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//! # `unstringify!` //! //! See [the documentation of the macro for more info]( //! https://docs.rs/unstringify/0.1.0/macro.unstringify.html) /// For compat with older versions of `rustc`. extern crate proc_macro; use ::proc_macro::{*, TokenTree as TT, }; #[macro_use] mod utils; struct Input { tokenized: TokenStream, metavar: Ident, template: TokenStream, } /// Reverse of [`stringify!`]: tokenize an input string literal. /// /// ## Basic example /// /// ```rust /// use ::unstringify::unstringify; /// /// unstringify!(r#" /// fn main () /// { /// println!("Hello, World!"); /// } /// "#); /// ``` /// /// Or, equivalently: /// /// ```rust /// use ::unstringify::unstringify; /// /// unstringify!(stringify! { /// fn main () /// { /// println!("Hello, World!"); /// } /// }); /// ``` /// /// <details><summary>A more interesting example</summary> /// /// A (non-procedural!) macro to evaluate the rust code snippets inside /// docstrings: /// /// ```rust,should_panic /// use ::unstringify::unstringify; /// /// macro_rules! eval_docstrings {( /// $( /// #[doc = $doc:tt] /// )* /// ) => ( /// extract_code_snippets! { /// @find_start /// $($doc)* /// } /// )} /// /// macro_rules! extract_code_snippets { /// ( /// @find_start /// r" ```rust" /// $($rest:tt)* /// ) => ( /// extract_code_snippets! { /// @accumulate_into [] /// $($rest)* /// } /// ); /// /// ( /// @find_start /// $otherwise_ignored:tt // ≠ " ```rust" /// $($rest:tt)* /// ) => ( /// extract_code_snippets! { /// @find_start /// $($rest)* /// } /// ); /// /// ( /// @find_start /// // No lines left /// ) => ( /// // The end. /// ); /// /// // End of code snippet found, /// // TIME TO EVALUATE THE CODE! /// ( /// @accumulate_into [ $($lines:tt)* ] /// r" ```" /// $($rest:tt)* /// ) => ( /// // evaluate the code... /// unstringify!(concat!( /// $($lines),* /// )); /// // ... and rince and repeat with the remaining docstrings /// extract_code_snippets! { /// @find_start /// $($rest)* /// } /// ); /// /// // Basic recursion step: accumulate a non-terminating line /// ( /// @accumulate_into [ $($lines:tt)* ] /// $current_line:tt // ≠ " ```" /// $($rest:tt)* /// ) => ( /// extract_code_snippets! { /// @accumulate_into [ $($lines)* $current_line ] /// $($rest)* /// } /// ); /// } /// /// eval_docstrings! { /// /// This is a comment. /// /// As ordinary as they make them. /// /// /// /// And yet... /// /// Sometimes... /// /// /// /// > A code snippet appears! /// /// /// /// ```rust /// /// panic!("Successfully managed to evaluate this panic (and thus panic)"); /// /// ``` /// /// /// /// Impressive, ain't it? /// } /// ``` /// /// ___ /// /// </details> /// /// ## Remarks /// /// This intuitive API very quickly encounters limitations, related not the /// macro itself, but rather to the way Rust expands macros. /// /// So, for instance, the following assertion fails: /// /// ```rust,should_panic /// # use ::unstringify::unstringify; /// # /// assert_eq!( /// stringify!(unstringify!("example")), /// "example", /// ); /// ``` /// /// Indeed, in the above code the macro `stringify!` is called _before_ /// `unstringify!`, so what happens is `stringify!` simply stringifies its input /// tokens, _verbatim_, without evaluating them: `'unstringify!("example")'`. 🤦 /// /// To solve that, [`unstringify!`] features "preprocessor" capabilities /// similar to [`::paste::paste!`](https://docs.rs/paste), that allow to /// circumvent this limitation, by doing: /// /// ```rust /// # use ::unstringify::unstringify; /// # /// assert_eq!