// SPDX-License-Identifier: Apache-2.0 OR MIT

use proc_macro2::{Delimiter, Group, Span, TokenStream};
use quote::{format_ident, quote, quote_spanned, ToTokens};
use syn::{
    parse_quote, punctuated::Punctuated, token, visit_mut::VisitMut, Attribute, Error, Field,
    Fields, FieldsNamed, FieldsUnnamed, Generics, Ident, Index, Item, Lifetime, LifetimeParam,
    Meta, Result, Token, Type, Variant, Visibility, WhereClause,
};

use super::{
    args::{parse_args, Args, ProjReplace, UnpinImpl},
    PIN,
};
use crate::utils::{
    determine_lifetime_name, determine_visibility, insert_lifetime_and_bound, ReplaceReceiver,
    SliceExt, Variants,
};

pub(super) fn parse_derive(input: TokenStream) -> Result<TokenStream> {
    let mut input: Item = syn::parse2(input)?;

    let mut cx;
    let mut generate = GenerateTokens::default();

    match &mut input {
        Item::Struct(input) => {
            let ident = &input.ident;
            let ty_generics = input.generics.split_for_impl().1;
            let self_ty = parse_quote!(#ident #ty_generics);
            let mut visitor = ReplaceReceiver(&self_ty);
            visitor.visit_item_struct_mut(input);
            cx = Context::new(&input.attrs, &input.vis, &input.ident, &mut input.generics, Struct)?;
            parse_struct(&mut cx, &input.fields, &mut generate)?;
        }
        Item::Enum(input) => {
            let ident = &input.ident;
            let ty_generics = input.generics.split_for_impl().1;
            let self_ty = parse_quote!(#ident #ty_generics);
            let mut visitor = ReplaceReceiver(&self_ty);
            visitor.visit_item_enum_mut(input);
            cx = Context::new(&input.attrs, &input.vis, &input.ident, &mut input.generics, Enum)?;
            parse_enum(&mut cx, input.brace_token, &input.variants, &mut generate)?;
        }
        _ => bail!(input, "#[pin_project] attribute may only be used on structs or enums"),
    }

    Ok(generate.into_tokens(&cx))
}

#[derive(Default)]
struct GenerateTokens {
    exposed: TokenStream,
    scoped: TokenStream,
}

impl GenerateTokens {
    fn extend(&mut self, expose: bool, tokens: TokenStream) {
        if expose {
            self.exposed.extend(tokens);
        } else {
            self.scoped.extend(tokens);
        }
    }

    fn into_tokens(self, cx: &Context<'_>) -> TokenStream {
        let mut tokens = self.exposed;
        let scoped = self.scoped;

        let unpin_impl = make_unpin_impl(cx);
        let drop_impl = make_drop_impl(cx);
        let allowed_lints = global_allowed_lints();

        tokens.extend(quote! {
            // All items except projected types are generated inside a `const` scope.
            // This makes it impossible for user code to refer to these types.
            // However, this prevents Rustdoc from displaying docs for any
            // of our types. In particular, users cannot see the
            // automatically generated `Unpin` impl for the '__UnpinStruct' types
            //
            // Previously, we provided a flag to correctly document the
            // automatically generated `Unpin` impl by using def-site hygiene,
            // but it is now removed.
            //
            // Refs:
            // - https://github.com/rust-lang/rust/issues/63281
            // - https://github.com/taiki-e/pin-project/pull/53#issuecomment-525906867
            // - https://github.com/taiki-e/pin-project/pull/70
            #[allow(
                unused_qualifications,
                #allowed_lints
                clippy::needless_lifetimes,
                clippy::semicolon_if_nothing_returned,
                clippy::use_self,
                clippy::used_underscore_binding
            )]
            const _: () = {
                #[allow(unused_extern_crates)]
                extern crate pin_project as _pin_project;
                #scoped
                #unpin_impl
                #drop_impl
            };
        });
        tokens
    }
}

/// Returns attributes that should be applied to all generated code.
fn global_allowed_lints() -> TokenStream {
    quote! {
        deprecated,
        explicit_outlives_requirements, // https://github.com/rust-lang/rust/issues/60993
        single_use_lifetimes, // https://github.com/rust-lang/rust/issues/55058
        unreachable_pub, // This lint warns `pub` field in private struct.
        unused_tuple_struct_fields,
        // This lint warns of `clippy::*` generated by external macros.
        // We allow this lint for compatibility with older compilers.
        clippy::unknown_clippy_lints,
        clippy::absolute_paths,
        clippy::min_ident_chars,
        clippy::pattern_type_mismatch,
        clippy::pub_with_shorthand,
        clippy::redundant_pub_crate, // This lint warns `pub(crate)` field in private struct.
        clippy::single_char_lifetime_names,
        clippy::type_repetition_in_bounds, // https://github.com/rust-lang/rust-clippy/issues/4326
    }
}

