Import thiserror-impl-1.0.16

Change-Id: If439e1c68f7afb772354f3c07ebfec7a94b01300

12 files changed, 1503 insertions, 0 deletions

diff --git a/.cargo_vcs_info.json b/.cargo_vcs_info.json
new file mode 100644
index 0000000..30d59f6
--- /dev/null
+++ b/.cargo_vcs_info.json
@@ -0,0 +1,5 @@
+{
+  "git": {
+    "sha1": "94e62a81bc6da67efb06a45a8f45caf49768d5e4"
+  }
+}
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..9197bc0
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,35 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies
+#
+# If you believe there's an error in this file please file an
+# issue against the rust-lang/cargo repository. If you're
+# editing this file be aware that the upstream Cargo.toml
+# will likely look very different (and much more reasonable)
+
+[package]
+edition = "2018"
+name = "thiserror-impl"
+version = "1.0.16"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+description = "Implementation detail of the `thiserror` crate"
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/dtolnay/thiserror"
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
+
+[lib]
+proc-macro = true
+[dependencies.proc-macro2]
+version = "1.0"
+
+[dependencies.quote]
+version = "1.0"
+
+[dependencies.syn]
+version = "1.0.11"
+[badges.travis-ci]
+repository = "dtolnay/thiserror"
diff --git a/Cargo.toml.orig b/Cargo.toml.orig
new file mode 100644
index 0000000..46363da
--- /dev/null
+++ b/Cargo.toml.orig
@@ -0,0 +1,22 @@
+[package]
+name = "thiserror-impl"
+version = "1.0.16"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+edition = "2018"
+license = "MIT OR Apache-2.0"
+description = "Implementation detail of the `thiserror` crate"
+repository = "https://github.com/dtolnay/thiserror"
+
+[lib]
+proc-macro = true
+
+[badges]
+travis-ci = { repository = "dtolnay/thiserror" }
+
+[dependencies]
+proc-macro2 = "1.0"
+quote = "1.0"
+syn = "1.0.11"
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
diff --git a/LICENSE-APACHE b/LICENSE-APACHE
new file mode 100644
index 0000000..16fe87b
--- /dev/null
+++ b/LICENSE-APACHE
@@ -0,0 +1,201 @@
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diff --git a/LICENSE-MIT b/LICENSE-MIT
new file mode 100644
index 0000000..31aa793
--- /dev/null
+++ b/LICENSE-MIT
@@ -0,0 +1,23 @@
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+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
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+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
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+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
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diff --git a/src/ast.rs b/src/ast.rs
new file mode 100644
index 0000000..e16401f
--- /dev/null
+++ b/src/ast.rs
@@ -0,0 +1,133 @@
+use crate::attr::{self, Attrs};
+use syn::{
+    Data, DataEnum, DataStruct, DeriveInput, Error, Fields, Generics, Ident, Index, Member, Result,
+    Type,
+};
+
+pub enum Input<'a> {
+    Struct(Struct<'a>),
+    Enum(Enum<'a>),
+}
+
+pub struct Struct<'a> {
+    pub original: &'a DeriveInput,
+    pub attrs: Attrs<'a>,
+    pub ident: Ident,
+    pub generics: &'a Generics,
+    pub fields: Vec<Field<'a>>,
+}
+
+pub struct Enum<'a> {
+    pub original: &'a DeriveInput,
+    pub attrs: Attrs<'a>,
+    pub ident: Ident,
+    pub generics: &'a Generics,
+    pub variants: Vec<Variant<'a>>,
+}
+
+pub struct Variant<'a> {
+    pub original: &'a syn::Variant,
+    pub attrs: Attrs<'a>,
+    pub ident: Ident,
+    pub fields: Vec<Field<'a>>,
+}
+
+pub struct Field<'a> {
+    pub original: &'a syn::Field,
+    pub attrs: Attrs<'a>,
+    pub member: Member,
+    pub ty: &'a Type,
+}
+
+impl<'a> Input<'a> {
+    pub fn from_syn(node: &'a DeriveInput) -> Result<Self> {
+        match &node.data {
+            Data::Struct(data) => Struct::from_syn(node, data).map(Input::Struct),
+            Data::Enum(data) => Enum::from_syn(node, data).map(Input::Enum),
+            Data::Union(_) => Err(Error::new_spanned(
+                node,
+                "union as errors are not supported",
+            )),
+        }
+    }
+}
+
+impl<'a> Struct<'a> {
+    fn from_syn(node: &'a DeriveInput, data: &'a DataStruct) -> Result<Self> {
+        let mut attrs = attr::get(&node.attrs)?;
+        let fields = Field::multiple_from_syn(&data.fields)?;
+        if let Some(display) = &mut attrs.display {
+            display.expand_shorthand(&fields);
+        }
+        Ok(Struct {
+            original: node,
+            attrs,
+            ident: node.ident.clone(),
+            generics: &node.generics,
+            fields,
+        })
+    }
+}
+
+impl<'a> Enum<'a> {
+    fn from_syn(node: &'a DeriveInput, data: &'a DataEnum) -> Result<Self> {
+        let attrs = attr::get(&node.attrs)?;
+        let variants = data
+            .variants
+            .iter()
+            .map(|node| {
+                let mut variant = Variant::from_syn(node)?;
+                if let display @ None = &mut variant.attrs.display {
+                    *display = attrs.display.clone();
+                }
+                if let Some(display) = &mut variant.attrs.display {
+                    display.expand_shorthand(&variant.fields);
+                } else if variant.attrs.transparent.is_none() {
+                    variant.attrs.transparent = attrs.transparent;
+                }
+                Ok(variant)
+            })
+            .collect::<Result<_>>()?;
