base.rs

use crate::expand::{self, AstFragment, Invocation};
use crate::module::DirectoryOwnership;

use rustc_ast::ast::{self, Attribute, Name, NodeId, PatKind};
use rustc_ast::mut_visit::{self, MutVisitor};
use rustc_ast::ptr::P;
use rustc_ast::token;
use rustc_ast::tokenstream::{self, TokenStream, TokenTree};
use rustc_ast::visit::{AssocCtxt, Visitor};
use rustc_attr::{self as attr, Deprecation, HasAttrs, Stability};
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::sync::{self, Lrc};
use rustc_errors::{DiagnosticBuilder, DiagnosticId};
use rustc_parse::{self, parser, MACRO_ARGUMENTS};
use rustc_session::parse::ParseSess;
use rustc_span::edition::Edition;
use rustc_span::hygiene::{AstPass, ExpnData, ExpnId, ExpnKind};
use rustc_span::source_map::SourceMap;
use rustc_span::symbol::{kw, sym, Ident, Symbol};
use rustc_span::{FileName, MultiSpan, Span, DUMMY_SP};
use smallvec::{smallvec, SmallVec};

use std::default::Default;
use std::iter;
use std::path::PathBuf;
use std::rc::Rc;

crate use rustc_span::hygiene::MacroKind;

#[derive(Debug, Clone)]
pub enum Annotatable {
    Item(P<ast::Item>),
    TraitItem(P<ast::AssocItem>),
    ImplItem(P<ast::AssocItem>),
    ForeignItem(P<ast::ForeignItem>),
    Stmt(P<ast::Stmt>),
    Expr(P<ast::Expr>),
    Arm(ast::Arm),
    Field(ast::Field),
    FieldPat(ast::FieldPat),
    GenericParam(ast::GenericParam),
    Param(ast::Param),
    StructField(ast::StructField),
    Variant(ast::Variant),
}

impl HasAttrs for Annotatable {
    fn attrs(&self) -> &[Attribute] {
        match *self {
            Annotatable::Item(ref item) => &item.attrs,
            Annotatable::TraitItem(ref trait_item) => &trait_item.attrs,
            Annotatable::ImplItem(ref impl_item) => &impl_item.attrs,
            Annotatable::ForeignItem(ref foreign_item) => &foreign_item.attrs,
            Annotatable::Stmt(ref stmt) => stmt.attrs(),
            Annotatable::Expr(ref expr) => &expr.attrs,
            Annotatable::Arm(ref arm) => &arm.attrs,
            Annotatable::Field(ref field) => &field.attrs,
            Annotatable::FieldPat(ref fp) => &fp.attrs,
            Annotatable::GenericParam(ref gp) => &gp.attrs,
            Annotatable::Param(ref p) => &p.attrs,
            Annotatable::StructField(ref sf) => &sf.attrs,
            Annotatable::Variant(ref v) => &v.attrs(),
        }
    }

    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
        match self {
            Annotatable::Item(item) => item.visit_attrs(f),
            Annotatable::TraitItem(trait_item) => trait_item.visit_attrs(f),
            Annotatable::ImplItem(impl_item) => impl_item.visit_attrs(f),
            Annotatable::ForeignItem(foreign_item) => foreign_item.visit_attrs(f),
            Annotatable::Stmt(stmt) => stmt.visit_attrs(f),
            Annotatable::Expr(expr) => expr.visit_attrs(f),
            Annotatable::Arm(arm) => arm.visit_attrs(f),
            Annotatable::Field(field) => field.visit_attrs(f),
            Annotatable::FieldPat(fp) => fp.visit_attrs(f),
            Annotatable::GenericParam(gp) => gp.visit_attrs(f),
            Annotatable::Param(p) => p.visit_attrs(f),
            Annotatable::StructField(sf) => sf.visit_attrs(f),
            Annotatable::Variant(v) => v.visit_attrs(f),
        }
    }
}