/// Returns attributes used on projected types.
fn proj_allowed_lints(cx: &Context<'_>) -> (TokenStream, TokenStream, TokenStream) {
    let global_allowed_lints = global_allowed_lints();
    let proj_mut_allowed_lints = if cx.project { Some(&global_allowed_lints) } else { None };
    let proj_mut = quote! {
        #[allow(
            dead_code, // This lint warns unused fields/variants.
            #proj_mut_allowed_lints
            clippy::missing_docs_in_private_items,
            clippy::mut_mut // This lint warns `&mut &mut <ty>`.
        )]
    };
    let proj_ref_allowed_lints = if cx.project_ref { Some(&global_allowed_lints) } else { None };
    let proj_ref = quote! {
        #[allow(
            dead_code, // This lint warns unused fields/variants.
            #proj_ref_allowed_lints
            clippy::missing_docs_in_private_items,
            clippy::ref_option_ref // This lint warns `&Option<&<ty>>`.
        )]
    };
    let proj_own_allowed_lints =
        if cx.project_replace.ident().is_some() { Some(&global_allowed_lints) } else { None };
    let variant_size_differences = if cx.kind == Enum {
        Some(quote! { variant_size_differences, clippy::large_enum_variant, })
    } else {
        None
    };
    let proj_own = quote! {
        #[allow(
            dead_code, // This lint warns unused fields/variants.
            #proj_own_allowed_lints
            #variant_size_differences
            clippy::missing_docs_in_private_items
        )]
    };
    (proj_mut, proj_ref, proj_own)
}

struct Context<'a> {
    /// The original type.
    orig: OriginalType<'a>,
    /// The projected types.
    proj: ProjectedType,
    /// Types of the pinned fields.
    pinned_fields: Vec<&'a Type>,
    /// Kind of the original type: struct or enum
    kind: TypeKind,

    /// `PinnedDrop` argument.
    pinned_drop: Option<Span>,
    /// `UnsafeUnpin` or `!Unpin` argument.
    unpin_impl: UnpinImpl,
    /// `project` argument.
    project: bool,
    /// `project_ref` argument.
    project_ref: bool,
    /// `project_replace [= <ident>]` argument.
    project_replace: ProjReplace,
}

impl<'a> Context<'a> {
    fn new(
        attrs: &'a [Attribute],
        vis: &'a Visibility,
        ident: &'a Ident,
        generics: &'a mut Generics,
        kind: TypeKind,
    ) -> Result<Self> {
        let Args { pinned_drop, unpin_impl, project, project_ref, project_replace } =
            parse_args(attrs)?;

        if let Some(name) = [project.as_ref(), project_ref.as_ref(), project_replace.ident()]
            .iter()
            .filter_map(Option::as_ref)
            .find(|name| **name == ident)
        {
            bail!(name, "name `{}` is the same as the original type name", name);
        }

        let mut lifetime_name = String::from("'pin");
        determine_lifetime_name(&mut lifetime_name, generics);
        let lifetime = Lifetime::new(&lifetime_name, Span::call_site());

        let ty_generics = generics.split_for_impl().1;
        let ty_generics_as_generics = parse_quote!(#ty_generics);
        let mut proj_generics = generics.clone();
        let pred = insert_lifetime_and_bound(
            &mut proj_generics,
            lifetime.clone(),
            &ty_generics_as_generics,
            ident,
        );
        let mut where_clause = generics.make_where_clause().clone();
        where_clause.predicates.push(pred);

        let own_ident = project_replace
            .ident()
            .cloned()
            .unwrap_or_else(|| format_ident!("__{}ProjectionOwned", ident));

        Ok(Self {
            kind,
            pinned_drop,
            unpin_impl,
            project: project.is_some(),
            project_ref: project_ref.is_some(),
            project_replace,
            proj: ProjectedType {
                vis: determine_visibility(vis),
                mut_ident: project.unwrap_or_else(|| format_ident!("__{}Projection", ident)),
                ref_ident: project_ref.unwrap_or_else(|| format_ident!("__{}ProjectionRef", ident)),
                own_ident,
                lifetime,
                generics: proj_generics,
                where_clause,
            },
            orig: OriginalType { attrs, vis, ident, generics },
            pinned_fields: vec![],
        })
    }
}

#[derive(Copy, Clone, PartialEq)]
enum TypeKind {
    Enum,
    Struct,
}
use self::TypeKind::{Enum, Struct};

struct OriginalType<'a> {
    /// Attributes of the original type.
    attrs: &'a [Attribute],
    /// Visibility of the original type.
    vis: &'a Visibility,
    /// Name of the original type.
    ident: &'a Ident,
    /// Generics of the original type.
    generics: &'a Generics,
}

struct ProjectedType {
    /// Visibility of the projected types.
    vis: Visibility,
    /// Name of the projected type returned by `project` method.
    mut_ident: Ident,
    /// Name of the projected type returned by `project_ref` method.
    ref_ident: Ident,
    /// Name of the projected type returned by `project_replace` method.
    own_ident: Ident,
    /// Lifetime on the generated projected types.
    lifetime: Lifetime,
    /// Generics of the projected types.
    generics: Generics,
    /// `where` clause of the projected types. This has an additional
    /// bound generated by `insert_lifetime_and_bound`
    where_clause: WhereClause,
}

struct ProjectedVariants {
    proj_variants: TokenStream,
    proj_ref_variants: TokenStream,
    proj_own_variants: TokenStream,
    proj_arms: TokenStream,
    proj_ref_arms: TokenStream,
    proj_own_arms: TokenStream,
}

