chore: checkpoint before Python removal

vendor/displaydoc/src/attr.rs

@@ -0,0 +1,137 @@
+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 num_doc_attrs = attrs
+            .iter()
+            .filter(|attr| attr.path().is_ident("doc"))
+            .count();
+
+        if !self.ignore_extra_doc_attributes && num_doc_attrs > 1 {
+            panic!("Multi-line 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.");
+        }
+
+        for attr in attrs {
+            if attr.path().is_ident("doc") {
+                let lit = match &attr.meta {
+                    Meta::NameValue(syn::MetaNameValue {
+                        value:
+                            syn::Expr::Lit(syn::ExprLit {
+                                lit: syn::Lit::Str(lit),
+                                ..
+                            }),
+                        ..
+                    }) => lit,
+                    _ => unimplemented!(),
+                };
+
+                // 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");
+
+                let lit = LitStr::new(doc_str.trim(), lit.span());
+
+                let mut display = Display {
+                    fmt: lit,
+                    args: TokenStream::new(),
+                };
+
+                display.expand_shorthand();
+                return Ok(Some(display));
+            }
+        }
+
+        Ok(None)
+    }
+
+    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 }))
+    }
+}

vendor/displaydoc/src/expand.rs

@@ -0,0 +1,409 @@
+use super::attr::AttrsHelper;
+use proc_macro2::{Span, TokenStream};
+use quote::{format_ident, quote};
+use syn::{
+    punctuated::Punctuated,
+    token::{Colon, Comma, PathSep, Plus, Where},
+    Data, DataEnum, DataStruct, DeriveInput, Error, Fields, Generics, Ident, Path, PathArguments,
+    PathSegment, PredicateType, Result, TraitBound, TraitBoundModifier, Type, TypeParam,
+    TypeParamBound, TypePath, WhereClause, WherePredicate,
+};
+
+use std::collections::HashMap;
+
+pub(crate) fn derive(input: &DeriveInput) -> Result<TokenStream> {
+    let impls = match &input.data {
+        Data::Struct(data) => impl_struct(input, data),
+        Data::Enum(data) => impl_enum(input, data),
+        Data::Union(_) => Err(Error::new_spanned(input, "Unions are not supported")),
+    }?;
+
+    let helpers = specialization();
+    Ok(quote! {
+        #[allow(non_upper_case_globals, unused_attributes, unused_qualifications)]
+        const _: () = {
+            #helpers
+            #impls
+        };
+    })
+}
+
+#[cfg(feature = "std")]
+fn specialization() -> TokenStream {
+    quote! {
+        trait DisplayToDisplayDoc {
+            fn __displaydoc_display(&self) -> Self;
+        }
+
+        impl<T: ::core::fmt::Display> DisplayToDisplayDoc for &T {
+            fn __displaydoc_display(&self) -> Self {
+                self
+            }
+        }
+
+        // If the `std` feature gets enabled we want to ensure that any crate
+        // using displaydoc can still reference the std crate, which is already
+        // being compiled in by whoever enabled the `std` feature in
+        // `displaydoc`, even if the crates using displaydoc are no_std.
+        extern crate std;
+
+        trait PathToDisplayDoc {
+            fn __displaydoc_display(&self) -> std::path::Display<'_>;
+        }
+
+        impl PathToDisplayDoc for std::path::Path {
+            fn __displaydoc_display(&self) -> std::path::Display<'_> {
+                self.display()
+            }
+        }
+
+        impl PathToDisplayDoc for std::path::PathBuf {
+            fn __displaydoc_display(&self) -> std::path::Display<'_> {
+                self.display()
+            }
+        }
+    }
+}
+
+#[cfg(not(feature = "std"))]
+fn specialization() -> TokenStream {
+    quote! {}
+}
+
+fn impl_struct(input: &DeriveInput, data: &DataStruct) -> Result<TokenStream> {
+    let ty = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let where_clause = generate_where_clause(&input.generics, where_clause);
+
+    let helper = AttrsHelper::new(&input.attrs);
+
+    let display = helper.display(&input.attrs)?.map(|display| {
+        let pat = match &data.fields {
+            Fields::Named(fields) => {
+                let var = fields.named.iter().map(|field| &field.ident);
+                quote!(Self { #(#var),* })
+            }
+            Fields::Unnamed(fields) => {
+                let var = (0..fields.unnamed.len()).map(|i| format_ident!("_{}", i));
+                quote!(Self(#(#var),*))
+            }
+            Fields::Unit => quote!(_),
+        };
+        quote! {
+            impl #impl_generics ::core::fmt::Display for #ty #ty_generics #where_clause {
+                fn fmt(&self, formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                    // NB: This destructures the fields of `self` into named variables (for unnamed
+                    // fields, it uses _0, _1, etc as above). The `#[allow(unused_variables)]`
+                    // section means it doesn't have to parse the individual field references out of
+                    // the docstring.