impl Annotatable {
    pub fn span(&self) -> Span {
        match *self {
            Annotatable::Item(ref item) => item.span,
            Annotatable::TraitItem(ref trait_item) => trait_item.span,
            Annotatable::ImplItem(ref impl_item) => impl_item.span,
            Annotatable::ForeignItem(ref foreign_item) => foreign_item.span,
            Annotatable::Stmt(ref stmt) => stmt.span,
            Annotatable::Expr(ref expr) => expr.span,
            Annotatable::Arm(ref arm) => arm.span,
            Annotatable::Field(ref field) => field.span,
            Annotatable::FieldPat(ref fp) => fp.pat.span,
            Annotatable::GenericParam(ref gp) => gp.ident.span,
            Annotatable::Param(ref p) => p.span,
            Annotatable::StructField(ref sf) => sf.span,
            Annotatable::Variant(ref v) => v.span,
        }
    }

    pub fn visit_with<'a, V: Visitor<'a>>(&'a self, visitor: &mut V) {
        match self {
            Annotatable::Item(item) => visitor.visit_item(item),
            Annotatable::TraitItem(item) => visitor.visit_assoc_item(item, AssocCtxt::Trait),
            Annotatable::ImplItem(item) => visitor.visit_assoc_item(item, AssocCtxt::Impl),
            Annotatable::ForeignItem(foreign_item) => visitor.visit_foreign_item(foreign_item),
            Annotatable::Stmt(stmt) => visitor.visit_stmt(stmt),
            Annotatable::Expr(expr) => visitor.visit_expr(expr),
            Annotatable::Arm(arm) => visitor.visit_arm(arm),
            Annotatable::Field(field) => visitor.visit_field(field),
            Annotatable::FieldPat(fp) => visitor.visit_field_pattern(fp),
            Annotatable::GenericParam(gp) => visitor.visit_generic_param(gp),
            Annotatable::Param(p) => visitor.visit_param(p),
            Annotatable::StructField(sf) => visitor.visit_struct_field(sf),
            Annotatable::Variant(v) => visitor.visit_variant(v),
        }
    }

    crate fn into_tokens(self) -> TokenStream {
        // `Annotatable` can be converted into tokens directly, but we
        // are packing it into a nonterminal as a piece of AST to make
        // the produced token stream look nicer in pretty-printed form.
        let nt = match self {
            Annotatable::Item(item) => token::NtItem(item),
            Annotatable::TraitItem(item) | Annotatable::ImplItem(item) => {
                token::NtItem(P(item.and_then(ast::AssocItem::into_item)))
            }
            Annotatable::ForeignItem(item) => {
                token::NtItem(P(item.and_then(ast::ForeignItem::into_item)))
            }
            Annotatable::Stmt(stmt) => token::NtStmt(stmt.into_inner()),
            Annotatable::Expr(expr) => token::NtExpr(expr),
            Annotatable::Arm(..)
            | Annotatable::Field(..)
            | Annotatable::FieldPat(..)
            | Annotatable::GenericParam(..)
            | Annotatable::Param(..)
            | Annotatable::StructField(..)
            | Annotatable::Variant(..) => panic!("unexpected annotatable"),
        };
        TokenTree::token(token::Interpolated(Lrc::new(nt)), DUMMY_SP).into()
    }

    pub fn expect_item(self) -> P<ast::Item> {
        match self {
            Annotatable::Item(i) => i,
            _ => panic!("expected Item"),
        }
    }

    pub fn map_item_or<F, G>(self, mut f: F, mut or: G) -> Annotatable
    where
        F: FnMut(P<ast::Item>) -> P<ast::Item>,
        G: FnMut(Annotatable) -> Annotatable,
    {
        match self {
            Annotatable::Item(i) => Annotatable::Item(f(i)),
            _ => or(self),
        }
    }

    pub fn expect_trait_item(self) -> P<ast::AssocItem> {
        match self {
            Annotatable::TraitItem(i) => i,
            _ => panic!("expected Item"),
        }
    }

    pub fn expect_impl_item(self) -> P<ast::AssocItem> {
        match self {
            Annotatable::ImplItem(i) => i,
            _ => panic!("expected Item"),
        }
    }