#[derive(Default)]
struct ProjectedFields {
    proj_pat: TokenStream,
    proj_body: TokenStream,
    proj_own_body: TokenStream,
    proj_fields: TokenStream,
    proj_ref_fields: TokenStream,
    proj_own_fields: TokenStream,
}

fn validate_struct(ident: &Ident, fields: &Fields) -> Result<()> {
    if fields.is_empty() {
        let msg = "#[pin_project] attribute may not be used on structs with zero fields";
        if let Fields::Unit = fields {
            bail!(ident, msg)
        }
        bail!(fields, msg)
    }
    Ok(())
}

fn validate_enum(brace_token: token::Brace, variants: &Variants) -> Result<()> {
    if variants.is_empty() {
        return Err(Error::new(
            brace_token.span.join(),
            "#[pin_project] attribute may not be used on enums without variants",
        ));
    }
    let has_field = variants.iter().try_fold(false, |has_field, v| {
        if let Some((_, e)) = &v.discriminant {
            bail!(e, "#[pin_project] attribute may not be used on enums with discriminants");
        } else if let Some(attr) = v.attrs.find(PIN) {
            bail!(attr, "#[pin] attribute may only be used on fields of structs or variants");
        } else if v.fields.is_empty() {
            Ok(has_field)
        } else {
            Ok(true)
        }
    })?;
    if has_field {
        Ok(())
    } else {
        bail!(variants, "#[pin_project] attribute may not be used on enums with zero fields");
    }
}

fn parse_struct<'a>(
    cx: &mut Context<'a>,
    fields: &'a Fields,
    generate: &mut GenerateTokens,
) -> Result<()> {
    // Do this first for a better error message.
    let packed_check = ensure_not_packed(&cx.orig, Some(fields))?;

    validate_struct(cx.orig.ident, fields)?;

    let ProjectedFields {
        proj_pat,
        proj_body,
        proj_fields,
        proj_ref_fields,
        proj_own_fields,
        proj_own_body,
    } = match fields {
        Fields::Named(_) => visit_fields(cx, None, fields, Delimiter::Brace)?,
        Fields::Unnamed(_) => visit_fields(cx, None, fields, Delimiter::Parenthesis)?,
        Fields::Unit => unreachable!(),
    };

    let proj_ident = &cx.proj.mut_ident;
    let proj_ref_ident = &cx.proj.ref_ident;
    let proj_own_ident = &cx.proj.own_ident;
    let vis = &cx.proj.vis;
    let mut orig_generics = cx.orig.generics.clone();
    let orig_where_clause = orig_generics.where_clause.take();
    let proj_generics = &cx.proj.generics;
    let proj_where_clause = &cx.proj.where_clause;

    // For tuple structs, we need to generate `(T1, T2) where Foo: Bar`
    // For non-tuple structs, we need to generate `where Foo: Bar { field1: T }`
    let (where_clause_fields, where_clause_ref_fields, where_clause_own_fields) = match fields {
        Fields::Named(_) => (
            quote!(#proj_where_clause #proj_fields),
            quote!(#proj_where_clause #proj_ref_fields),
            quote!(#orig_where_clause #proj_own_fields),
        ),
        Fields::Unnamed(_) => (
            quote!(#proj_fields #proj_where_clause;),
            quote!(#proj_ref_fields #proj_where_clause;),
            quote!(#proj_own_fields #orig_where_clause;),
        ),
        Fields::Unit => unreachable!(),
    };

    let (proj_attrs, proj_ref_attrs, proj_own_attrs) = proj_allowed_lints(cx);
    generate.extend(cx.project, quote! {
        #proj_attrs
        #vis struct #proj_ident #proj_generics #where_clause_fields
    });
    generate.extend(cx.project_ref, quote! {
        #proj_ref_attrs
        #vis struct #proj_ref_ident #proj_generics #where_clause_ref_fields
    });
    if cx.project_replace.span().is_some() {
        generate.extend(cx.project_replace.ident().is_some(), quote! {
            #proj_own_attrs
            #vis struct #proj_own_ident #orig_generics #where_clause_own_fields
        });
    }

    let proj_mut_body = quote! {
        let Self #proj_pat = self.get_unchecked_mut();
        #proj_ident #proj_body
    };
    let proj_ref_body = quote! {
        let Self #proj_pat = self.get_ref();
        #proj_ref_ident #proj_body
    };
    let proj_own_body = quote! {
        let Self #proj_pat = &mut *__self_ptr;
        #proj_own_body
    };
    generate.extend(false, make_proj_impl(cx, &proj_mut_body, &proj_ref_body, &proj_own_body));

    generate.extend(false, packed_check);
    Ok(())
}

fn parse_enum<'a>(
    cx: &mut Context<'a>,
    brace_token: token::Brace,
    variants: &'a Punctuated<Variant, Token![,]>,
    generate: &mut GenerateTokens,
) -> Result<()> {
    if let ProjReplace::Unnamed { span } = &cx.project_replace {
        return Err(Error::new(
            *span,
            "`project_replace` argument requires a value when used on enums",
        ));
    }