+                    #[allow(unused_variables)]
+                    let #pat = self;
+                    #display
+                }
+            }
+        }
+    });
+
+    Ok(quote! { #display })
+}
+
+/// Create a `where` predicate for `ident`, without any [bound][TypeParamBound]s yet.
+fn new_empty_where_type_predicate(ident: Ident) -> PredicateType {
+    let mut path_segments = Punctuated::<PathSegment, PathSep>::new();
+    path_segments.push_value(PathSegment {
+        ident,
+        arguments: PathArguments::None,
+    });
+    PredicateType {
+        lifetimes: None,
+        bounded_ty: Type::Path(TypePath {
+            qself: None,
+            path: Path {
+                leading_colon: None,
+                segments: path_segments,
+            },
+        }),
+        colon_token: Colon {
+            spans: [Span::call_site()],
+        },
+        bounds: Punctuated::<TypeParamBound, Plus>::new(),
+    }
+}
+
+/// Create a `where` clause that we can add [WherePredicate]s to.
+fn new_empty_where_clause() -> WhereClause {
+    WhereClause {
+        where_token: Where {
+            span: Span::call_site(),
+        },
+        predicates: Punctuated::<WherePredicate, Comma>::new(),
+    }
+}
+
+enum UseGlobalPrefix {
+    LeadingColon,
+    #[allow(dead_code)]
+    NoLeadingColon,
+}
+
+/// Create a path with segments composed of [Idents] *without* any [PathArguments].
+fn join_paths(name_segments: &[&str], use_global_prefix: UseGlobalPrefix) -> Path {
+    let mut segments = Punctuated::<PathSegment, PathSep>::new();
+    assert!(!name_segments.is_empty());
+    segments.push_value(PathSegment {
+        ident: Ident::new(name_segments[0], Span::call_site()),
+        arguments: PathArguments::None,
+    });
+    for name in name_segments[1..].iter() {
+        segments.push_punct(PathSep {
+            spans: [Span::call_site(), Span::mixed_site()],
+        });
+        segments.push_value(PathSegment {
+            ident: Ident::new(name, Span::call_site()),
+            arguments: PathArguments::None,
+        });
+    }
+    Path {
+        leading_colon: match use_global_prefix {
+            UseGlobalPrefix::LeadingColon => Some(PathSep {
+                spans: [Span::call_site(), Span::mixed_site()],
+            }),
+            UseGlobalPrefix::NoLeadingColon => None,
+        },
+        segments,
+    }
+}
+
+/// Push `new_type_predicate` onto the end of `where_clause`.
+fn append_where_clause_type_predicate(
+    where_clause: &mut WhereClause,
+    new_type_predicate: PredicateType,
+) {
+    // Push a comma at the end if there are already any `where` predicates.
+    if !where_clause.predicates.is_empty() {
+        where_clause.predicates.push_punct(Comma {
+            spans: [Span::call_site()],
+        });
+    }
+    where_clause
+        .predicates
+        .push_value(WherePredicate::Type(new_type_predicate));
+}
+
+/// Add a requirement for [core::fmt::Display] to a `where` predicate for some type.
+fn add_display_constraint_to_type_predicate(
+    predicate_that_needs_a_display_impl: &mut PredicateType,
+) {
+    // Create a `Path` of `::core::fmt::Display`.