    pub fn expect_foreign_item(self) -> P<ast::ForeignItem> {
        match self {
            Annotatable::ForeignItem(i) => i,
            _ => panic!("expected foreign item"),
        }
    }

    pub fn expect_stmt(self) -> ast::Stmt {
        match self {
            Annotatable::Stmt(stmt) => stmt.into_inner(),
            _ => panic!("expected statement"),
        }
    }

    pub fn expect_expr(self) -> P<ast::Expr> {
        match self {
            Annotatable::Expr(expr) => expr,
            _ => panic!("expected expression"),
        }
    }

    pub fn expect_arm(self) -> ast::Arm {
        match self {
            Annotatable::Arm(arm) => arm,
            _ => panic!("expected match arm"),
        }
    }

    pub fn expect_field(self) -> ast::Field {
        match self {
            Annotatable::Field(field) => field,
            _ => panic!("expected field"),
        }
    }

    pub fn expect_field_pattern(self) -> ast::FieldPat {
        match self {
            Annotatable::FieldPat(fp) => fp,
            _ => panic!("expected field pattern"),
        }
    }

    pub fn expect_generic_param(self) -> ast::GenericParam {
        match self {
            Annotatable::GenericParam(gp) => gp,
            _ => panic!("expected generic parameter"),
        }
    }

    pub fn expect_param(self) -> ast::Param {
        match self {
            Annotatable::Param(param) => param,
            _ => panic!("expected parameter"),
        }
    }

    pub fn expect_struct_field(self) -> ast::StructField {
        match self {
            Annotatable::StructField(sf) => sf,
            _ => panic!("expected struct field"),
        }
    }

    pub fn expect_variant(self) -> ast::Variant {
        match self {
            Annotatable::Variant(v) => v,
            _ => panic!("expected variant"),
        }
    }

    pub fn derive_allowed(&self) -> bool {
        match *self {
            Annotatable::Item(ref item) => match item.kind {
                ast::ItemKind::Struct(..) | ast::ItemKind::Enum(..) | ast::ItemKind::Union(..) => {
                    true
                }
                _ => false,
            },
            _ => false,
        }
    }
}

/// Result of an expansion that may need to be retried.
/// Consider using this for non-`MultiItemModifier` expanders as well.
pub enum ExpandResult<T, U> {
    /// Expansion produced a result (possibly dummy).
    Ready(T),
    /// Expansion could not produce a result and needs to be retried.
    /// The string is an explanation that will be printed if we are stuck in an infinite retry loop.
    Retry(U, String),
}

// `meta_item` is the attribute, and `item` is the item being modified.
pub trait MultiItemModifier {
    fn expand(
        &self,
        ecx: &mut ExtCtxt<'_>,
        span: Span,
        meta_item: &ast::MetaItem,
        item: Annotatable,
    ) -> ExpandResult<Vec<Annotatable>, Annotatable>;
}

impl<F> MultiItemModifier for F
where
    F: Fn(&mut ExtCtxt<'_>, Span, &ast::MetaItem, Annotatable) -> Vec<Annotatable>,
{
    fn expand(
        &self,
        ecx: &mut ExtCtxt<'_>,
        span: Span,
        meta_item: &ast::MetaItem,
        item: Annotatable,
    ) -> ExpandResult<Vec<Annotatable>, Annotatable> {
        ExpandResult::Ready(self(ecx, span, meta_item, item))
    }
}

pub trait ProcMacro {
    fn expand<'cx>(&self, ecx: &'cx mut ExtCtxt<'_>, span: Span, ts: TokenStream) -> TokenStream;
}

impl<F> ProcMacro for F
where
    F: Fn(TokenStream) -> TokenStream,
{
    fn expand<'cx>(&self, _ecx: &'cx mut ExtCtxt<'_>, _span: Span, ts: TokenStream) -> TokenStream {
        // FIXME setup implicit context in TLS before calling self.
        (*self)(ts)
    }
}

pub trait AttrProcMacro {
    fn expand<'cx>(
        &self,
        ecx: &'cx mut ExtCtxt<'_>,
        span: Span,
        annotation: TokenStream,
        annotated: TokenStream,
    ) -> TokenStream;
}

impl<F> AttrProcMacro for F
where
    F: Fn(TokenStream, TokenStream) -> TokenStream,
{
    fn expand<'cx>(
        &self,
        _ecx: &'cx mut ExtCtxt<'_>,
        _span: Span,
        annotation: TokenStream,
        annotated: TokenStream,
    ) -> TokenStream {
        // FIXME setup implicit context in TLS before calling self.
        (*self)(annotation, annotated)
    }
}