+        Ok(Enum {
+            original: node,
+            attrs,
+            ident: node.ident.clone(),
+            generics: &node.generics,
+            variants,
+        })
+    }
+}
+
+impl<'a> Variant<'a> {
+    fn from_syn(node: &'a syn::Variant) -> Result<Self> {
+        Ok(Variant {
+            original: node,
+            attrs: attr::get(&node.attrs)?,
+            ident: node.ident.clone(),
+            fields: Field::multiple_from_syn(&node.fields)?,
+        })
+    }
+}
+
+impl<'a> Field<'a> {
+    fn multiple_from_syn(fields: &'a Fields) -> Result<Vec<Self>> {
+        fields
+            .iter()
+            .enumerate()
+            .map(|(i, field)| Field::from_syn(i, field))
+            .collect()
+    }
+
+    fn from_syn(i: usize, node: &'a syn::Field) -> Result<Self> {
+        Ok(Field {
+            original: node,
+            attrs: attr::get(&node.attrs)?,
+            member: node
+                .ident
+                .clone()
+                .map(Member::Named)
+                .unwrap_or_else(|| Member::Unnamed(Index::from(i))),
+            ty: &node.ty,
+        })
+    }
+}
diff --git a/src/attr.rs b/src/attr.rs
new file mode 100644
index 0000000..30ecfc8
--- /dev/null
+++ b/src/attr.rs
@@ -0,0 +1,181 @@
+use proc_macro2::{Delimiter, Group, TokenStream, TokenTree};
+use quote::{format_ident, quote, ToTokens};
+use std::iter::FromIterator;
+use syn::parse::{Nothing, ParseStream};
+use syn::{
+    braced, bracketed, parenthesized, token, Attribute, Error, Ident, Index, LitInt, LitStr,
+    Result, Token,
+};
+
+pub struct Attrs<'a> {
+    pub display: Option<Display<'a>>,
+    pub source: Option<&'a Attribute>,
+    pub backtrace: Option<&'a Attribute>,
+    pub from: Option<&'a Attribute>,
+    pub transparent: Option<&'a Attribute>,
+}
+
+#[derive(Clone)]
+pub struct Display<'a> {
+    pub original: &'a Attribute,
+    pub fmt: LitStr,
+    pub args: TokenStream,
+    pub has_bonus_display: bool,
+}
+
+pub fn get(input: &[Attribute]) -> Result<Attrs> {
+    let mut attrs = Attrs {
+        display: None,
+        source: None,
+        backtrace: None,
+        from: None,
+        transparent: None,
+    };
+
+    for attr in input {
+        if attr.path.is_ident("error") {
+            parse_error_attribute(&mut attrs, attr)?;
+        } else if attr.path.is_ident("source") {
+            require_empty_attribute(attr)?;
+            if attrs.source.is_some() {
+                return Err(Error::new_spanned(attr, "duplicate #[source] attribute"));
+            }
+            attrs.source = Some(attr);
+        } else if attr.path.is_ident("backtrace") {
+            require_empty_attribute(attr)?;
+            if attrs.backtrace.is_some() {
+                return Err(Error::new_spanned(attr, "duplicate #[backtrace] attribute"));
+            }
+            attrs.backtrace = Some(attr);
+        } else if attr.path.is_ident("from") {
+            if !attr.tokens.is_empty() {
+                // Assume this is meant for derive_more crate or something.
+                continue;
+            }
+            if attrs.from.is_some() {
+                return Err(Error::new_spanned(attr, "duplicate #[from] attribute"));
+            }
+            attrs.from = Some(attr);
+        }
+    }
+
+    Ok(attrs)
+}
+
+fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Result<()> {
+    syn::custom_keyword!(transparent);
+
+    attr.parse_args_with(|input: ParseStream| {
+        if input.parse::<Option<transparent>>()?.is_some() {
+            if attrs.transparent.is_some() {
+                return Err(Error::new_spanned(
+                    attr,
+                    "duplicate #[error(transparent)] attribute",
+                ));
+            }
+            attrs.transparent = Some(attr);
+            return Ok(());
+        }
+
+        let display = Display {
+            original: attr,
+            fmt: input.parse()?,
+            args: parse_token_expr(input, false)?,
+            has_bonus_display: false,
+        };
+        if attrs.display.is_some() {
+            return Err(Error::new_spanned(
+                attr,
+                "only one #[error(...)] attribute is allowed",
+            ));
+        }
+        attrs.display = Some(display);
+        Ok(())
+    })
+}
+
+fn parse_token_expr(input: ParseStream, mut begin_expr: bool) -> Result<TokenStream> {
+    let mut tokens = Vec::new();
+    while !input.is_empty() {
+        if begin_expr && input.peek(Token![.]) {
+            if input.peek2(Ident) {
+                input.parse::<Token![.]>()?;
+                begin_expr = false;
+                continue;
+            }
+            if input.peek2(LitInt) {
+                input.parse::<Token![.]>()?;
+                let int: Index = input.parse()?;
+                let ident = format_ident!("_{}", int.index, span = int.span);
+                tokens.push(TokenTree::Ident(ident));
+                begin_expr = false;
+                continue;
+            }
+        }
+
+        begin_expr = input.peek(Token![break])
+            || input.peek(Token![continue])
+            || input.peek(Token![if])
+            || input.peek(Token![in])
+            || input.peek(Token![match])
+            || input.peek(Token![mut])
+            || input.peek(Token![return])
+            || input.peek(Token![while])
+            || input.peek(Token![+])
+            || input.peek(Token![&])
+            || input.peek(Token![!])
