`ParseStream::peek`] is that here we
/// automatically construct an appropriate error message based on the token
/// alternatives that get peeked. If you are producing your own error message,
/// go ahead and use `ParseStream::peek` instead.
///
/// Use [`ParseStream::lookahead1`] to construct this object.
///
/// [`ParseStream::peek`]: crate::parse::ParseBuffer::peek
/// [`ParseStream::lookahead1`]: crate::parse::ParseBuffer::lookahead1
///
/// Consuming tokens from the source stream after constructing a lookahead
/// object does not also advance the lookahead object.
///
/// # Example
///
/// ```
/// use syn::{ConstParam, Ident, Lifetime, LifetimeParam, Result, Token, TypeParam};
/// use syn::parse::{Parse, ParseStream};
///
/// // A generic parameter, a single one of the comma-separated elements inside
/// // angle brackets in:
/// //
/// // fn f&lt;T: Clone, &#39;a, &#39;b: &#39;a, const N: usize&gt;() { ... }
/// //
/// // On invalid input, lookahead gives us a reasonable error message.
/// //
/// // error: expected one of: identifier, lifetime, `const`
/// // |
/// // 5 | fn f&lt;!Sized&gt;() {}
/// // | ^
/// enum GenericParam {
/// Type(TypeParam),
/// Lifetime(LifetimeParam),
/// Const(ConstParam),
/// }
///
/// impl Parse for GenericParam {
/// fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
/// let lookahead = input.lookahead1();
/// if lookahead.peek(Ident) {
/// input.parse().map(GenericParam::Type)
/// } else if lookahead.peek(Lifetime) {
/// input.parse().map(GenericParam::Lifetime)
/// } else if lookahead.peek(Token![const]) {
/// input.parse().map(GenericParam::Const)
/// } else {
/// Err(lookahead.error())
/// }
/// }
/// }
/// ```
</span><span class="kw">pub struct </span>Lookahead1&lt;<span class="lifetime">&#39;a</span>&gt; {
scope: Span,
cursor: Cursor&lt;<span class="lifetime">&#39;a</span>&gt;,
comparisons: RefCell&lt;Vec&lt;<span class="kw-2">&amp;</span><span class="lifetime">&#39;static </span>str&gt;&gt;,
}
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new(scope: Span, cursor: Cursor) -&gt; Lookahead1 {
Lookahead1 {
scope,
cursor,
comparisons: RefCell::new(Vec::new()),
}
}
<span class="kw">fn </span>peek_impl(
lookahead: <span class="kw-2">&amp;</span>Lookahead1,
peek: <span class="kw">fn</span>(Cursor) -&gt; bool,
display: <span class="kw">fn</span>() -&gt; <span class="kw-2">&amp;</span><span class="lifetime">&#39;static </span>str,
) -&gt; bool {
<span class="kw">if </span>peek(lookahead.cursor) {
<span class="kw">return </span><span class="bool-val">true</span>;
}
lookahead.comparisons.borrow_mut().push(display());
<span class="bool-val">false
</span>}
<span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; Lookahead1&lt;<span class="lifetime">&#39;a</span>&gt; {
<span class="doccomment">/// Looks at the next token in the parse stream to determine whether it
/// matches the requested type of token.
///
/// # Syntax
///
/// Note that this method does not use turbofish syntax. Pass the peek type
/// inside of parentheses.
///
/// - `input.peek(Token![struct])`
/// - `input.peek(Token![==])`
/// - `input.peek(Ident)`&amp;emsp;*(does not accept keywords)*
/// - `input.peek(Ident::peek_any)`
/// - `input.peek(Lifetime)`
/// - `input.peek(token::Brace)`
</span><span class="kw">pub fn </span>peek&lt;T: Peek&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, token: T) -&gt; bool {
<span class="kw">let _ </span>= token;
peek_impl(<span class="self">self</span>, T::Token::peek, T::Token::display)
}
<span class="doccomment">/// Triggers an error at the current position of the parse stream.
///
/// The error message will identify all of the expected token types that
/// have been peeked against this lookahead instance.
</span><span class="kw">pub fn </span>error(<span class="self">self</span>) -&gt; Error {
<span class="kw">let </span>comparisons = <span class="self">self</span>.comparisons.borrow();
<span class="kw">match </span>comparisons.len() {
<span class="number">0 </span>=&gt; {
<span class="kw">if </span><span class="self">self</span>.cursor.eof() {
Error::new(<span class="self">self</span>.scope, <span class="string">&quot;unexpected end of input&quot;</span>)
} <span class="kw">else </span>{
Error::new(<span class="self">self</span>.cursor.span(), <span class="string">&quot;unexpected token&quot;</span>)
}
}
<span class="number">1 </span>=&gt; {
<span class="kw">let </span>message = <span class="macro">format!</span>(<span class="string">&quot;expected {}&quot;</span>, comparisons[<span class="number">0</span>]);
error::new_at(<span class="self">self</span>.scope, <span class="self">self</span>.cursor, message)
}
<span class="number">2 </span>=&gt; {
<span class="kw">let </span>message = <span class="macro">format!</span>(<span class="string">&quot;expected {} or {}&quot;</span>, comparisons[<span class="number">0</span>], comparisons[<span class="number">1</span>]);
error::new_at(<span class="self">self</span>.scope, <span class="self">self</span>.cursor, message)
}
<span class="kw">_ </span>=&gt; {
<span class="kw">let </span>join = comparisons.join(<span class="string">&quot;, &quot;</span>);
<span class="kw">let </span>message = <span class="macro">format!</span>(<span class="string">&quot;expected one of: {}&quot;</span>, join);
error::new_at(<span class="self">self</span>.scope, <span class="self">self</span>.cursor, message)
}
}
}
}
<span class="doccomment">/// Types that can be parsed by looking at just one token.
///
/// Use [`ParseStream::peek`] to peek one of these types in a parse stream
/// without consuming it from the stream.
///
/// This trait is sealed and cannot be implemented for types outside of Syn.
///
/// [`ParseStream::peek`]: crate::parse::ParseBuffer::peek
</span><span class="kw">pub trait </span>Peek: Sealed {
<span class="comment">// Not public API.
</span><span class="attribute">#[doc(hidden)]
</span><span class="kw">type </span>Token: Token;
}
<span class="kw">impl</span>&lt;F: Copy + FnOnce(TokenMarker) -&gt; T, T: Token&gt; Peek <span class="kw">for </span>F {
<span class="kw">type </span>Token = T;
}
<span class="kw">pub enum </span>TokenMarker {}
<span class="kw">impl</span>&lt;S&gt; IntoSpans&lt;S&gt; <span class="kw">for </span>TokenMarker {
<span class="kw">fn </span>into_spans(<span class="self">self</span>) -&gt; S {
<span class="kw">match </span><span class="self">self </span>{}
}
}
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>is_delimiter(cursor: Cursor, delimiter: Delimiter) -&gt; bool {
cursor.group(delimiter).is_some()
}
<span class="kw">impl</span>&lt;F: Copy + FnOnce(TokenMarker) -&gt; T, T: Token&gt; Sealed <span class="kw">for </span>F {}
</code></pre></div>
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