+    let display_path = join_paths(&["core", "fmt", "Display"], UseGlobalPrefix::LeadingColon);
+
+    let display_bound = TypeParamBound::Trait(TraitBound {
+        paren_token: None,
+        modifier: TraitBoundModifier::None,
+        lifetimes: None,
+        path: display_path,
+    });
+    if !predicate_that_needs_a_display_impl.bounds.is_empty() {
+        predicate_that_needs_a_display_impl.bounds.push_punct(Plus {
+            spans: [Span::call_site()],
+        });
+    }
+
+    predicate_that_needs_a_display_impl
+        .bounds
+        .push_value(display_bound);
+}
+
+/// Map each declared generic type parameter to the set of all trait boundaries declared on it.
+///
+/// These boundaries may come from the declaration site:
+///     pub enum E<T: MyTrait> { ... }
+/// or a `where` clause after the parameter declarations:
+///     pub enum E<T> where T: MyTrait { ... }
+/// This method will return the boundaries from both of those cases.
+fn extract_trait_constraints_from_source(
+    where_clause: &WhereClause,
+    type_params: &[&TypeParam],
+) -> HashMap<Ident, Vec<TraitBound>> {
+    // Add trait bounds provided at the declaration site of type parameters for the struct/enum.
+    let mut param_constraint_mapping: HashMap<Ident, Vec<TraitBound>> = type_params
+        .iter()
+        .map(|type_param| {
+            let trait_bounds: Vec<TraitBound> = type_param
+                .bounds
+                .iter()
+                .flat_map(|bound| match bound {
+                    TypeParamBound::Trait(trait_bound) => Some(trait_bound),
+                    _ => None,
+                })
+                .cloned()
+                .collect();
+            (type_param.ident.clone(), trait_bounds)
+        })
+        .collect();
+
+    // Add trait bounds from `where` clauses, which may be type parameters or types containing
+    // those parameters.
+    for predicate in where_clause.predicates.iter() {
+        // We only care about type and not lifetime constraints here.
+        if let WherePredicate::Type(ref pred_ty) = predicate {
+            let ident = match &pred_ty.bounded_ty {
+                Type::Path(TypePath { path, qself: None }) => match path.get_ident() {
+                    None => continue,
+                    Some(ident) => ident,
+                },
+                _ => continue,
+            };
+            // We ignore any type constraints that aren't direct references to type
+            // parameters of the current enum of struct definition. No types can be
+            // constrained in a `where` clause unless they are a type parameter or a generic
+            // type instantiated with one of the type parameters, so by only allowing single
+            // identifiers, we can be sure that the constrained type is a type parameter
+            // that is contained in `param_constraint_mapping`.
+            if let Some((_, ref mut known_bounds)) = param_constraint_mapping
+                .iter_mut()
+                .find(|(id, _)| *id == ident)
+            {
+                for bound in pred_ty.bounds.iter() {
+                    // We only care about trait bounds here.
+                    if let TypeParamBound::Trait(ref bound) = bound {
+                        known_bounds.push(bound.clone());
+                    }
+                }
+            }
+        }
+    }
+
+    param_constraint_mapping
+}
+
+/// Hygienically add `where _: Display` to the set of [TypeParamBound]s for `ident`, creating such
+/// a set if necessary.
+fn ensure_display_in_where_clause_for_type(where_clause: &mut WhereClause, ident: Ident) {
+    for pred_ty in where_clause
+        .predicates
+        .iter_mut()
+        // Find the `where` predicate constraining the current type param, if it exists.
+        .flat_map(|predicate| match predicate {
+            WherePredicate::Type(pred_ty) => Some(pred_ty),
+            // We're looking through type constraints, not lifetime constraints.
+            _ => None,
+        })
+    {
+        // Do a complicated destructuring in order to check if the type being constrained in this
+        // `where` clause is the type we're looking for, so we can use the mutable reference to
+        // `pred_ty` if so.
+        let matches_desired_type = matches!(
+            &pred_ty.bounded_ty,
+            Type::Path(TypePath { path, .. }) if Some(&ident) == path.get_ident());
+        if matches_desired_type {
+            add_display_constraint_to_type_predicate(pred_ty);
+            return;
+        }
+    }
+
+    // If there is no `where` predicate for the current type param, we will construct one.
+    let mut new_type_predicate = new_empty_where_type_predicate(ident);
+    add_display_constraint_to_type_predicate(&mut new_type_predicate);
+    append_where_clause_type_predicate(where_clause, new_type_predicate);
+}
+
+/// For all declared type parameters, add a [core::fmt::Display] constraint, unless the type
+/// parameter already has any type constraint.