+            || input.peek(Token![^])
+            || input.peek(Token![,])
+            || input.peek(Token![/])
+            || input.peek(Token![=])
+            || input.peek(Token![>])
+            || input.peek(Token![<])
+            || input.peek(Token![|])
+            || input.peek(Token![%])
+            || input.peek(Token![;])
+            || input.peek(Token![*])
+            || input.peek(Token![-]);
+
+        let token: TokenTree = if input.peek(token::Paren) {
+            let content;
+            let delimiter = parenthesized!(content in input);
+            let nested = parse_token_expr(&content, true)?;
+            let mut group = Group::new(Delimiter::Parenthesis, nested);
+            group.set_span(delimiter.span);
+            TokenTree::Group(group)
+        } else if input.peek(token::Brace) {
+            let content;
+            let delimiter = braced!(content in input);
+            let nested = parse_token_expr(&content, true)?;
+            let mut group = Group::new(Delimiter::Brace, nested);
+            group.set_span(delimiter.span);
+            TokenTree::Group(group)
+        } else if input.peek(token::Bracket) {
+            let content;
+            let delimiter = bracketed!(content in input);
+            let nested = parse_token_expr(&content, true)?;
+            let mut group = Group::new(Delimiter::Bracket, nested);
+            group.set_span(delimiter.span);
+            TokenTree::Group(group)
+        } else {
+            input.parse()?
+        };
+        tokens.push(token);
+    }
+    Ok(TokenStream::from_iter(tokens))
+}
+
+fn require_empty_attribute(attr: &Attribute) -> Result<()> {
+    syn::parse2::<Nothing>(attr.tokens.clone())?;
+    Ok(())
+}
+
+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)
+        });
+    }
+}
diff --git a/src/expand.rs b/src/expand.rs
new file mode 100644
index 0000000..3da8f41
--- /dev/null
+++ b/src/expand.rs
@@ -0,0 +1,407 @@
+use crate::ast::{Enum, Field, Input, Struct};
+use proc_macro2::TokenStream;
+use quote::{format_ident, quote, quote_spanned, ToTokens};
+use syn::spanned::Spanned;
+use syn::{Data, DeriveInput, Member, PathArguments, Result, Type, Visibility};
+
+pub fn derive(node: &DeriveInput) -> Result<TokenStream> {
+    let input = Input::from_syn(node)?;
+    input.validate()?;
+    Ok(match input {
+        Input::Struct(input) => impl_struct(input),
+        Input::Enum(input) => impl_enum(input),
+    })
+}
+
+fn impl_struct(input: Struct) -> TokenStream {
+    let ty = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let source_body = if input.attrs.transparent.is_some() {
+        let only_field = &input.fields[0].member;
+        Some(quote! {
+            std::error::Error::source(self.#only_field.as_dyn_error())
+        })
+    } else if let Some(source_field) = input.source_field() {
+        let source = &source_field.member;
+        let asref = if type_is_option(source_field.ty) {
+            Some(quote_spanned!(source.span()=> .as_ref()?))