/// Represents a thing that maps token trees to Macro Results
pub trait TTMacroExpander {
    fn expand<'cx>(
        &self,
        ecx: &'cx mut ExtCtxt<'_>,
        span: Span,
        input: TokenStream,
    ) -> Box<dyn MacResult + 'cx>;
}

pub type MacroExpanderFn =
    for<'cx> fn(&'cx mut ExtCtxt<'_>, Span, TokenStream) -> Box<dyn MacResult + 'cx>;

impl<F> TTMacroExpander for F
where
    F: for<'cx> Fn(&'cx mut ExtCtxt<'_>, Span, TokenStream) -> Box<dyn MacResult + 'cx>,
{
    fn expand<'cx>(
        &self,
        ecx: &'cx mut ExtCtxt<'_>,
        span: Span,
        mut input: TokenStream,
    ) -> Box<dyn MacResult + 'cx> {
        struct AvoidInterpolatedIdents;

        impl MutVisitor for AvoidInterpolatedIdents {
            fn visit_tt(&mut self, tt: &mut tokenstream::TokenTree) {
                if let tokenstream::TokenTree::Token(token) = tt {
                    if let token::Interpolated(nt) = &token.kind {
                        if let token::NtIdent(ident, is_raw) = **nt {
                            *tt = tokenstream::TokenTree::token(
                                token::Ident(ident.name, is_raw),
                                ident.span,
                            );
                        }
                    }
                }
                mut_visit::noop_visit_tt(tt, self)
            }

            fn visit_mac(&mut self, mac: &mut ast::MacCall) {
                mut_visit::noop_visit_mac(mac, self)
            }
        }
        AvoidInterpolatedIdents.visit_tts(&mut input);
        (*self)(ecx, span, input)
    }
}

// Use a macro because forwarding to a simple function has type system issues
macro_rules! make_stmts_default {
    ($me:expr) => {
        $me.make_expr().map(|e| {
            smallvec![ast::Stmt {
                id: ast::DUMMY_NODE_ID,
                span: e.span,
                kind: ast::StmtKind::Expr(e),
            }]
        })
    };
}

/// The result of a macro expansion. The return values of the various
/// methods are spliced into the AST at the callsite of the macro.
pub trait MacResult {
    /// Creates an expression.
    fn make_expr(self: Box<Self>) -> Option<P<ast::Expr>> {
        None
    }
    /// Creates zero or more items.
    fn make_items(self: Box<Self>) -> Option<SmallVec<[P<ast::Item>; 1]>> {
        None
    }

    /// Creates zero or more impl items.
    fn make_impl_items(self: Box<Self>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
        None
    }

    /// Creates zero or more trait items.
    fn make_trait_items(self: Box<Self>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
        None
    }

    /// Creates zero or more items in an `extern {}` block
    fn make_foreign_items(self: Box<Self>) -> Option<SmallVec<[P<ast::ForeignItem>; 1]>> {
        None
    }

    /// Creates a pattern.
    fn make_pat(self: Box<Self>) -> Option<P<ast::Pat>> {
        None
    }