    // #[repr(packed)] cannot be apply on enums and will be rejected by rustc.
    // However, we should not rely on the behavior of rustc that rejects this.
    // https://github.com/taiki-e/pin-project/pull/324#discussion_r612388001
    //
    // Do this first for a better error message.
    ensure_not_packed(&cx.orig, None)?;

    validate_enum(brace_token, variants)?;

    let ProjectedVariants {
        proj_variants,
        proj_ref_variants,
        proj_own_variants,
        proj_arms,
        proj_ref_arms,
        proj_own_arms,
    } = visit_variants(cx, variants)?;

    let proj_ident = &cx.proj.mut_ident;
    let proj_ref_ident = &cx.proj.ref_ident;
    let proj_own_ident = &cx.proj.own_ident;
    let vis = &cx.proj.vis;
    let mut orig_generics = cx.orig.generics.clone();
    let orig_where_clause = orig_generics.where_clause.take();
    let proj_generics = &cx.proj.generics;
    let proj_where_clause = &cx.proj.where_clause;

    let (proj_attrs, proj_ref_attrs, proj_own_attrs) = proj_allowed_lints(cx);
    if cx.project {
        generate.extend(true, quote! {
            #proj_attrs
            #vis enum #proj_ident #proj_generics #proj_where_clause {
                #proj_variants
            }
        });
    }
    if cx.project_ref {
        generate.extend(true, quote! {
            #proj_ref_attrs
            #vis enum #proj_ref_ident #proj_generics #proj_where_clause {
                #proj_ref_variants
            }
        });
    }
    if cx.project_replace.ident().is_some() {
        generate.extend(true, quote! {
            #proj_own_attrs
            #vis enum #proj_own_ident #orig_generics #orig_where_clause {
                #proj_own_variants
            }
        });
    }

    let proj_mut_body = quote! {
        match self.get_unchecked_mut() {
            #proj_arms
        }
    };
    let proj_ref_body = quote! {
        match self.get_ref() {
            #proj_ref_arms
        }
    };
    let proj_own_body = quote! {
        match &mut *__self_ptr {
            #proj_own_arms
        }
    };
    generate.extend(false, make_proj_impl(cx, &proj_mut_body, &proj_ref_body, &proj_own_body));

    Ok(())
}

fn visit_variants<'a>(cx: &mut Context<'a>, variants: &'a Variants) -> Result<ProjectedVariants> {
    let mut proj_variants = TokenStream::new();
    let mut proj_ref_variants = TokenStream::new();
    let mut proj_own_variants = TokenStream::new();
    let mut proj_arms = TokenStream::new();
    let mut proj_ref_arms = TokenStream::new();
    let mut proj_own_arms = TokenStream::new();

    for Variant { ident, fields, .. } in variants {
        let ProjectedFields {
            proj_pat,
            proj_body,
            proj_fields,
            proj_ref_fields,
            proj_own_fields,
            proj_own_body,
        } = match fields {
            Fields::Named(_) => visit_fields(cx, Some(ident), fields, Delimiter::Brace)?,
            Fields::Unnamed(_) => visit_fields(cx, Some(ident), fields, Delimiter::Parenthesis)?,
            Fields::Unit => ProjectedFields {
                proj_own_body: proj_own_body(cx, Some(ident), None, &[]),
                ..Default::default()
            },
        };

        let proj_ident = &cx.proj.mut_ident;
        let proj_ref_ident = &cx.proj.ref_ident;
        proj_variants.extend(quote! {
            #ident #proj_fields,
        });
        proj_ref_variants.extend(quote! {
            #ident #proj_ref_fields,
        });
        proj_own_variants.extend(quote! {
            #ident #proj_own_fields,
        });
        proj_arms.extend(quote! {
            Self::#ident #proj_pat => #proj_ident::#ident #proj_body,
        });
        proj_ref_arms.extend(quote! {
            Self::#ident #proj_pat => #proj_ref_ident::#ident #proj_body,
        });
        proj_own_arms.extend(quote! {
            Self::#ident #proj_pat => { #proj_own_body }
        });
    }

    Ok(ProjectedVariants {
        proj_variants,
        proj_ref_variants,
        proj_own_variants,
        proj_arms,
        proj_ref_arms,
        proj_own_arms,
    })
}

fn visit_fields<'a>(
    cx: &mut Context<'a>,
    variant_ident: Option<&Ident>,
    fields: &'a Fields,
    delim: Delimiter,
) -> Result<ProjectedFields> {
    fn surround(delim: Delimiter, tokens: TokenStream) -> TokenStream {
        Group::new(delim, tokens).into_token_stream()
    }

    let mut proj_pat = TokenStream::new();
    let mut proj_body = TokenStream::new();
    let mut proj_fields = TokenStream::new();
    let mut proj_ref_fields = TokenStream::new();
    let mut proj_own_fields = TokenStream::new();
    let mut proj_move = TokenStream::new();
    let mut pinned_bindings = Vec::with_capacity(fields.len());

    for (i, Field { attrs, vis, ident, colon_token, ty, .. }) in fields.iter().enumerate() {
        let binding = ident.clone().unwrap_or_else(|| format_ident!("_{}", i));
        proj_pat.extend(quote!(#binding,));
        let lifetime = &cx.proj.lifetime;
        if attrs.position_exact(PIN)?.is_some() {
            proj_fields.extend(quote! {
                #vis #ident #colon_token ::pin_project::__private::Pin<&#lifetime mut (#ty)>,
            });
            proj_ref_fields.extend(quote! {
                #vis #ident #colon_token ::pin_project::__private::Pin<&#lifetime (#ty)>,
            });
            proj_own_fields.extend(quote! {
                #vis #ident #colon_token ::pin_project::__private::PhantomData<#ty>,
            });
            proj_body.extend(quote! {
                #ident #colon_token _pin_project::__private::Pin::new_unchecked(#binding),
            });
            proj_move.extend(quote! {
                #ident #colon_token _pin_project::__private::PhantomData,
            });