+        } else {
+            None
+        };
+        let dyn_error = quote_spanned!(source.span()=> self.#source #asref.as_dyn_error());
+        Some(quote! {
+            std::option::Option::Some(#dyn_error)
+        })
+    } else {
+        None
+    };
+    let source_method = source_body.map(|body| {
+        quote! {
+            fn source(&self) -> std::option::Option<&(dyn std::error::Error + 'static)> {
+                use thiserror::private::AsDynError;
+                #body
+            }
+        }
+    });
+
+    let backtrace_method = input.backtrace_field().map(|backtrace_field| {
+        let backtrace = &backtrace_field.member;
+        let body = if let Some(source_field) = input.source_field() {
+            let source = &source_field.member;
+            let source_backtrace = if type_is_option(source_field.ty) {
+                quote_spanned! {source.span()=>
+                    self.#source.as_ref().and_then(|source| source.as_dyn_error().backtrace())
+                }
+            } else {
+                quote_spanned! {source.span()=>
+                    self.#source.as_dyn_error().backtrace()
+                }
+            };
+            let combinator = if type_is_option(backtrace_field.ty) {
+                quote! {
+                    #source_backtrace.or(self.#backtrace.as_ref())
+                }
+            } else {
+                quote! {
+                    std::option::Option::Some(#source_backtrace.unwrap_or(&self.#backtrace))
+                }
+            };
+            quote! {
+                use thiserror::private::AsDynError;
+                #combinator
+            }
+        } else if type_is_option(backtrace_field.ty) {
+            quote! {
+                self.#backtrace.as_ref()
+            }
+        } else {
+            quote! {
+                std::option::Option::Some(&self.#backtrace)
+            }
+        };
+        quote! {
+            fn backtrace(&self) -> std::option::Option<&std::backtrace::Backtrace> {
+                #body
+            }
+        }
+    });
+
+    let display_body = if input.attrs.transparent.is_some() {
+        let only_field = &input.fields[0].member;
+        Some(quote! {
+            std::fmt::Display::fmt(&self.#only_field, __formatter)
+        })
+    } else if let Some(display) = &input.attrs.display {
+        let use_as_display = if display.has_bonus_display {
+            Some(quote! {
+                #[allow(unused_imports)]
+                use thiserror::private::{DisplayAsDisplay, PathAsDisplay};
+            })
+        } else {
+            None
+        };
+        let pat = fields_pat(&input.fields);
+        Some(quote! {
+            #use_as_display
+            #[allow(unused_variables, deprecated)]
+            let Self #pat = self;
+            #display
+        })
+    } else {
+        None
+    };
+    let display_impl = display_body.map(|body| {
+        quote! {
+            impl #impl_generics std::fmt::Display for #ty #ty_generics #where_clause {
+                fn fmt(&self, __formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+                    #body
+                }
+            }
+        }
+    });
+
+    let from_impl = input.from_field().map(|from_field| {
+        let backtrace_field = input.backtrace_field();
+        let from = from_field.ty;
+        let body = from_initializer(from_field, backtrace_field);
+        quote! {
+            impl #impl_generics std::convert::From<#from> for #ty #ty_generics #where_clause {
+                #[allow(deprecated)]
+                fn from(source: #from) -> Self {
+                    #ty #body
+                }
+            }
+        }
+    });
+
+    let error_trait = spanned_error_trait(input.original);
+
+    quote! {
+        impl #impl_generics #error_trait for #ty #ty_generics #where_clause {
+            #source_method
+            #backtrace_method
+        }
+        #display_impl
+        #from_impl
+    }
+}
+
+fn impl_enum(input: Enum) -> TokenStream {
+    let ty = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let source_method = if input.has_source() {
+        let arms = input.variants.iter().map(|variant| {
+            let ident = &variant.ident;
+            if variant.attrs.transparent.is_some() {
+                let only_field = &variant.fields[0].member;
+                let source = quote!(std::error::Error::source(transparent.as_dyn_error()));
+                quote! {
+                    #ty::#ident {#only_field: transparent} => #source,
+                }
+            } else if let Some(source_field) = variant.source_field() {
+                let source = &source_field.member;
+                let asref = if type_is_option(source_field.ty) {
+                    Some(quote_spanned!(source.span()=> .as_ref()?))
+                } else {
+                    None
+                };
+                let dyn_error = quote_spanned!(source.span()=> source #asref.as_dyn_error());
+                quote! {
+                    #ty::#ident {#source: source, ..} => std::option::Option::Some(#dyn_error),
+                }
+            } else {
+                quote! {
+                    #ty::#ident {..} => std::option::Option::None,
+                }
+            }
+        });
+        Some(quote! {
+            fn source(&self) -> std::option::Option<&(dyn std::error::Error + 'static)> {
+                use thiserror::private::AsDynError;
+                #[allow(deprecated)]
+                match self {
+                    #(#arms)*
+                }
+            }
+        })
+    } else {
+        None
+    };
+
+    let backtrace_method = if input.has_backtrace() {
+        let arms = input.variants.iter().map(|variant| {
+            let ident = &variant.ident;
+            match (variant.backtrace_field(), variant.source_field()) {
+                (Some(backtrace_field), Some(source_field))
+                    if backtrace_field.attrs.backtrace.is_none() =>
+                {
+                    let backtrace = &backtrace_field.member;
+                    let source = &source_field.member;
+                    let source_backtrace = if type_is_option(source_field.ty) {
+                        quote_spanned! {source.span()=>
+                            source.as_ref().and_then(|source| source.as_dyn_error().backtrace())
+                        }
+                    } else {
+                        quote_spanned! {source.span()=>
+                            source.as_dyn_error().backtrace()
+                        }
+                    };
+                    let combinator = if type_is_option(backtrace_field.ty) {
+                        quote! {
+                            #source_backtrace.or(backtrace.as_ref())
+                        }
+                    } else {
+                        quote! {
+                            std::option::Option::Some(#source_backtrace.unwrap_or(backtrace))
+                        }
+                    };
+                    quote! {
+                        #ty::#ident {
+                            #backtrace: backtrace,
+                            #source: source,
+                            ..