( /// unstringify!(let $tokens = unstringify!("example") in { /// stringify!($tokens) /// }), /// "example", /// ); /// ``` /// /// ___ /// /// Also, for the same reason but reversed this time, the input fed to /// [`unstringify!`] cannot be eagerly macro-expanded. /// /// This means that the following fails: /// /// ```rust,compile_fail /// # use ::unstringify::unstringify; /// # /// macro_rules! my_macro {() => ("fn main () {}")} /// /// unstringify!(my_macro!()); /// ``` /// /// - The workaround is to define things such as `my_macro!` using, for /// instance, the callback pattern: /// /// ```rust /// # use ::unstringify::unstringify; /// # /// macro_rules! my_macro {( /// => $callback:ident ! /// ) => ( /// $callback! { "fn main () {}" } /// )} /// /// my_macro!(=> unstringify!); /// ``` /// /// That being said, the astute reader may retort: /// /// > But wait, doesn't your second example within this documentation showcase /// > `unstringify!(stringify! { ... })`? /// /// And indeed it does. This is achieved by hard-coding the (basic) logic of /// `stringify!` and `concat!` inside the [`unstringify!`] macro (for instance, /// when [`unstringify!`] stumbles upon a `stringify! { ... }` (which is _not_, I /// repeat, a _verbatim_ string literal), it decides to simply emit the inner /// `...`). #[proc_macro] pub fn unstringify (input: TokenStream) -> TokenStream { match tokenize_string_literal_or_concat_or_stringify( input.clone().into_iter().peekable() ) { | Ok((tokenized, mut remaining)) => if remaining.next().is_none() { return tokenized; }, | _ => {} } let Input { tokenized, metavar, template, } = match let_unstringify(input) { | Ok(it) => it, | Err((span, err_msg)) => { macro_rules! spanned {($expr:expr) => ( match $expr { mut expr => { expr.set_span(span); expr }} )} return ts![ Ident::new("compile_error", span), spanned!(Punct::new('!', Spacing::Alone)), spanned!(ts![ ( Literal::string(&*err_msg), )]), spanned!(Punct::new(';', Spacing::Alone)), ]; }, }; map_replace(&metavar.to_string(), &tokenized, template) } /// `let $var = unstringify!("...") in { ... }` fn let_unstringify (input: TokenStream) -> Result< Input, (Span, ::std::borrow::Cow<'static, str>), > { let mut tokens = input.into_iter().peekable(); unwrap_next_token! { if let TT::Ident(ident) = tokens.next(), if (ident.to_string() == "let") {} else { failwith!("expected `let`"); } } unwrap_next_token! { if let TT::Punct(p) = tokens.next(), if (p.as_char() == '$') {} else { failwith!("expected `$`"); } } let metavar = unwrap_next_token! { if let TT::Ident(it) = tokens.next(), { it } else { failwith!("expected an identifier"); } }; unwrap_next_token! { if let TT::Punct(p) = tokens.next(), if (p.as_char() == '=') {} else { failwith!("expected `=`"); } } unwrap_next_token! { if let TT::Ident(ident) = tokens.next(), if (ident.to_string() == "unstringify") {} else { failwith!("expected `unstringify`"); } } unwrap_next_token! { if let TT::Punct(p) = tokens.next(), if (p.as_char() == '!') {} else { failwith!("expected `!`"); } } let tokenized: TokenStream = { let tokenize_args = unwrap_next_token! { if let TT::Group(group) = tokens.next(), if (matches!(group.delimiter(), Delimiter::Parenthesis)) { group.stream().into_iter() } else { failwith!("expected `( ... )`"); } }; let (tokenized, mut remaining) = tokenize_string_literal_or_concat_or_stringify( tokenize_args.into_iter().peekable(), )? ; if let Some(extraneous_tt) = remaining.next() { return Err(( extraneous_tt.span(), "extraneous token(s)".into(), )); } tokenized }; unwrap_next_token! { if let TT::Ident(in_) = tokens.next(), if (in_.to_string() == "in") {} else { failwith!