+fn ensure_where_clause_has_display_for_all_unconstrained_members(
+    where_clause: &mut WhereClause,
+    type_params: &[&TypeParam],
+) {
+    let param_constraint_mapping = extract_trait_constraints_from_source(where_clause, type_params);
+
+    for (ident, known_bounds) in param_constraint_mapping.into_iter() {
+        // If the type parameter has any constraints already, we don't want to touch it, to avoid
+        // breaking use cases where a type parameter only needs to impl `Debug`, for example.
+        if known_bounds.is_empty() {
+            ensure_display_in_where_clause_for_type(where_clause, ident);
+        }
+    }
+}
+
+/// Generate a `where` clause that ensures all generic type parameters `impl`
+/// [core::fmt::Display] unless already constrained.
+///
+/// This approach allows struct/enum definitions deriving [crate::Display] to avoid hardcoding
+/// a [core::fmt::Display] constraint into every type parameter.
+///
+/// If the type parameter isn't already constrained, we add a `where _: Display` clause to our
+/// display implementation to expect to be able to format every enum case or struct member.
+///
+/// In fact, we would preferably only require `where _: Display` or `where _: Debug` where the
+/// format string actually requires it. However, while [`std::fmt` defines a formal syntax for
+/// `format!()`][format syntax], it *doesn't* expose the actual logic to parse the format string,
+/// which appears to live in [`rustc_parse_format`]. While we use the [`syn`] crate to parse rust
+/// syntax, it also doesn't currently provide any method to introspect a `format!()` string. It
+/// would be nice to contribute this upstream in [`syn`].
+///
+/// [format syntax]: std::fmt#syntax
+/// [`rustc_parse_format`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse_format/index.html
+fn generate_where_clause(generics: &Generics, where_clause: Option<&WhereClause>) -> WhereClause {
+    let mut where_clause = where_clause.cloned().unwrap_or_else(new_empty_where_clause);
+    let type_params: Vec<&TypeParam> = generics.type_params().collect();
+    ensure_where_clause_has_display_for_all_unconstrained_members(&mut where_clause, &type_params);
+    where_clause
+}
+
+fn impl_enum(input: &DeriveInput, data: &DataEnum) -> Result<TokenStream> {
+    let ty = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let where_clause = generate_where_clause(&input.generics, where_clause);
+
+    let helper = AttrsHelper::new(&input.attrs);
+
+    let displays = data
+        .variants
+        .iter()
+        .map(|variant| helper.display_with_input(&input.attrs, &variant.attrs))
+        .collect::<Result<Vec<_>>>()?;
+
+    if data.variants.is_empty() {
+        Ok(quote! {
+            impl #impl_generics ::core::fmt::Display for #ty #ty_generics #where_clause {
+                fn fmt(&self, formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                    unreachable!("empty enums cannot be instantiated and thus cannot be printed")
+                }
+            }
+        })
+    } else if displays.iter().any(Option::is_some) {
+        let arms = data
+            .variants
+            .iter()
+            .zip(displays)
+            .map(|(variant, display)| {
+                let display =
+                    display.ok_or_else(|| Error::new_spanned(variant, "missing doc comment"))?;
+                let ident = &variant.ident;
+                Ok(match &variant.fields {
+                    Fields::Named(fields) => {
+                        let var = fields.named.iter().map(|field| &field.ident);
+                        quote!(Self::#ident { #(#var),* } => { #display })
+                    }
+                    Fields::Unnamed(fields) => {
+                        let var = (0..fields.unnamed.len()).map(|i| format_ident!("_{}", i));
+                        quote!(Self::#ident(#(#var),*) => { #display })
+                    }
+                    Fields::Unit => quote!(Self::#ident => { #display }),
+                })
+            })
+            .collect::<Result<Vec<_>>>()?;
+        Ok(quote! {
+            impl #impl_generics ::core::fmt::Display for #ty #ty_generics #where_clause {
+                fn fmt(&self, formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                    #[allow(unused_variables)]
+                    match self {
+                        #(#arms,)*
+                    }
+                }
+            }
+        })
+    } else {
+        Err(Error::new_spanned(input, "Missing doc comments"))
+    }
+}

vendor/displaydoc/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");
+    }
+}

vendor/displaydoc/src/lib.rs

@@ -0,0 +1,186 @@
+//! This library provides a convenient derive macro for the standard library's
+//! [`core::fmt::Display`] trait.