+                        } => {
+                            use thiserror::private::AsDynError;
+                            #combinator
+                        }
+                    }
+                }
+                (Some(backtrace_field), _) => {
+                    let backtrace = &backtrace_field.member;
+                    let body = if type_is_option(backtrace_field.ty) {
+                        quote!(backtrace.as_ref())
+                    } else {
+                        quote!(std::option::Option::Some(backtrace))
+                    };
+                    quote! {
+                        #ty::#ident {#backtrace: backtrace, ..} => #body,
+                    }
+                }
+                (None, _) => quote! {
+                    #ty::#ident {..} => std::option::Option::None,
+                },
+            }
+        });
+        Some(quote! {
+            fn backtrace(&self) -> std::option::Option<&std::backtrace::Backtrace> {
+                #[allow(deprecated)]
+                match self {
+                    #(#arms)*
+                }
+            }
+        })
+    } else {
+        None
+    };
+
+    let display_impl = if input.has_display() {
+        let use_as_display = if input.variants.iter().any(|v| {
+            v.attrs
+                .display
+                .as_ref()
+                .map_or(false, |display| display.has_bonus_display)
+        }) {
+            Some(quote! {
+                #[allow(unused_imports)]
+                use thiserror::private::{DisplayAsDisplay, PathAsDisplay};
+            })
+        } else {
+            None
+        };
+        let void_deref = if input.variants.is_empty() {
+            Some(quote!(*))
+        } else {
+            None
+        };
+        let arms = input.variants.iter().map(|variant| {
+            let display = match &variant.attrs.display {
+                Some(display) => display.to_token_stream(),
+                None => {
+                    let only_field = match &variant.fields[0].member {
+                        Member::Named(ident) => ident.clone(),
+                        Member::Unnamed(index) => format_ident!("_{}", index),
+                    };
+                    quote!(std::fmt::Display::fmt(#only_field, __formatter))
+                }
+            };
+            let ident = &variant.ident;
+            let pat = fields_pat(&variant.fields);
+            quote! {
+                #ty::#ident #pat => #display
+            }
+        });
+        Some(quote! {
+            impl #impl_generics std::fmt::Display for #ty #ty_generics #where_clause {
+                fn fmt(&self, __formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+                    #use_as_display
+                    #[allow(unused_variables, deprecated)]
+                    match #void_deref self {
+                        #(#arms,)*
+                    }
+                }
+            }
+        })
+    } else {
+        None
+    };
+
+    let from_impls = input.variants.iter().filter_map(|variant| {
+        let from_field = variant.from_field()?;
+        let backtrace_field = variant.backtrace_field();
+        let variant = &variant.ident;
+        let from = from_field.ty;
+        let body = from_initializer(from_field, backtrace_field);
+        Some(quote! {
+            impl #impl_generics std::convert::From<#from> for #ty #ty_generics #where_clause {
+                #[allow(deprecated)]
+                fn from(source: #from) -> Self {
+                    #ty::#variant #body
+                }
+            }
+        })
+    });
+
+    let error_trait = spanned_error_trait(input.original);
+
+    quote! {
+        impl #impl_generics #error_trait for #ty #ty_generics #where_clause {
+            #source_method
+            #backtrace_method
+        }
+        #display_impl
+        #(#from_impls)*
+    }
+}
+
+fn fields_pat(fields: &[Field]) -> TokenStream {
+    let mut members = fields.iter().map(|field| &field.member).peekable();
+    match members.peek() {
+        Some(Member::Named(_)) => quote!({ #(#members),* }),
+        Some(Member::Unnamed(_)) => {
+            let vars = members.map(|member| match member {
+                Member::Unnamed(member) => format_ident!("_{}", member),
+                Member::Named(_) => unreachable!(),
+            });
+            quote!((#(#vars),*))
+        }
+        None => quote!({}),
+    }
+}
+
+fn from_initializer(from_field: &Field, backtrace_field: Option<&Field>) -> TokenStream {
+    let from_member = &from_field.member;
+    let backtrace = backtrace_field.map(|backtrace_field| {
+        let backtrace_member = &backtrace_field.member;
+        if type_is_option(backtrace_field.ty) {
+            quote! {
+                #backtrace_member: std::option::Option::Some(std::backtrace::Backtrace::capture()),
+            }
+        } else {
+            quote! {
+                #backtrace_member: std::backtrace::Backtrace::capture(),
+            }
+        }
+    });
+    quote!({
+        #from_member: source,
+        #backtrace
+    })
+}
+
+fn type_is_option(ty: &Type) -> bool {
+    let path = match ty {
+        Type::Path(ty) => &ty.path,
+        _ => return false,
+    };
+
+    let last = path.segments.last().unwrap();
+    if last.ident != "Option" {
+        return false;
+    }
+
+    match &last.arguments {
+        PathArguments::AngleBracketed(bracketed) => bracketed.args.len() == 1,
+        _ => false,
+    }
+}
+
+fn spanned_error_trait(input: &DeriveInput) -> TokenStream {
+    let vis_span = match &input.vis {
+        Visibility::Public(vis) => Some(vis.pub_token.span()),
+        Visibility::Crate(vis) => Some(vis.crate_token.span()),
+        Visibility::Restricted(vis) => Some(vis.pub_token.span()),
+        Visibility::Inherited => None,
+    };
+    let data_span = match &input.data {
+        Data::Struct(data) => data.struct_token.span(),
+        Data::Enum(data) => data.enum_token.span(),
+        Data::Union(data) => data.union_token.span(),
+    };
+    let first_span = vis_span.unwrap_or(data_span);
+    let last_span = input.ident.span();
+    let path = quote_spanned!(first_span=> std::error::);
+    let error = quote_spanned!(last_span=> Error);
+    quote!(#path #error)
+}
diff --git a/src/fmt.rs b/src/fmt.rs
new file mode 100644
index 0000000..ea1b518
--- /dev/null
+++ b/src/fmt.rs
@@ -0,0 +1,138 @@
+use crate::ast::Field;
+use crate::attr::Display;
+use proc_macro2::TokenTree;
+use quote::{format_ident, quote_spanned};
+use std::collections::HashSet as Set;
+use syn::ext::IdentExt;
+use syn::parse::{ParseStream, Parser};
+use syn::{Ident, Index, LitStr, Member, Result, Token};
+
+impl Display<'_> {
+    // Transform `"error {var}"` to `"error {}", var`.
