displaydoc 0.2.6

A derive macro for implementing the display Trait via a doc comment and string interpolation
Documentation 
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use proc_macro2::TokenStream;
use quote::{quote, ToTokens};
use syn::{Attribute, LitStr, Meta, Result};

#[derive(Clone)]
pub(crate) struct Display {
    pub(crate) fmt: LitStr,
    pub(crate) args: TokenStream,
}

pub(crate) struct VariantDisplay {
    pub(crate) r#enum: Option<Display>,
    pub(crate) variant: Display,
}

impl ToTokens for Display {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        let fmt = &self.fmt;
        let args = &self.args;
        tokens.extend(quote! {
            write!(formatter, #fmt #args)
        });
    }
}

impl ToTokens for VariantDisplay {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        if let Some(ref r#enum) = self.r#enum {
            r#enum.to_tokens(tokens);
            tokens.extend(quote! { ?; write!(formatter, ": ")?; });
        }
        self.variant.to_tokens(tokens);
    }
}

pub(crate) struct AttrsHelper {
    ignore_extra_doc_attributes: bool,
    prefix_enum_doc_attributes: bool,
}

impl AttrsHelper {
    pub(crate) fn new(attrs: &[Attribute]) -> Self {
        let ignore_extra_doc_attributes = attrs
            .iter()
            .any(|attr| attr.path().is_ident("ignore_extra_doc_attributes"));
        let prefix_enum_doc_attributes = attrs
            .iter()
            .any(|attr| attr.path().is_ident("prefix_enum_doc_attributes"));

        Self {
            ignore_extra_doc_attributes,
            prefix_enum_doc_attributes,
        }
    }

    pub(crate) fn display(&self, attrs: &[Attribute]) -> Result<Option<Display>> {
        let displaydoc_attr = attrs.iter().find(|attr| attr.path().is_ident("displaydoc"));

        if let Some(displaydoc_attr) = displaydoc_attr {
            let lit = displaydoc_attr
                .parse_args()
                .expect("#[displaydoc(\"foo\")] must contain string arguments");
            let mut display = Display {
                fmt: lit,
                args: TokenStream::new(),
            };

            display.expand_shorthand();
            return Ok(Some(display));
        }

        let literals = attrs
            .iter()
            .filter(|attr| attr.path().is_ident("doc"))
            .map(|attr| match &attr.meta {
                Meta::NameValue(syn::MetaNameValue {
                    value:
                        syn::Expr::Lit(syn::ExprLit {
                            lit: syn::Lit::Str(lit),
                            ..
                        }),
                    ..
                }) => lit,
                _ => unimplemented!(),
            });

        let span = match literals.clone().next() {
            Some(lit) => lit.span(),
            None => return Ok(None),
        };

        let strs = literals.map(|lit| {
            // Make an attempt at cleaning up multiline doc comments.
            let doc_str = lit
                .value()
                .lines()
                .map(|line| line.trim().trim_start_matches('*').trim())
                .collect::<Vec<&str>>()
                .join("\n")
                .trim()
                .to_string();
            (!doc_str.is_empty()).then(|| doc_str)
        });

        let joined = if self.ignore_extra_doc_attributes {
            strs.take_while(|x| x.is_some()).collect::<Option<Vec<_>>>()
        } else {
            strs.collect::<Option<Vec<_>>>()
        }.unwrap_or_else(|| {
            panic!("Paragraph breaks in multi-line doc comments are disabled by default by displaydoc. Please consider using block doc comments (/** */) or adding the #[ignore_extra_doc_attributes] attribute to your type next to the derive");
        }).join(" ");

        let mut display = Display {
            fmt: LitStr::new(&joined, span),
            args: TokenStream::new(),
        };

        display.expand_shorthand();
        Ok(Some(display))
    }

    pub(crate) fn display_with_input(
        &self,
        r#enum: &[Attribute],
        variant: &[Attribute],
    ) -> Result<Option<VariantDisplay>> {
        let r#enum = if self.prefix_enum_doc_attributes {
            let result = self
                .display(r#enum)?
                .expect("Missing doc comment on enum with #[prefix_enum_doc_attributes]. Please remove the attribute or add a doc comment to the enum itself.");

            Some(result)
        } else {
            None
        };

        Ok(self
            .display(variant)?
            .map(|variant| VariantDisplay { r#enum, variant }))
    }
}