    /// Creates zero or more statements.
    ///
    /// By default this attempts to create an expression statement,
    /// returning None if that fails.
    fn make_stmts(self: Box<Self>) -> Option<SmallVec<[ast::Stmt; 1]>> {
        make_stmts_default!(self)
    }

    fn make_ty(self: Box<Self>) -> Option<P<ast::Ty>> {
        None
    }

    fn make_arms(self: Box<Self>) -> Option<SmallVec<[ast::Arm; 1]>> {
        None
    }

    fn make_fields(self: Box<Self>) -> Option<SmallVec<[ast::Field; 1]>> {
        None
    }

    fn make_field_patterns(self: Box<Self>) -> Option<SmallVec<[ast::FieldPat; 1]>> {
        None
    }

    fn make_generic_params(self: Box<Self>) -> Option<SmallVec<[ast::GenericParam; 1]>> {
        None
    }

    fn make_params(self: Box<Self>) -> Option<SmallVec<[ast::Param; 1]>> {
        None
    }

    fn make_struct_fields(self: Box<Self>) -> Option<SmallVec<[ast::StructField; 1]>> {
        None
    }

    fn make_variants(self: Box<Self>) -> Option<SmallVec<[ast::Variant; 1]>> {
        None
    }
}

macro_rules! make_MacEager {
    ( $( $fld:ident: $t:ty, )* ) => {
        /// `MacResult` implementation for the common case where you've already
        /// built each form of AST that you might return.
        #[derive(Default)]
        pub struct MacEager {
            $(
                pub $fld: Option<$t>,
            )*
        }

        impl MacEager {
            $(
                pub fn $fld(v: $t) -> Box<dyn MacResult> {
                    Box::new(MacEager {
                        $fld: Some(v),
                        ..Default::default()
                    })
                }
            )*
        }
    }
}

make_MacEager! {
    expr: P<ast::Expr>,
    pat: P<ast::Pat>,
    items: SmallVec<[P<ast::Item>; 1]>,
    impl_items: SmallVec<[P<ast::AssocItem>; 1]>,
    trait_items: SmallVec<[P<ast::AssocItem>; 1]>,
    foreign_items: SmallVec<[P<ast::ForeignItem>; 1]>,
    stmts: SmallVec<[ast::Stmt; 1]>,
    ty: P<ast::Ty>,
}

impl MacResult for MacEager {
    fn make_expr(self: Box<Self>) -> Option<P<ast::Expr>> {
        self.expr
    }

    fn make_items(self: Box<Self>) -> Option<SmallVec<[P<ast::Item>; 1]>> {
        self.items
    }

    fn make_impl_items(self: Box<Self>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
        self.impl_items
    }

    fn make_trait_items(self: Box<Self>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
        self.trait_items
    }

    fn make_foreign_items(self: Box<Self>) -> Option<SmallVec<[P<ast::ForeignItem>; 1]>> {
        self.foreign_items
    }

    fn make_stmts(self: Box<Self>) -> Option<SmallVec<[ast::Stmt; 1]>> {
        match self.stmts.as_ref().map_or(0, |s| s.len()) {
            0 => make_stmts_default!(self),
            _ => self.stmts,
        }
    }

    fn make_pat(self: Box<Self>) -> Option<P<ast::Pat>> {
        if let Some(p) = self.pat {
            return Some(p);
        }
        if let Some(e) = self.expr {
            if let ast::ExprKind::Lit(_) = e.kind {
                return Some(P(ast::Pat {
                    id: ast::DUMMY_NODE_ID,
                    span: e.span,
                    kind: PatKind::Lit(e),
                }));
            }
        }
        None
    }

    fn make_ty(self: Box<Self>) -> Option<P<ast::Ty>> {
        self.ty
    }
}

/// Fill-in macro expansion result, to allow compilation to continue
/// after hitting errors.
#[derive(Copy, Clone)]
pub struct DummyResult {
    is_error: bool,
    span: Span,
}

impl DummyResult {
    /// Creates a default MacResult that can be anything.
    ///
    /// Use this as a return value after hitting any errors and
    /// calling `span_err`.
    pub fn any(span: Span) -> Box<dyn MacResult + 'static> {
        Box::new(DummyResult { is_error: true, span })
    }

    /// Same as `any`, but must be a valid fragment, not error.
    pub fn any_valid(span: Span) -> Box<dyn MacResult + 'static> {
        Box::new(DummyResult { is_error: false, span })
    }