+    pub fn expand_shorthand(&mut self, fields: &[Field]) {
+        let raw_args = self.args.clone();
+        let mut named_args = explicit_named_args.parse2(raw_args).unwrap();
+        let fields: Set<Member> = fields.iter().map(|f| f.member.clone()).collect();
+
+        let span = self.fmt.span();
+        let fmt = self.fmt.value();
+        let mut read = fmt.as_str();
+        let mut out = String::new();
+        let mut args = self.args.clone();
+        let mut has_bonus_display = false;
+
+        let mut has_trailing_comma = false;
+        if let Some(TokenTree::Punct(punct)) = args.clone().into_iter().last() {
+            if punct.as_char() == ',' {
+                has_trailing_comma = true;
+            }
+        }
+
+        while let Some(brace) = read.find('{') {
+            out += &read[..brace + 1];
+            read = &read[brace + 1..];
+            if read.starts_with('{') {
+                out.push('{');
+                read = &read[1..];
+                continue;
+            }
+            let next = match read.chars().next() {
+                Some(next) => next,
+                None => return,
+            };
+            let member = match next {
+                '0'..='9' => {
+                    let int = take_int(&mut read);
+                    let member = match int.parse::<u32>() {
+                        Ok(index) => Member::Unnamed(Index { index, span }),
+                        Err(_) => return,
+                    };
+                    if !fields.contains(&member) {
+                        out += &int;
+                        continue;
+                    }
+                    member
+                }
+                'a'..='z' | 'A'..='Z' | '_' => {
+                    let ident = take_ident(&mut read);
+                    Member::Named(Ident::new(&ident, span))
+                }
+                _ => continue,
+            };
+            let local = match &member {
+                Member::Unnamed(index) => format_ident!("_{}", index),
+                Member::Named(ident) => ident.clone(),
+            };
+            let mut formatvar = local.clone();
+            if formatvar.to_string().starts_with('_') {
+                // Work around leading underscore being rejected by 1.40 and
+                // older compilers. https://github.com/rust-lang/rust/pull/66847
+                formatvar = format_ident!("field_{}", formatvar);
+            }
+            out += &formatvar.to_string();
+            if !named_args.insert(formatvar.clone()) {
+                // Already specified in the format argument list.
+                continue;
+            }
+            if !has_trailing_comma {
+                args.extend(quote_spanned!(span=> ,));
+            }
+            args.extend(quote_spanned!(span=> #formatvar = #local));
+            if read.starts_with('}') && fields.contains(&member) {
+                has_bonus_display = true;
+                args.extend(quote_spanned!(span=> .as_display()));
+            }
+            has_trailing_comma = false;
+        }
+
+        out += read;
+        self.fmt = LitStr::new(&out, self.fmt.span());
+        self.args = args;
+        self.has_bonus_display = has_bonus_display;
+    }
+}
+
+fn explicit_named_args(input: ParseStream) -> Result<Set<Ident>> {
+    let mut named_args = Set::new();
+
+    while !input.is_empty() {
+        if input.peek(Token![,]) && input.peek2(Ident::peek_any) && input.peek3(Token![=]) {
+            input.parse::<Token![,]>()?;
+            let ident: Ident = input.parse()?;
+            input.parse::<Token![=]>()?;
+            named_args.insert(ident);
+        } else {
+            input.parse::<TokenTree>()?;
+        }
+    }
+
+    Ok(named_args)
+}
+
+fn take_int(read: &mut &str) -> String {
+    let mut int = String::new();
+    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
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..95b9e39
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,21 @@
+#![allow(clippy::block_in_if_condition_stmt, clippy::range_plus_one)]
+
+extern crate proc_macro;
+
+mod ast;
+mod attr;
+mod expand;
+mod fmt;
+mod prop;
+mod valid;
+
+use proc_macro::TokenStream;
+use syn::{parse_macro_input, DeriveInput};
+
+#[proc_macro_derive(Error, attributes(backtrace, error, from, source))]
+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()
+}
diff --git a/src/prop.rs b/src/prop.rs
new file mode 100644
index 0000000..e011848
--- /dev/null
+++ b/src/prop.rs
@@ -0,0 +1,111 @@
+use crate::ast::{Enum, Field, Struct, Variant};
+use syn::{Member, Type};
+
+impl Struct<'_> {
+    pub(crate) fn from_field(&self) -> Option<&Field> {
+        from_field(&self.fields)
+    }
+
+    pub(crate) fn source_field(&self) -> Option<&Field> {
+        source_field(&self.fields)
+    }
+
+    pub(crate) fn backtrace_field(&self) -> Option<&Field> {
+        backtrace_field(&self.fields)
+    }
+}
+
+impl Enum<'_> {
+    pub(crate) fn has_source(&self) -> bool {
+        self.variants
+            .iter()
+            .any(|variant| variant.source_field().is_some() || variant.attrs.transparent.is_some())
