1 files changed, 140 insertions, 0 deletions
diff --git a/src/attr.rs b/src/attr.rs
new file mode 100644
index 0000000..2d23730
--- /dev/null
+++ b/src/attr.rs
@@ -0,0 +1,140 @@
+use crate::error::Result;
+use crate::segment::{self, Segment};
+use proc_macro::{Delimiter, Group, Span, TokenStream, TokenTree};
+use std::iter;
+use std::mem;
+use std::str::FromStr;
+
+pub fn expand_attr(
+    attr: TokenStream,
+    span: Span,
+    contains_paste: &mut bool,
+) -> Result<TokenStream> {
+    let mut tokens = attr.clone().into_iter();
+    match tokens.next() {
+        Some(TokenTree::Ident(..)) => {}
+        _ => return Ok(attr),
+    }
+
+    let group = match tokens.next() {
+        Some(TokenTree::Punct(ref punct)) if punct.as_char() == '=' => {
+            let mut count = 0;
+            if tokens.inspect(|_| count += 1).all(|tt| is_stringlike(&tt)) && count > 1 {
+                *contains_paste = true;
+                return do_paste_name_value_attr(attr, span);
+            }
+            return Ok(attr);
+        }
+        Some(TokenTree::Group(group)) => group,
+        _ => return Ok(attr),
+    };
+
+    if group.delimiter() != Delimiter::Parenthesis {
+        return Ok(attr);
+    }
+
+    // There can't be anything else after the first group in a valid attribute.
+    if tokens.next().is_some() {
+        return Ok(attr);
+    }
+
+    let mut group_contains_paste = false;
+    let mut expanded = TokenStream::new();
+    let mut nested_attr = TokenStream::new();
+    for tt in group.stream().into_iter() {
+        match &tt {
+            TokenTree::Punct(punct) if punct.as_char() == ',' => {
+                expanded.extend(expand_attr(
+                    nested_attr,
+                    group.span(),
+                    &mut group_contains_paste,
+                )?);
+                expanded.extend(iter::once(tt));
+                nested_attr = TokenStream::new();
+            }
+            _ => nested_attr.extend(iter::once(tt)),
+        }
+    }
+
+    if !nested_attr.is_empty() {
+        expanded.extend(expand_attr(
+            nested_attr,
+            group.span(),
+            &mut group_contains_paste,
+        )?);
+    }
+
+    if group_contains_paste {
+        *contains_paste = true;
+        let mut group = Group::new(Delimiter::Parenthesis, expanded);
+        group.set_span(span);
+        Ok(attr
+            .into_iter()
+            // Just keep the initial ident in `#[ident(...)]`.
+            .take(1)
+            .chain(iter::once(TokenTree::Group(group)))
+            .collect())
+    } else {
+        Ok(attr)
+    }
+}
+
+fn do_paste_name_value_attr(attr: TokenStream, span: Span) -> Result<TokenStream> {
+    let mut expanded = TokenStream::new();
+    let mut tokens = attr.into_iter().peekable();
+    expanded.extend(tokens.by_ref().take(2)); // `doc =`
+
+    let mut segments = segment::parse(&mut tokens)?;
+
+    for segment in &mut segments {
+        if let Segment::String(string) = segment {
+            if let Some(open_quote) = string.value.find('"') {
+                if open_quote == 0 {
+                    string.value.truncate(string.value.len() - 1);
+                    string.value.remove(0);
+                } else {
+                    let begin = open_quote + 1;
+                    let end = string.value.rfind('"').unwrap();
+                    let raw_string = mem::replace(&mut string.value, String::new());
+                    for ch in raw_string[begin..end].chars() {
+                        string.value.extend(ch.escape_default());
+                    }
+                }
+            }
+        }
+    }
+
+    let mut lit = segment::paste(&segments)?;
+    lit.insert(0, '"');
+    lit.push('"');
+
+    let mut lit = TokenStream::from_str(&lit)
+        .unwrap()
+        .into_iter()
+        .next()
+        .unwrap();
+    lit.set_span(span);
+    expanded.extend(iter::once(lit));
+    Ok(expanded)
+}
+
+fn is_stringlike(token: &TokenTree) -> bool {
+    match token {
+        TokenTree::Ident(_) => true,
+        TokenTree::Literal(literal) => {
+            let repr = literal.to_string();
+            !repr.starts_with('b') && !repr.starts_with('\'')
+        }
+        TokenTree::Group(group) => {
+            if group.delimiter() != Delimiter::None {
+                return false;
+            }
+            let mut inner = group.stream().into_iter();
+            match inner.next() {
+                Some(first) => inner.next().is_none() && is_stringlike(&first),
+                None => false,
+            }
+        }
+        TokenTree::Punct(punct) => punct.as_char() == '\'' || punct.as_char() == ':',
+    }
+}