Remove dependency on syn

Cargo.toml

@@ -29,5 +29,3 @@ version_check = "0.9.2"
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = "1.0"
-syn-mid = "0.5"

src/ast.rs

@@ -0,0 +1,132 @@
+use proc_macro2::{Delimiter, Literal, Span, TokenStream, TokenTree};
+use quote::ToTokens;
+use std::iter::Peekable;
+
+use crate::utils::{parse_as_empty, tt_span};
+
+pub(crate) struct Func {
+    pub(crate) attrs: Vec<Attribute>,
+    pub(crate) sig: Vec<TokenTree>,
+    pub(crate) body: TokenTree,
+    pub(crate) print_const: bool,
+}
+
+pub(crate) fn parse_input(input: TokenStream) -> Result<Func, TokenStream> {
+    let mut input = input.into_iter().peekable();
+
+    let attrs = parse_attrs(&mut input)?;
+    let sig = parse_signature(&mut input);
+    let body = input.next();
+    parse_as_empty(input)?;
+
+    if body.is_none()
+        || !sig.iter().any(|tt| if let TokenTree::Ident(i) = tt { i == "fn" } else { false })
+    {
+        return Err(error!(
+            Span::call_site(),
+            "#[const_fn] attribute may only be used on functions"
+        ));
+    }
+    if !sig.iter().any(|tt| if let TokenTree::Ident(i) = tt { i == "const" } else { false }) {
+        let span = sig
+            .iter()
+            .position(|tt| if let TokenTree::Ident(i) = tt { i == "fn" } else { false })
+            .map(|i| sig[i].span())
+            .unwrap();
+        return Err(error!(span, "#[const_fn] attribute may only be used on const functions"));
+    }
+
+    Ok(Func { attrs, sig, body: body.unwrap(), print_const: true })
+}
+
+impl ToTokens for Func {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        self.attrs.iter().for_each(|attr| attr.to_tokens(tokens));
+        if self.print_const {
+            self.sig.iter().for_each(|attr| attr.to_tokens(tokens));
+        } else {
+            self.sig
+                .iter()
+                .filter(|tt| if let TokenTree::Ident(i) = tt { i != "const" } else { true })
+                .for_each(|tt| tt.to_tokens(tokens));
+        }
+        self.body.to_tokens(tokens);
+    }
+}
+
+fn parse_signature(input: &mut Peekable<impl Iterator<Item = TokenTree>>) -> Vec<TokenTree> {
+    let mut sig = Vec::new();
+    loop {
+        match input.peek() {
+            Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Brace => break,
+            None => break,
+            _ => sig.push(input.next().unwrap()),
+        }
+    }
+    sig
+}
+
+fn parse_attrs(
+    input: &mut Peekable<impl Iterator<Item = TokenTree>>,
+) -> Result<Vec<Attribute>, TokenStream> {
+    let mut attrs = Vec::new();
+    loop {
+        let pound_token = match input.peek() {
+            Some(TokenTree::Punct(p)) if p.as_char() == '#' => input.next().unwrap(),
+            _ => break,
+        };
+        let group = match input.peek() {
+            Some(TokenTree::Group(g)) if g.delimiter() == Delimiter::Bracket => {
+                input.next().unwrap()
+            }
+            tt => return Err(error!(tt_span(tt), "expected `[`")),
+        };
+        attrs.push(Attribute { pound_token, group });
+    }
+    Ok(attrs)
+}
+
+pub(crate) struct Attribute {
+    // `#`
+    pub(crate) pound_token: TokenTree,
+    // `[...]`
+    pub(crate) group: TokenTree,
+}
+
+impl ToTokens for Attribute {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        self.pound_token.to_tokens(tokens);
+        self.group.to_tokens(tokens);
+    }
+}
+
+pub(crate) struct LitStr {
+    token: Literal,
+    value: String,
+}
+
+impl LitStr {
+    pub(crate) fn new(token: &Literal) -> Result<Self, TokenStream> {
+        let value = token.to_string();
+        // unlike `syn::LitStr`, only accepts `"..."`
+        if value.starts_with('"') && value.ends_with('"') {
+            Ok(Self { token: token.clone(), value })
+        } else {
+            Err(error!(token.span(), "expected string literal"))
+        }
+    }
+
+    pub(crate) fn value(&self) -> &str {
+        &self.value[1..self.value.len() - 1]
+    }
+
+    pub(crate) fn span(&self) -> Span {
+        self.token.span()
+    }
+}
+
+impl ToTokens for LitStr {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        self.token.to_tokens(tokens);
+    }
+}

src/lib.rs

@@ -47,109 +47,124 @@
 #[allow(unused_extern_crates)]
 extern crate proc_macro;
 