    /// A plain dummy expression.
    pub fn raw_expr(sp: Span, is_error: bool) -> P<ast::Expr> {
        P(ast::Expr {
            id: ast::DUMMY_NODE_ID,
            kind: if is_error { ast::ExprKind::Err } else { ast::ExprKind::Tup(Vec::new()) },
            span: sp,
            attrs: ast::AttrVec::new(),
        })
    }

    /// A plain dummy pattern.
    pub fn raw_pat(sp: Span) -> ast::Pat {
        ast::Pat { id: ast::DUMMY_NODE_ID, kind: PatKind::Wild, span: sp }
    }

    /// A plain dummy type.
    pub fn raw_ty(sp: Span, is_error: bool) -> P<ast::Ty> {
        P(ast::Ty {
            id: ast::DUMMY_NODE_ID,
            kind: if is_error { ast::TyKind::Err } else { ast::TyKind::Tup(Vec::new()) },
            span: sp,
        })
    }
}

impl MacResult for DummyResult {
    fn make_expr(self: Box<DummyResult>) -> Option<P<ast::Expr>> {
        Some(DummyResult::raw_expr(self.span, self.is_error))
    }

    fn make_pat(self: Box<DummyResult>) -> Option<P<ast::Pat>> {
        Some(P(DummyResult::raw_pat(self.span)))
    }

    fn make_items(self: Box<DummyResult>) -> Option<SmallVec<[P<ast::Item>; 1]>> {
        Some(SmallVec::new())
    }

    fn make_impl_items(self: Box<DummyResult>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
        Some(SmallVec::new())
    }

    fn make_trait_items(self: Box<DummyResult>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
        Some(SmallVec::new())
    }

    fn make_foreign_items(self: Box<Self>) -> Option<SmallVec<[P<ast::ForeignItem>; 1]>> {
        Some(SmallVec::new())
    }

    fn make_stmts(self: Box<DummyResult>) -> Option<SmallVec<[ast::Stmt; 1]>> {
        Some(smallvec![ast::Stmt {
            id: ast::DUMMY_NODE_ID,
            kind: ast::StmtKind::Expr(DummyResult::raw_expr(self.span, self.is_error)),
            span: self.span,
        }])
    }

    fn make_ty(self: Box<DummyResult>) -> Option<P<ast::Ty>> {
        Some(DummyResult::raw_ty(self.span, self.is_error))
    }

    fn make_arms(self: Box<DummyResult>) -> Option<SmallVec<[ast::Arm; 1]>> {
        Some(SmallVec::new())
    }

    fn make_fields(self: Box<DummyResult>) -> Option<SmallVec<[ast::Field; 1]>> {
        Some(SmallVec::new())
    }

    fn make_field_patterns(self: Box<DummyResult>) -> Option<SmallVec<[ast::FieldPat; 1]>> {
        Some(SmallVec::new())
    }

    fn make_generic_params(self: Box<DummyResult>) -> Option<SmallVec<[ast::GenericParam; 1]>> {
        Some(SmallVec::new())
    }

    fn make_params(self: Box<DummyResult>) -> Option<SmallVec<[ast::Param; 1]>> {
        Some(SmallVec::new())
    }

    fn make_struct_fields(self: Box<DummyResult>) -> Option<SmallVec<[ast::StructField; 1]>> {
        Some(SmallVec::new())
    }

    fn make_variants(self: Box<DummyResult>) -> Option<SmallVec<[ast::Variant; 1]>> {
        Some(SmallVec::new())
    }
}

/// A syntax extension kind.
pub enum SyntaxExtensionKind {
    /// A token-based function-like macro.
    Bang(
        /// An expander with signature TokenStream -> TokenStream.
        Box<dyn ProcMacro + sync::Sync + sync::Send>,
    ),

    /// An AST-based function-like macro.
    LegacyBang(
        /// An expander with signature TokenStream -> AST.
        Box<dyn TTMacroExpander + sync::Sync + sync::Send>,
    ),