            cx.pinned_fields.push(ty);
            pinned_bindings.push(binding);
        } else {
            proj_fields.extend(quote! {
                #vis #ident #colon_token &#lifetime mut (#ty),
            });
            proj_ref_fields.extend(quote! {
                #vis #ident #colon_token &#lifetime (#ty),
            });
            proj_own_fields.extend(quote! {
                #vis #ident #colon_token #ty,
            });
            proj_body.extend(quote! {
                #binding,
            });
            proj_move.extend(quote! {
                #ident #colon_token _pin_project::__private::ptr::read(#binding),
            });
        }
    }

    let proj_pat = surround(delim, proj_pat);
    let proj_body = surround(delim, proj_body);
    let proj_fields = surround(delim, proj_fields);
    let proj_ref_fields = surround(delim, proj_ref_fields);
    let proj_own_fields = surround(delim, proj_own_fields);

    let proj_move = Group::new(delim, proj_move);
    let proj_own_body = proj_own_body(cx, variant_ident, Some(&proj_move), &pinned_bindings);

    Ok(ProjectedFields {
        proj_pat,
        proj_body,
        proj_own_body,
        proj_fields,
        proj_ref_fields,
        proj_own_fields,
    })
}

/// Generates the processing that `project_replace` does for the struct or each variant.
///
/// Note: `pinned_fields` must be in declaration order.
fn proj_own_body(
    cx: &Context<'_>,
    variant_ident: Option<&Ident>,
    proj_move: Option<&Group>,
    pinned_fields: &[Ident],
) -> TokenStream {
    let ident = &cx.proj.own_ident;
    let proj_own = match variant_ident {
        Some(variant_ident) => quote!(#ident::#variant_ident),
        None => quote!(#ident),
    };

    // The fields of the struct and the active enum variant are dropped
    // in declaration order.
    // Refs: https://doc.rust-lang.org/reference/destructors.html
    let pinned_fields = pinned_fields.iter().rev();

    quote! {
        // First, extract all the unpinned fields.
        let __result = #proj_own #proj_move;

        // Now create guards to drop all the pinned fields.
        //
        // Due to a compiler bug (https://github.com/rust-lang/rust/issues/47949)
        // this must be in its own scope, or else `__result` will not be dropped
        // if any of the destructors panic.
        {
            #(
                let __guard = _pin_project::__private::UnsafeDropInPlaceGuard::new(#pinned_fields);
            )*
        }

        // Finally, return the result.
        __result
    }
}

/// Creates `Unpin` implementation for the original type.
///
/// The kind of `Unpin` impl generated depends on `unpin_impl` field:
/// - `UnpinImpl::Unsafe` - Implements `Unpin` via `UnsafeUnpin` impl.
/// - `UnpinImpl::Negative` - Generates `Unpin` impl with bounds that will never be true.
/// - `UnpinImpl::Default` - Generates `Unpin` impl that requires `Unpin` for all pinned fields.
fn make_unpin_impl(cx: &Context<'_>) -> TokenStream {
    match cx.unpin_impl {
        UnpinImpl::Unsafe(span) => {
            let mut proj_generics = cx.proj.generics.clone();
            let orig_ident = cx.orig.ident;
            let lifetime = &cx.proj.lifetime;

            // Make the error message highlight `UnsafeUnpin` argument.
            proj_generics.make_where_clause().predicates.push(parse_quote_spanned! { span =>
                _pin_project::__private::PinnedFieldsOf<
                    _pin_project::__private::Wrapper<#lifetime, Self>
                >: _pin_project::UnsafeUnpin
            });

            let (impl_generics, _, where_clause) = proj_generics.split_for_impl();
            let ty_generics = cx.orig.generics.split_for_impl().1;

            quote_spanned! { span =>
                impl #impl_generics _pin_project::__private::Unpin for #orig_ident #ty_generics
                #where_clause
                {
                }
            }
        }
        UnpinImpl::Negative(span) => {
            let mut proj_generics = cx.proj.generics.clone();
            let orig_ident = cx.orig.ident;
            let lifetime = &cx.proj.lifetime;

            // TODO: Using `<unsized type>: Sized` here allow emulating real negative_impls...
            // https://github.com/taiki-e/pin-project/issues/340#issuecomment-2428002670
            proj_generics.make_where_clause().predicates.push(parse_quote! {
                _pin_project::__private::PinnedFieldsOf<_pin_project::__private::Wrapper<
                    #lifetime, _pin_project::__private::PhantomPinned
                >>: _pin_project::__private::Unpin
            });

            let (impl_generics, _, where_clause) = proj_generics.split_for_impl();
            let ty_generics = cx.orig.generics.split_for_impl().1;

            // For interoperability with `forbid(unsafe_code)`, `unsafe` token should be
            // call-site span.
            let unsafety = <Token![unsafe]>::default();
            quote_spanned! { span =>
                #[doc(hidden)]
                impl #impl_generics _pin_project::__private::Unpin for #orig_ident #ty_generics
                #where_clause
                {
                }