+#[macro_use]
+mod utils;
+
+mod ast;
+
 use proc_macro::TokenStream;
-use proc_macro2::TokenStream as TokenStream2;
+use proc_macro2::{Delimiter, TokenStream as TokenStream2, TokenTree};
 use quote::{quote, ToTokens};
 use std::str::FromStr;
-use syn::{
-    parse::{Parse, ParseStream},
-    parse_quote, Error, *,
-};
-use syn_mid::ItemFn;
+
+use crate::utils::{parse_as_empty, tt_span};
 
 /// An attribute for easy generation of a const function with conditional compilations.
 /// See crate level documentation for details.
 #[proc_macro_attribute]
-pub fn const_fn(args: TokenStream, function: TokenStream) -> TokenStream {
-    let arg: Arg = syn::parse_macro_input!(args);
-
-    let mut item: ItemFn = syn::parse_macro_input!(function);
-
-    if item.sig.constness.is_none() {
-        return Error::new_spanned(
-            item.sig.fn_token,
-            "#[const_fn] attribute may only be used on const functions",
-        )
-        .to_compile_error()
-        .into();
-    }
+pub fn const_fn(args: TokenStream, input: TokenStream) -> TokenStream {
+    let arg = match parse_arg(args.into()) {
+        Ok(a) => a,
+        Err(e) => return e.into(),
+    };
+    let mut func = match ast::parse_input(input.into()) {
+        Ok(i) => i,
+        Err(e) => return e.into(),
+    };
 
     match arg {
         Arg::Cfg(c) => {
             let mut tokens = quote!(#[cfg(#c)]);
-            tokens.extend(item.to_token_stream());
-            item.attrs.push(parse_quote!(#[cfg(not(#c))]));
-            item.sig.constness = None;
-            tokens.extend(item.into_token_stream());
+            tokens.extend(func.to_token_stream());
+            tokens.extend(quote!(#[cfg(not(#c))]));
+            func.print_const = false;
+            tokens.extend(func.into_token_stream());
             tokens.into()
         }
-        Arg::Feature(f, e, s) => {
-            let mut tokens = quote!(#[cfg(#f #e #s)]);
-            tokens.extend(item.to_token_stream());
-            item.attrs.push(parse_quote!(#[cfg(not(#f #e #s))]));
-            item.sig.constness = None;
-            tokens.extend(item.into_token_stream());
+        Arg::Feature(f) => {
+            let mut tokens = quote!(#[cfg(#f)]);
+            tokens.extend(func.to_token_stream());
+            tokens.extend(quote!(#[cfg(not(#f))]));
+            func.print_const = false;
+            tokens.extend(func.into_token_stream());
             tokens.into()
         }
         Arg::Version(req) => {
             if req.major > 1 || req.minor > VERSION.minor {
-                item.sig.constness = None;
+                func.print_const = false;
             }
-            item.into_token_stream().into()
+            func.into_token_stream().into()
         }
         Arg::Nightly => {
             if !VERSION.nightly {
-                item.sig.constness = None;
+                func.print_const = false;
             }
-            item.into_token_stream().into()
+            func.into_token_stream().into()
         }
     }
 }
 
-mod kw {
-    syn::custom_keyword!(nightly);
-    syn::custom_keyword!(feature);
-    syn::custom_keyword!(cfg);
-}
-
 enum Arg {
-    // `const_fn("1.36")`
+    // `const_fn("...")`
     Version(VersionReq),
     // `const_fn(nightly)`
     Nightly,
     // `const_fn(cfg(...))`
     Cfg(TokenStream2),
     // `const_fn(feature = "...")`
-    Feature(kw::feature, Token![=], LitStr),
+    Feature(TokenStream2),
 }
 
