1 files changed, 159 insertions, 0 deletions
diff --git a/src/fmt.rs b/src/fmt.rs
new file mode 100644
index 0000000..3334a82
--- /dev/null
+++ b/src/fmt.rs
@@ -0,0 +1,159 @@
+use crate::attr::Display;
+use proc_macro2::TokenStream;
+use quote::quote_spanned;
+use syn::{Ident, LitStr};
+
+macro_rules! peek_next {
+    ($read:ident) => {
+        match $read.chars().next() {
+            Some(next) => next,
+            None => return,
+        }
+    };
+}
+
+impl Display {
+    // Transform `"error {var}"` to `"error {}", var`.
+    pub(crate) fn expand_shorthand(&mut self) {
+        let span = self.fmt.span();
+        let fmt = self.fmt.value();
+        let mut read = fmt.as_str();
+        let mut out = String::new();
+        let mut args = TokenStream::new();
+
+        while let Some(brace) = read.find('{') {
+            out += &read[..=brace];
+            read = &read[brace + 1..];
+
+            // skip cases where we find a {{
+            if read.starts_with('{') {
+                out.push('{');
+                read = &read[1..];
+                continue;
+            }
+
+            let next = peek_next!(read);
+
+            let var = match next {
+                '0'..='9' => take_int(&mut read),
+                'a'..='z' | 'A'..='Z' | '_' => take_ident(&mut read),
+                _ => return,
+            };
+
+            let ident = Ident::new(&var, span);
+
+            let next = peek_next!(read);
+
+            let arg = if cfg!(feature = "std") && next == '}' {
+                quote_spanned!(span=> , #ident.__displaydoc_display())
+            } else {
+                quote_spanned!(span=> , #ident)
+            };
+
+            args.extend(arg);
+        }
+
+        out += read;
+        self.fmt = LitStr::new(&out, self.fmt.span());
+        self.args = args;
+    }
+}
+
+fn take_int(read: &mut &str) -> String {
+    let mut int = String::new();
+    int.push('_');
+    for (i, ch) in read.char_indices() {
+        match ch {
+            '0'..='9' => int.push(ch),
+            _ => {
+                *read = &read[i..];
+                break;
+            }
+        }
+    }
+    int
+}
+
+fn take_ident(read: &mut &str) -> String {
+    let mut ident = String::new();
+    for (i, ch) in read.char_indices() {
+        match ch {
+            'a'..='z' | 'A'..='Z' | '0'..='9' | '_' => ident.push(ch),
+            _ => {
+                *read = &read[i..];
+                break;
+            }
+        }
+    }
+    ident
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+    use proc_macro2::Span;
+
+    fn assert(input: &str, fmt: &str, args: &str) {
+        let mut display = Display {
+            fmt: LitStr::new(input, Span::call_site()),
+            args: TokenStream::new(),
+        };
+        display.expand_shorthand();
+        assert_eq!(fmt, display.fmt.value());
+        assert_eq!(args, display.args.to_string());
+    }
+
+    #[test]
+    fn test_expand() {
+        assert("fn main() {{ }}", "fn main() {{ }}", "");
+    }
+
+    #[test]
+    #[cfg_attr(not(feature = "std"), ignore)]
+    fn test_std_expand() {
+        assert(
+            "{v} {v:?} {0} {0:?}",
+            "{} {:?} {} {:?}",
+            ", v . __displaydoc_display () , v , _0 . __displaydoc_display () , _0",
+        );
+        assert("error {var}", "error {}", ", var . __displaydoc_display ()");
+
+        assert(
+            "error {var1}",
+            "error {}",
+            ", var1 . __displaydoc_display ()",
+        );
+
+        assert(
+            "error {var1var}",
+            "error {}",
+            ", var1var . __displaydoc_display ()",
+        );
+
+        assert(
+            "The path {0}",
+            "The path {}",
+            ", _0 . __displaydoc_display ()",
+        );
+        assert("The path {0:?}", "The path {:?}", ", _0");
+    }
+
+    #[test]
+    #[cfg_attr(feature = "std", ignore)]
+    fn test_nostd_expand() {
+        assert(
+            "{v} {v:?} {0} {0:?}",
+            "{} {:?} {} {:?}",
+            ", v , v , _0 , _0",
+        );
+        assert("error {var}", "error {}", ", var");
+
+        assert("The path {0}", "The path {}", ", _0");
+        assert("The path {0:?}", "The path {:?}", ", _0");
+
+        assert("error {var1}", "error {}", ", var1");
+
+        assert("error {var1var}", "error {}", ", var1var");
+    }
+}