+    }
+
+    pub(crate) fn has_backtrace(&self) -> bool {
+        self.variants
+            .iter()
+            .any(|variant| variant.backtrace_field().is_some())
+    }
+
+    pub(crate) fn has_display(&self) -> bool {
+        self.attrs.display.is_some()
+            || self.attrs.transparent.is_some()
+            || self
+                .variants
+                .iter()
+                .any(|variant| variant.attrs.display.is_some())
+            || self
+                .variants
+                .iter()
+                .all(|variant| variant.attrs.transparent.is_some())
+    }
+}
+
+impl Variant<'_> {
+    pub(crate) fn from_field(&self) -> Option<&Field> {
+        from_field(&self.fields)
+    }
+
+    pub(crate) fn source_field(&self) -> Option<&Field> {
+        source_field(&self.fields)
+    }
+
+    pub(crate) fn backtrace_field(&self) -> Option<&Field> {
+        backtrace_field(&self.fields)
+    }
+}
+
+impl Field<'_> {
+    pub(crate) fn is_backtrace(&self) -> bool {
+        type_is_backtrace(self.ty)
+    }
+}
+
+fn from_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
+    for field in fields {
+        if field.attrs.from.is_some() {
+            return Some(&field);
+        }
+    }
+    None
+}
+
+fn source_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
+    for field in fields {
+        if field.attrs.from.is_some() || field.attrs.source.is_some() {
+            return Some(&field);
+        }
+    }
+    for field in fields {
+        match &field.member {
+            Member::Named(ident) if ident == "source" => return Some(&field),
+            _ => {}
+        }
+    }
+    None
+}
+
+fn backtrace_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
+    for field in fields {
+        if field.attrs.backtrace.is_some() {
+            return Some(&field);
+        }
+    }
+    for field in fields {
+        if field.is_backtrace() {
+            return Some(&field);
+        }
+    }
+    None
+}
+
+fn type_is_backtrace(ty: &Type) -> bool {
+    let path = match ty {
+        Type::Path(ty) => &ty.path,
+        _ => return false,
+    };
+
+    let last = path.segments.last().unwrap();
+    last.ident == "Backtrace" && last.arguments.is_empty()
+}
diff --git a/src/valid.rs b/src/valid.rs
new file mode 100644
index 0000000..0d5ba36
--- /dev/null
+++ b/src/valid.rs
@@ -0,0 +1,226 @@
+use crate::ast::{Enum, Field, Input, Struct, Variant};
+use crate::attr::Attrs;
+use quote::ToTokens;
+use std::collections::BTreeSet as Set;
+use syn::{Error, GenericArgument, Member, PathArguments, Result, Type};
+
+impl Input<'_> {
+    pub(crate) fn validate(&self) -> Result<()> {
+        match self {
+            Input::Struct(input) => input.validate(),
+            Input::Enum(input) => input.validate(),
+        }
+    }
+}
+
+impl Struct<'_> {
+    fn validate(&self) -> Result<()> {
+        check_non_field_attrs(&self.attrs)?;
+        if let Some(transparent) = self.attrs.transparent {
+            if self.fields.len() != 1 {
+                return Err(Error::new_spanned(
+                    transparent,
+                    "#[error(transparent)] requires exactly one field",
+                ));
+            }
+            if let Some(source) = self.fields.iter().filter_map(|f| f.attrs.source).next() {
+                return Err(Error::new_spanned(
+                    source,
+                    "transparent error struct can't contain #[source]",
+                ));
+            }
+        }
+        check_field_attrs(&self.fields)?;
+        for field in &self.fields {
+            field.validate()?;
+        }
+        Ok(())
+    }
+}
+
+impl Enum<'_> {
+    fn validate(&self) -> Result<()> {
+        check_non_field_attrs(&self.attrs)?;
+        let has_display = self.has_display();
+        for variant in &self.variants {
+            variant.validate()?;
+            if has_display && variant.attrs.display.is_none() && variant.attrs.transparent.is_none()
+            {
+                return Err(Error::new_spanned(
+                    variant.original,
+                    "missing #[error(\"...\")] display attribute",
+                ));
+            }
+        }
+        let mut from_types = Set::new();
+        for variant in &self.variants {
+            if let Some(from_field) = variant.from_field() {
+                let repr = from_field.ty.to_token_stream().to_string();
+                if !from_types.insert(repr) {
+                    return Err(Error::new_spanned(
+                        from_field.original,
+                        "cannot derive From because another variant has the same source type",
+                    ));
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
+impl Variant<'_> {
+    fn validate(&self) -> Result<()> {
+        check_non_field_attrs(&self.attrs)?;
+        if self.attrs.transparent.is_some() {
+            if self.fields.len() != 1 {
+                return Err(Error::new_spanned(
+                    self.original,
+                    "#[error(transparent)] requires exactly one field",