+//!
+//! [`core::fmt::Display`]: https://doc.rust-lang.org/std/fmt/trait.Display.html
+//!
+//! ```toml
+//! [dependencies]
+//! displaydoc = "0.2"
+//! ```
+//!
+//! *Compiler support: requires rustc 1.56+*
+//!
+//! <br>
+//!
+//! ## Example
+//!
+//! *Demonstration alongside the [`Error`][std::error::Error] derive macro from [`thiserror`](https://docs.rs/thiserror/1.0.25/thiserror/index.html),
+//! to propagate source locations from [`io::Error`][std::io::Error] with the `#[source]` attribute:*
+//! ```rust
+//! use std::io;
+//! use displaydoc::Display;
+//! use thiserror::Error;
+//!
+//! #[derive(Display, Error, Debug)]
+//! pub enum DataStoreError {
+//!     /// data store disconnected
+//!     Disconnect(#[source] io::Error),
+//!     /// the data for key `{0}` is not available
+//!     Redaction(String),
+//!     /// invalid header (expected {expected:?}, found {found:?})
+//!     InvalidHeader {
+//!         expected: String,
+//!         found: String,
+//!     },
+//!     /// unknown data store error
+//!     Unknown,
+//! }
+//!
+//! let error = DataStoreError::Redaction("CLASSIFIED CONTENT".to_string());
+//! assert!("the data for key `CLASSIFIED CONTENT` is not available" == &format!("{}", error));
+//! ```
+//! *Note that although [`io::Error`][std::io::Error] implements `Display`, we do not add it to the
+//! generated message for `DataStoreError::Disconnect`, since it is already made available via
+//! `#[source]`. See further context on avoiding duplication in error reports at the rust blog
+//! [here](https://github.com/yaahc/blog.rust-lang.org/blob/master/posts/inside-rust/2021-05-15-What-the-error-handling-project-group-is-working-towards.md#duplicate-information-issue).*
+//!
+//! <br>
+//!
+//! ## Details
+//!
+//! - A `fmt::Display` impl is generated for your enum if you provide
+//!   a docstring comment on each variant as shown above in the example. The
+//!   `Display` derive macro supports a shorthand for interpolating fields from
+//!   the error:
+//!     - `/// {var}` ⟶ `write!("{}", self.var)`
+//!     - `/// {0}` ⟶ `write!("{}", self.0)`
+//!     - `/// {var:?}` ⟶ `write!("{:?}", self.var)`
+//!     - `/// {0:?}` ⟶ `write!("{:?}", self.0)`
+//! - This also works with structs and [generic types][crate::Display#generic-type-parameters]:
+//! ```rust
+//! # use displaydoc::Display;
+//! /// oh no, an error: {0}
+//! #[derive(Display)]
+//! pub struct Error<E>(pub E);
+//!
+//! let error: Error<&str> = Error("muahaha i am an error");
+//! assert!("oh no, an error: muahaha i am an error" == &format!("{}", error));
+//! ```
+//!
+//! - Two optional attributes can be added to your types next to the derive:
+//!
+//!     - `#[ignore_extra_doc_attributes]` makes the macro ignore any doc
+//!       comment attributes (or `///` lines) after the first. Multi-line
+//!       comments using `///` are otherwise treated as an error, so use this
+//!       attribute or consider switching to block doc comments (`/** */`).
+//!
+//!     - `#[prefix_enum_doc_attributes]` combines the doc comment message on
+//!       your enum itself with the messages for each variant, in the format
+//!       “enum: variant”. When added to an enum, the doc comment on the enum
+//!       becomes mandatory. When added to any other type, it has no effect.
+//!
+//! - In case you want to have an independent doc comment, the
+//!   `#[displaydoc("...")` atrribute may be used on the variant or struct to
+//!   override it.
+//!
+//! <br>
+//!
+//! ## FAQ
+//!