                // Generate a dummy impl of `UnsafeUnpin`, to ensure that the user cannot implement it.
                //
                // To ensure that users don't accidentally write a non-functional `UnsafeUnpin`
                // impls, we emit one ourselves. If the user ends up writing an `UnsafeUnpin`
                // impl, they'll get a "conflicting implementations of trait" error when
                // coherence checks are run.
                #[doc(hidden)]
                #unsafety impl #impl_generics _pin_project::UnsafeUnpin for #orig_ident #ty_generics
                #where_clause
                {
                }
            }
        }
        UnpinImpl::Default => {
            let mut impl_where_clause = cx.orig.generics.where_clause.clone().unwrap();

            // Generate a field in our new struct for every
            // pinned field in the original type.
            let fields = cx.pinned_fields.iter().enumerate().map(|(i, ty)| {
                let field_ident = format_ident!("__field{}", i);
                quote!(#field_ident: #ty)
            });

            // We could try to determine the subset of type parameters
            // and lifetimes that are actually used by the pinned fields
            // (as opposed to those only used by unpinned fields).
            // However, this would be tricky and error-prone, since
            // it's possible for users to create types that would alias
            // with generic parameters (e.g. 'struct T').
            //
            // Instead, we generate a use of every single type parameter
            // and lifetime used in the original struct. For type parameters,
            // we generate code like this:
            //
            // ```
            // struct AlwaysUnpin<T: ?Sized>(PhantomData<T>) {}
            // impl<T: ?Sized> Unpin for AlwaysUnpin<T> {}
            //
            // ...
            // _field: AlwaysUnpin<(A, B, C)>
            // ```
            //
            // This ensures that any unused type parameters
            // don't end up with `Unpin` bounds.
            let lifetime_fields = cx.orig.generics.lifetimes().enumerate().map(
                |(i, LifetimeParam { lifetime, .. })| {
                    let field_ident = format_ident!("__lifetime{}", i);
                    quote!(#field_ident: &#lifetime ())
                },
            );

            let orig_ident = cx.orig.ident;
            let struct_ident = format_ident!("__{}", orig_ident);
            let vis = cx.orig.vis;
            let lifetime = &cx.proj.lifetime;
            let type_params = cx.orig.generics.type_params().map(|t| &t.ident);
            let proj_generics = &cx.proj.generics;
            let (impl_generics, proj_ty_generics, _) = proj_generics.split_for_impl();
            let (_, ty_generics, ty_where_clause) = cx.orig.generics.split_for_impl();

            impl_where_clause.predicates.push(parse_quote! {
                _pin_project::__private::PinnedFieldsOf<#struct_ident #proj_ty_generics>:
                    _pin_project::__private::Unpin
            });

            quote! {
                // This needs to have the same visibility as the original type,
                // due to the limitations of the 'public in private' error.
                //
                // Our goal is to implement the public trait `Unpin` for
                // a potentially public user type. Because of this, rust
                // requires that any types mentioned in the where clause of
                // our `Unpin` impl also be public. This means that our generated
                // `__UnpinStruct` type must also be public.
                // However, we ensure that the user can never actually reference
                // this 'public' type by creating this type in the inside of `const`.
                #[allow(missing_debug_implementations, unnameable_types)]
                #vis struct #struct_ident #proj_generics #ty_where_clause {
                    __pin_project_use_generics: _pin_project::__private::AlwaysUnpin<
                        #lifetime, (#(_pin_project::__private::PhantomData<#type_params>),*)
                    >,

                    #(#fields,)*
                    #(#lifetime_fields,)*
                }

                impl #impl_generics _pin_project::__private::Unpin for #orig_ident #ty_generics
                #impl_where_clause
                {
                }

                // Generate a dummy impl of `UnsafeUnpin`, to ensure that the user cannot implement it.
                //
                // To ensure that users don't accidentally write a non-functional `UnsafeUnpin`
                // impls, we emit one ourselves. If the user ends up writing an `UnsafeUnpin`
                // impl, they'll get a "conflicting implementations of trait" error when
                // coherence checks are run.
                #[doc(hidden)]
                unsafe impl #impl_generics _pin_project::UnsafeUnpin for #orig_ident #ty_generics
                #impl_where_clause
                {
                }
            }
        }
    }
}

/// Creates `Drop` implementation for the original type.
///
/// The kind of `Drop` impl generated depends on `pinned_drop` field:
/// - `Some` - implements `Drop` via `PinnedDrop` impl.
/// - `None` - generates code that ensures that `Drop` trait is not implemented,
///            instead of generating `Drop` impl.
fn make_drop_impl(cx: &Context<'_>) -> TokenStream {
    let ident = cx.orig.ident;
    let (impl_generics, ty_generics, where_clause) = cx.orig.generics.split_for_impl();