+                ));
+            }
+            if let Some(source) = self.fields.iter().filter_map(|f| f.attrs.source).next() {
+                return Err(Error::new_spanned(
+                    source,
+                    "transparent variant can't contain #[source]",
+                ));
+            }
+        }
+        check_field_attrs(&self.fields)?;
+        for field in &self.fields {
+            field.validate()?;
+        }
+        Ok(())
+    }
+}
+
+impl Field<'_> {
+    fn validate(&self) -> Result<()> {
+        if let Some(display) = &self.attrs.display {
+            return Err(Error::new_spanned(
+                display.original,
+                "not expected here; the #[error(...)] attribute belongs on top of a struct or an enum variant",
+            ));
+        }
+        Ok(())
+    }
+}
+
+fn check_non_field_attrs(attrs: &Attrs) -> Result<()> {
+    if let Some(from) = &attrs.from {
+        return Err(Error::new_spanned(
+            from,
+            "not expected here; the #[from] attribute belongs on a specific field",
+        ));
+    }
+    if let Some(source) = &attrs.source {
+        return Err(Error::new_spanned(
+            source,
+            "not expected here; the #[source] attribute belongs on a specific field",
+        ));
+    }
+    if let Some(backtrace) = &attrs.backtrace {
+        return Err(Error::new_spanned(
+            backtrace,
+            "not expected here; the #[backtrace] attribute belongs on a specific field",
+        ));
+    }
+    if let Some(display) = &attrs.display {
+        if attrs.transparent.is_some() {
+            return Err(Error::new_spanned(
+                display.original,
+                "cannot have both #[error(transparent)] and a display attribute",
+            ));
+        }
+    }
+    Ok(())
+}
+
+fn check_field_attrs(fields: &[Field]) -> Result<()> {
+    let mut from_field = None;
+    let mut source_field = None;
+    let mut backtrace_field = None;
+    let mut has_backtrace = false;
+    for field in fields {
+        if let Some(from) = field.attrs.from {
+            if from_field.is_some() {
+                return Err(Error::new_spanned(from, "duplicate #[from] attribute"));
+            }
+            from_field = Some(field);
+        }
+        if let Some(source) = field.attrs.source {
+            if source_field.is_some() {
+                return Err(Error::new_spanned(source, "duplicate #[source] attribute"));
+            }
+            source_field = Some(field);
+        }
+        if let Some(backtrace) = field.attrs.backtrace {
+            if backtrace_field.is_some() {
+                return Err(Error::new_spanned(
+                    backtrace,
+                    "duplicate #[backtrace] attribute",
+                ));
+            }
+            backtrace_field = Some(field);
+            has_backtrace = true;
+        }
+        if let Some(transparent) = field.attrs.transparent {
+            return Err(Error::new_spanned(
+                transparent,
+                "#[error(transparent)] needs to go outside the enum or struct, not on an individual field",
+            ));
+        }
+        has_backtrace |= field.is_backtrace();
+    }
+    if let (Some(from_field), Some(source_field)) = (from_field, source_field) {
+        if !same_member(from_field, source_field) {
+            return Err(Error::new_spanned(
+                from_field.attrs.from,
+                "#[from] is only supported on the source field, not any other field",
+            ));
+        }
+    }
+    if let Some(from_field) = from_field {
+        if fields.len() > 1 + has_backtrace as usize {
+            return Err(Error::new_spanned(
+                from_field.attrs.from,
+                "deriving From requires no fields other than source and backtrace",
+            ));
+        }
+    }
+    if let Some(source_field) = source_field.or(from_field) {
+        if contains_non_static_lifetime(source_field) {
+            return Err(Error::new_spanned(
+                source_field.original,
+                "non-static lifetimes are not allowed in the source of an error",
+            ));
+        }
+    }
+    Ok(())
+}
+
+fn same_member(one: &Field, two: &Field) -> bool {
+    match (&one.member, &two.member) {
+        (Member::Named(one), Member::Named(two)) => one == two,
+        (Member::Unnamed(one), Member::Unnamed(two)) => one.index == two.index,
+        _ => unreachable!(),
+    }
+}
+
+fn contains_non_static_lifetime(field: &Field) -> bool {
+    let ty = match field.ty {
+        Type::Path(ty) => ty,
+        _ => return false, // maybe implement later if there are common other cases
+    };
+    let bracketed = match &ty.path.segments.last().unwrap().arguments {
+        PathArguments::AngleBracketed(bracketed) => bracketed,
+        _ => return false,
+    };
+    for arg in &bracketed.args {
+        if let GenericArgument::Lifetime(lifetime) = arg {
+            if lifetime.ident != "static" {
+                return true;
+            }
+        }
+    }
+    false
+}