+//! 1. **Is this crate `no_std` compatible?**
+//!     * Yes! This crate implements the [`core::fmt::Display`] trait, not the [`std::fmt::Display`] trait, so it should work in `std` and `no_std` environments. Just add `default-features = false`.
+//!
+//! 2. **Does this crate work with `Path` and `PathBuf` via the `Display` trait?**
+//!     * Yuuup. This crate uses @dtolnay's [autoref specialization technique](https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md) to add a special trait for types to get the display impl. It then specializes for `Path` and `PathBuf`, and when either of these types are found, it calls `self.display()` to get a `std::path::Display<'_>` type which can be used with the `Display` format specifier!
+#![doc(html_root_url = "https://docs.rs/displaydoc/0.2.3")]
+#![cfg_attr(docsrs, feature(doc_cfg))]
+#![warn(
+    rust_2018_idioms,
+    unreachable_pub,
+    bad_style,
+    dead_code,
+    improper_ctypes,
+    non_shorthand_field_patterns,
+    no_mangle_generic_items,
+    overflowing_literals,
+    path_statements,
+    patterns_in_fns_without_body,
+    unconditional_recursion,
+    unused,
+    unused_allocation,
+    unused_comparisons,
+    unused_parens,
+    while_true
+)]
+#![allow(clippy::try_err)]
+
+#[allow(unused_extern_crates)]
+extern crate proc_macro;
+
+mod attr;
+mod expand;
+mod fmt;
+
+use proc_macro::TokenStream;
+use syn::{parse_macro_input, DeriveInput};
+
+/// [Custom `#[derive(...)]` macro](https://doc.rust-lang.org/edition-guide/rust-2018/macros/custom-derive.html)
+/// for implementing [`fmt::Display`][core::fmt::Display] via doc comment attributes.
+///
+/// ### Generic Type Parameters
+///
+/// Type parameters to an enum or struct using this macro should *not* need to
+/// have an explicit `Display` constraint at the struct or enum definition
+/// site. A `Display` implementation for the `derive`d struct or enum is
+/// generated assuming each type parameter implements `Display`, but that should
+/// be possible without adding the constraint to the struct definition itself:
+/// ```rust
+/// use displaydoc::Display;
+///
+/// /// oh no, an error: {0}
+/// #[derive(Display)]
+/// pub struct Error<E>(pub E);
+///
+/// // No need to require `E: Display`, since `displaydoc::Display` adds that implicitly.
+/// fn generate_error<E>(e: E) -> Error<E> { Error(e) }
+///
+/// assert!("oh no, an error: muahaha" == &format!("{}", generate_error("muahaha")));
+/// ```
+///
+/// ### Using [`Debug`][core::fmt::Debug] Implementations with Type Parameters
+/// However, if a type parameter must instead be constrained with the
+/// [`Debug`][core::fmt::Debug] trait so that some field may be printed with
+/// `{:?}`, that constraint must currently still also be specified redundantly
+/// at the struct or enum definition site. If a struct or enum field is being
+/// formatted with `{:?}` via [`displaydoc`][crate], and a generic type
+/// parameter must implement `Debug` to do that, then that struct or enum
+/// definition will need to propagate the `Debug` constraint to every type
+/// parameter it's instantiated with:
+/// ```rust
+/// use core::fmt::Debug;
+/// use displaydoc::Display;
+///
+/// /// oh no, an error: {0:?}
+/// #[derive(Display)]
+/// pub struct Error<E: Debug>(pub E);
+///
+/// // `E: Debug` now has to propagate to callers.
+/// fn generate_error<E: Debug>(e: E) -> Error<E> { Error(e) }
+///
+/// assert!("oh no, an error: \"cool\"" == &format!("{}", generate_error("cool")));
+///
+/// // Try this with a struct that doesn't impl `Display` at all, unlike `str`.
+/// #[derive(Debug)]
+/// pub struct Oh;
+/// assert!("oh no, an error: Oh" == &format!("{}", generate_error(Oh)));
+/// ```
+#[proc_macro_derive(
+    Display,
+    attributes(ignore_extra_doc_attributes, prefix_enum_doc_attributes, displaydoc)
+)]
+pub fn derive_error(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    expand::derive(&input)
+        .unwrap_or_else(|err| err.to_compile_error())
+        .into()
+}