    if let Some(span) = cx.pinned_drop {
        // For interoperability with `forbid(unsafe_code)`, `unsafe` token should be
        // call-site span.
        let unsafety = <Token![unsafe]>::default();
        quote_spanned! { span =>
            impl #impl_generics _pin_project::__private::Drop for #ident #ty_generics
            #where_clause
            {
                #[allow(clippy::missing_inline_in_public_items)]
                fn drop(&mut self) {
                    #unsafety {
                        // Safety - we're in 'drop', so we know that 'self' will
                        // never move again.
                        let __pinned_self = _pin_project::__private::Pin::new_unchecked(self);
                        // We call `pinned_drop` only once. Since `PinnedDrop::drop`
                        // is an unsafe method and a private API, it is never called again in safe
                        // code *unless the user uses a maliciously crafted macro*.
                        _pin_project::__private::PinnedDrop::drop(__pinned_self);
                    }
                }
            }
        }
    } else {
        // If the user does not provide a `PinnedDrop` impl,
        // we need to ensure that they don't provide a `Drop` impl of their
        // own.
        // Based on https://github.com/upsuper/assert-impl/blob/f503255b292ab0ba8d085b657f4065403cfa46eb/src/lib.rs#L80-L87
        //
        // We create a new identifier for each struct, so that the traits
        // for different types do not conflict with each other.
        //
        // Another approach would be to provide an empty Drop impl,
        // which would conflict with a user-provided Drop impl.
        // However, this would trigger the compiler's special handling
        // of Drop types (e.g. fields cannot be moved out of a Drop type).
        // This approach prevents the creation of needless Drop impls,
        // giving users more flexibility.
        let trait_ident = format_ident!("{}MustNotImplDrop", ident);

        quote! {
            // There are two possible cases:
            // 1. The user type does not implement Drop. In this case,
            // the first blanket impl will not apply to it. This code
            // will compile, as there is only one impl of MustNotImplDrop for the user type
            // 2. The user type does impl Drop. This will make the blanket impl applicable,
            // which will then conflict with the explicit MustNotImplDrop impl below.
            // This will result in a compilation error, which is exactly what we want.
            trait #trait_ident {}
            #[allow(clippy::drop_bounds, drop_bounds)]
            impl<T: _pin_project::__private::Drop> #trait_ident for T {}
            impl #impl_generics #trait_ident for #ident #ty_generics #where_clause {}

            // Generate a dummy impl of `PinnedDrop`, to ensure that the user cannot implement it.
            // Since the user did not pass `PinnedDrop` to `#[pin_project]`, any `PinnedDrop`
            // impl will not actually be called. Unfortunately, we can't detect this situation
            // directly from either the `#[pin_project]` or `#[pinned_drop]` attributes, since
            // we don't know what other attributes/impl may exist.
            //
            // To ensure that users don't accidentally write a non-functional `PinnedDrop`
            // impls, we emit one ourselves. If the user ends up writing a `PinnedDrop` impl,
            // they'll get a "conflicting implementations of trait" error when coherence
            // checks are run.
            #[doc(hidden)]
            impl #impl_generics _pin_project::__private::PinnedDrop for #ident #ty_generics
            #where_clause
            {
                unsafe fn drop(self: _pin_project::__private::Pin<&mut Self>) {}
            }
        }
    }
}

/// Creates an implementation of the projection methods.
///
/// On structs, both the `project` and `project_ref` methods are always generated,
/// and the `project_replace` method is only generated if `ProjReplace::span` is `Some`.
///
/// On enums, only methods that the returned projected type is named will be generated.
fn make_proj_impl(
    cx: &Context<'_>,
    proj_body: &TokenStream,
    proj_ref_body: &TokenStream,
    proj_own_body: &TokenStream,
) -> TokenStream {
    let vis = &cx.proj.vis;
    let lifetime = &cx.proj.lifetime;
    let orig_ident = cx.orig.ident;
    let proj_ident = &cx.proj.mut_ident;
    let proj_ref_ident = &cx.proj.ref_ident;
    let proj_own_ident = &cx.proj.own_ident;

    let orig_ty_generics = cx.orig.generics.split_for_impl().1;
    let proj_ty_generics = cx.proj.generics.split_for_impl().1;
    let (impl_generics, ty_generics, where_clause) = cx.orig.generics.split_for_impl();
    // TODO: For enums and project_replace, dead_code warnings should not be
    // allowed because methods are not generated unless explicitly specified.
    // However, there is currently no good way to allow warnings for generated
    // code, so we allow warnings for all methods for now.
    let allow_dead_code = quote! { #[allow(dead_code)] };

    let mut project = Some(quote! {
        #allow_dead_code
        #[inline]
        #vis fn project<#lifetime>(
            self: _pin_project::__private::Pin<&#lifetime mut Self>,
        ) -> #proj_ident #proj_ty_generics {
            unsafe {
                #proj_body
            }
        }
    });
    let mut project_ref = Some(quote! {
        #allow_dead_code
        #[allow(clippy::missing_const_for_fn)]
        #[inline]
        #vis fn project_ref<#lifetime>(
            self: _pin_project::__private::Pin<&#lifetime Self>,
        ) -> #proj_ref_ident #proj_ty_generics {
            unsafe {
                #proj_ref_body
            }
        }
    });
    let mut project_replace = cx.project_replace.span().map(|span| {
        // It is enough to only set the span of the signature.
        let sig = quote_spanned! { span =>
            #vis fn project_replace(
                self: _pin_project::__private::Pin<&mut Self>,
                __replacement: Self,
            ) -> #proj_own_ident #orig_ty_generics
        };
        quote! {
            #allow_dead_code
            #[inline]
            #sig {
                unsafe {
                    let __self_ptr: *mut Self = self.get_unchecked_mut();