("expected `;`"); } } let rest = unwrap_next_token! { if let TT::Group(group) = tokens.next(), { group.stream() } else { failwith!("expected `{ ... }` or `( ... )` or `[ ... ]`"); } }; if let Some(extraneous_tt) = tokens.next() { return Err(( extraneous_tt.span(), "extraneous token(s)".into(), )); } Ok(Input { tokenized, metavar, template: rest, }) } fn map_replace ( metavar: &'_ String, tokenized: &'_ TokenStream, tokens: TokenStream ) -> TokenStream { let mut tokens = tokens.into_iter().peekable(); let mut ret = TokenStream::new(); loop { match (tokens.next(), tokens.peek()) { | ( Some(TT::Punct(dollar)), Some(TT::Ident(ident)), ) if dollar.as_char() == '$' && ident.to_string() == *metavar => { drop(tokens.next()); ret.extend(tokenized.clone()); }, | (Some(TT::Group(group)), _) => { ret.extend(Some(TT::Group(Group::new( group.delimiter(), map_replace(metavar, tokenized, group.stream()), )))); }, | (None, _) => break, | (tt, _) => ret.extend(tt), } } ret } type Tokens = ::core::iter::Peekable<token_stream::IntoIter>; /// Input may be a: /// /// - a string literal (terminal); /// /// - a `stringify! { ... }` call (verbatim); /// /// - a `concat!(...)` call whose args can be any of these three options: /// recurse. /// /// To recurse, especially when wanting to parse a comma-separated sequence of /// expressions, we return not only the successfully parsed-and-then-tokenized /// input, we also return the trailing tokens, to keep iterating the logic /// if they start with a `,`. fn tokenize_string_literal_or_concat_or_stringify ( mut tokens: Tokens, ) -> Result< (TokenStream, Tokens), (Span, ::std::borrow::Cow<'static, str>), > {Ok({ let recurse = tokenize_string_literal_or_concat_or_stringify; macro_rules! err_msg {() => ( "expected \ a string literal, \ a verbatim `stringify!` call, \ or a verbatim `concat!` call.\ " )} let mut s: String; let ret = match tokens.next() { // Un-group the `$var` metavariables. | Some(TT::Group(group)) if matches!(group.delimiter(), Delimiter::None) => { let mut flattened = group.stream(); flattened.extend(tokens); return recurse(flattened.into_iter().peekable()); }, | Some(TT::Literal(lit)) if { s = lit.to_string(); utils::extracted_string_literal(&mut s) } => match s.parse::<TokenStream>() { | Ok(ts) => ts, | Err(err) => return Err(( lit.span(), format!("Invalid tokens: {}", err).into(), )), }, | Some(TT::Ident(ident)) if matches!( tokens.peek(), Some(TT::Punct(p)) if p.as_char() == '!' ) => { drop(tokens.next()); let group_contents = unwrap_next_token! { if let TT::Group(group) = tokens.next(), { group.stream() } else { failwith!("\ expected `{ ... }` or `( ... )` or `[ ... ]`\ "); } }; match ident.to_string().as_str() { | "stringify" => group_contents, | "concat" => { let mut ret = TokenStream::new(); let mut current = group_contents.into_iter().peekable(); loop { let (parsed, mut remaining) = recurse(current)?; ret.extend(parsed); if remaining.peek().is_none() { break ret; } unwrap_next_token! { if let TT::Punct(p) = remaining.next(), if (p.as_char() == ',') {} else { failwith!("expected nothing or `,`"); } } if remaining.peek().is_none() { break ret; } current = remaining; } }, | _ => return Err(( ident.span(), "expected `stringify` or `concat`".into(), )), } }, | Some(bad_tt) => return Err(( bad_tt.span(), err_msg!().into(), )), | None => return Err(( Span::call_site(), concat!("Unexpected end of input: ", err_msg!()).into(), )), }; (ret, tokens) })}