-impl Parse for Arg {
-    fn parse(input: ParseStream<'_>) -> Result<Self> {
-        let lookahead = input.lookahead1();
-        if lookahead.peek(kw::nightly) {
-            let _: kw::nightly = input.parse()?;
-            Ok(Arg::Nightly)
-        } else if lookahead.peek(kw::cfg) {
-            let _: kw::cfg = input.parse()?;
-            let content;
-            let _: token::Paren = syn::parenthesized!(content in input);
-            let t: TokenStream2 = content.parse()?;
-            Ok(Arg::Cfg(t))
-        } else if lookahead.peek(kw::feature) {
-            let f: kw::feature = input.parse()?;
-            let e: Token![=] = input.parse()?;
-            let s: LitStr = input.parse()?;
-            Ok(Arg::Feature(f, e, s))
-        } else if lookahead.peek(LitStr) {
-            let s: LitStr = input.parse()?;
-            match s.value().parse::<VersionReq>() {
-                Ok(req) => Ok(Arg::Version(req)),
-                Err(e) => Err(Error::new(s.span(), e)),
+fn parse_arg(tokens: TokenStream2) -> Result<Arg, TokenStream2> {
+    let tokens2 = tokens.clone();
+    let mut iter = tokens.into_iter();
+
+    let next = iter.next();
+    match &next {
+        Some(TokenTree::Ident(i)) => match i.to_string().as_str() {
+            "nightly" => {
+                parse_as_empty(iter)?;
+                return Ok(Arg::Nightly);
+            }
+            "cfg" => {
+                return match iter.next().as_ref() {
+                    Some(TokenTree::Group(g)) if g.delimiter() == Delimiter::Parenthesis => {
+                        parse_as_empty(iter)?;
+                        Ok(Arg::Cfg(g.stream()))
+                    }
+                    tt => Err(error!(tt_span(tt), "expected `(`")),
+                };
+            }
+            "feature" => {
+                return match iter.next().as_ref() {
+                    Some(TokenTree::Punct(p)) if p.as_char() == '=' => match iter.next().as_ref() {
+                        Some(TokenTree::Literal(l)) if l.to_string().starts_with('"') => {
+                            parse_as_empty(iter)?;
+                            Ok(Arg::Feature(tokens2))
+                        }
+                        tt => Err(error!(tt_span(tt), "expected `=`")),
+                    },
+                    tt => Err(error!(tt_span(tt), "expected `=`")),
+                };
+            }
+            _ => {}
+        },
+        Some(TokenTree::Literal(l)) => {
+            if let Ok(l) = ast::LitStr::new(l) {
+                parse_as_empty(iter)?;
+                return match l.value().parse::<VersionReq>() {
+                    Ok(req) => Ok(Arg::Version(req)),
+                    Err(e) => Err(error!(l.span(), "{}", e)),
+                };
             }
-        } else {
-            Err(lookahead.error())
         }
+        _ => {}
     }
+
+    Err(error!(
+        tt_span(next.as_ref()),
+        "expected one of: `nightly`, `cfg`, `feature`, string literal"
+    ))
 }
 
 struct VersionReq {
@@ -160,7 +175,7 @@ struct VersionReq {
 impl FromStr for VersionReq {
     type Err = String;
 
-    fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
         let mut pieces = s.split('.');
         let major = pieces
             .next()

src/utils.rs

@@ -0,0 +1,24 @@
+use proc_macro2::{Span, TokenStream, TokenTree};
+
+macro_rules! error {
+    ($span:expr, $msg:expr) => {{
+        let msg = $msg;
+        quote::quote_spanned!($span=> compile_error! { #msg })
+    }};
+    ($span:expr, $($tt:tt)*) => {
+        error!($span, format!($($tt)*))
+    };
+}
+
+pub(crate) fn tt_span(tt: Option<&TokenTree>) -> Span {
+    tt.map_or_else(Span::call_site, TokenTree::span)
+}
+
+pub(crate) fn parse_as_empty(
+    mut tokens: impl Iterator<Item = TokenTree>,
+) -> Result<(), TokenStream> {
+    match tokens.next() {
+        Some(tt) => Err(error!(tt.span(), "unexpected token: {}", tt)),
+        None => Ok(()),
+    }
+}