                    // Destructors will run in reverse order, so next create a guard to overwrite
                    // `self` with the replacement value without calling destructors.
                    let __guard = _pin_project::__private::UnsafeOverwriteGuard::new(
                        __self_ptr,
                        __replacement,
                    );

                    #proj_own_body
                }
            }
        }
    });

    if cx.kind == Enum {
        if !cx.project {
            project = None;
        }
        if !cx.project_ref {
            project_ref = None;
        }
        if cx.project_replace.ident().is_none() {
            project_replace = None;
        }
    }

    quote! {
        impl #impl_generics #orig_ident #ty_generics #where_clause {
            #project
            #project_ref
            #project_replace
        }
    }
}

/// Checks that the `[repr(packed)]` attribute is not included.
///
/// This currently does two checks:
/// - Checks the attributes of structs to ensure there is no `[repr(packed)]`.
/// - Generates a function that borrows fields without an unsafe block and
///   forbidding `unaligned_references` lint.
fn ensure_not_packed(orig: &OriginalType<'_>, fields: Option<&Fields>) -> Result<TokenStream> {
    for attr in orig.attrs {
        if let Meta::List(ref list) = attr.meta {
            if list.path.is_ident("repr") {
                for repr in list.parse_args_with(Punctuated::<Meta, Token![,]>::parse_terminated)? {
                    if repr.path().is_ident("packed") {
                        let msg = if fields.is_none() {
                            // #[repr(packed)] cannot be apply on enums and will be rejected by rustc.
                            // However, we should not rely on the behavior of rustc that rejects this.
                            // https://github.com/taiki-e/pin-project/pull/324#discussion_r612388001
                            "#[repr(packed)] attribute should be applied to a struct or union"
                        } else if repr.require_name_value().is_ok() {
                            // #[repr(packed = "")] is not valid format of #[repr(packed)] and will be
                            // rejected by rustc.
                            // However, we should not rely on the behavior of rustc that rejects this.
                            // https://github.com/taiki-e/pin-project/pull/324#discussion_r612388001
                            "#[repr(packed)] attribute should not be name-value pair"
                        } else {
                            "#[pin_project] attribute may not be used on #[repr(packed)] types"
                        };
                        bail!(repr, msg);
                    }
                }
            }
        }
    }

    let fields = match fields {
        Some(fields) => fields,
        None => return Ok(TokenStream::new()),
    };

    // Workaround for https://github.com/taiki-e/pin-project/issues/32
    // Through the tricky use of proc macros, it's possible to bypass
    // the above check for the `repr` attribute.
    // To ensure that it's impossible to use pin projections on a `#[repr(packed)]`
    // struct, we generate code like this:
    //
    // ```
    // #[forbid(unaligned_references)]
    // fn assert_not_repr_packed(val: &MyStruct) {
    //     let _field_1 = &val.field_1;
    //     let _field_2 = &val.field_2;
    //     ...
    //     let _field_n = &val.field_n;
    // }
    // ```
    //
    // Taking a reference to a packed field is UB, and applying
    // `#[forbid(unaligned_references)]` makes sure that doing this is a hard error.
    //
    // If the struct ends up having `#[repr(packed)]` applied somehow,
    // this will generate an (unfriendly) error message. Under all reasonable
    // circumstances, we'll detect the `#[repr(packed)]` attribute, and generate
    // a much nicer error above.
    //
    // There is one exception: If the type of a struct field has an alignment of 1
    // (e.g. u8), it is always safe to take a reference to it, even if the struct
    // is `#[repr(packed)]`. If the struct is composed entirely of types of
    // alignment 1, our generated method will not trigger an error if the
    // struct is `#[repr(packed)]`.
    //
    // Fortunately, this should have no observable consequence - `#[repr(packed)]`
    // is essentially a no-op on such a type. Nevertheless, we include a test
    // to ensure that the compiler doesn't ever try to copy the fields on
    // such a struct when trying to drop it - which is reason we prevent
    // `#[repr(packed)]` in the first place.
    //
    // See also https://github.com/taiki-e/pin-project/pull/34.
    //
    // Note:
    // - Lint-based tricks aren't perfect, but they're much better than nothing:
    //   https://github.com/taiki-e/pin-project-lite/issues/26
    //
    // - Enable both unaligned_references and safe_packed_borrows lints
    //   because unaligned_references lint does not exist in older compilers:
    //   https://github.com/taiki-e/pin-project-lite/pull/55
    //   https://github.com/rust-lang/rust/pull/82525
    let mut field_refs = vec![];
    match fields {
        Fields::Named(FieldsNamed { named, .. }) => {
            for Field { ident, .. } in named {
                field_refs.push(quote!(&this.#ident));
            }
        }
        Fields::Unnamed(FieldsUnnamed { unnamed, .. }) => {
            for (index, _) in unnamed.iter().enumerate() {
                let index = Index::from(index);
                field_refs.push(quote!(&this.#index));
            }
        }
        Fields::Unit => {}
    }

    let (impl_generics, ty_generics, where_clause) = orig.generics.split_for_impl();
    let ident = orig.ident;
    Ok(quote! {
        #[forbid(unaligned_references, safe_packed_borrows)]
        fn __assert_not_repr_packed #impl_generics (this: &#ident #ty_generics) #where_clause {
            #(let _ = #field_refs;)*
        }
    })
}