    /// A token-based attribute macro.
    Attr(
        /// An expander with signature (TokenStream, TokenStream) -> TokenStream.
        /// The first TokenSteam is the attribute itself, the second is the annotated item.
        /// The produced TokenSteam replaces the input TokenSteam.
        Box<dyn AttrProcMacro + sync::Sync + sync::Send>,
    ),

    /// An AST-based attribute macro.
    LegacyAttr(
        /// An expander with signature (AST, AST) -> AST.
        /// The first AST fragment is the attribute itself, the second is the annotated item.
        /// The produced AST fragment replaces the input AST fragment.
        Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
    ),

    /// A trivial attribute "macro" that does nothing,
    /// only keeps the attribute and marks it as inert,
    /// thus making it ineligible for further expansion.
    NonMacroAttr {
        /// Suppresses the `unused_attributes` lint for this attribute.
        mark_used: bool,
    },

    /// A token-based derive macro.
    Derive(
        /// An expander with signature TokenStream -> TokenStream (not yet).
        /// The produced TokenSteam is appended to the input TokenSteam.
        Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
    ),

    /// An AST-based derive macro.
    LegacyDerive(
        /// An expander with signature AST -> AST.
        /// The produced AST fragment is appended to the input AST fragment.
        Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
    ),
}

/// A struct representing a macro definition in "lowered" form ready for expansion.
pub struct SyntaxExtension {
    /// A syntax extension kind.
    pub kind: SyntaxExtensionKind,
    /// Span of the macro definition.
    pub span: Span,
    /// Whitelist of unstable features that are treated as stable inside this macro.
    pub allow_internal_unstable: Option<Lrc<[Symbol]>>,
    /// Suppresses the `unsafe_code` lint for code produced by this macro.
    pub allow_internal_unsafe: bool,
    /// Enables the macro helper hack (`ident!(...)` -> `$crate::ident!(...)`) for this macro.
    pub local_inner_macros: bool,
    /// The macro's stability info.
    pub stability: Option<Stability>,
    /// The macro's deprecation info.
    pub deprecation: Option<Deprecation>,
    /// Names of helper attributes registered by this macro.
    pub helper_attrs: Vec<Symbol>,
    /// Edition of the crate in which this macro is defined.
    pub edition: Edition,
    /// Built-in macros have a couple of special properties like availability
    /// in `#[no_implicit_prelude]` modules, so we have to keep this flag.
    pub is_builtin: bool,
    /// We have to identify macros providing a `Copy` impl early for compatibility reasons.
    pub is_derive_copy: bool,
}

impl SyntaxExtension {
    /// Returns which kind of macro calls this syntax extension.
    pub fn macro_kind(&self) -> MacroKind {
        match self.kind {
            SyntaxExtensionKind::Bang(..) | SyntaxExtensionKind::LegacyBang(..) => MacroKind::Bang,
            SyntaxExtensionKind::Attr(..)
            | SyntaxExtensionKind::LegacyAttr(..)
            | SyntaxExtensionKind::NonMacroAttr { .. } => MacroKind::Attr,
            SyntaxExtensionKind::Derive(..) | SyntaxExtensionKind::LegacyDerive(..) => {
                MacroKind::Derive
            }
        }
    }

    /// Constructs a syntax extension with default properties.
    pub fn default(kind: SyntaxExtensionKind, edition: Edition) -> SyntaxExtension {
        SyntaxExtension {
            span: DUMMY_SP,
            allow_internal_unstable: None,
            allow_internal_unsafe: false,
            local_inner_macros: false,
            stability: None,
            deprecation: None,
            helper_attrs: Vec::new(),
            edition,
            is_builtin: false,
            is_derive_copy: false,
            kind,
        }
    }

    /// Constructs a syntax extension with the given properties
    /// and other properties converted from attributes.
    pub fn new(
        sess: &ParseSess,
        kind: SyntaxExtensionKind,
        span: Span,
        helper_attrs: Vec<Symbol>,
        edition: Edition,
        name: Name,
        attrs: &[ast::Attribute],
    ) -> SyntaxExtension {
        let allow_internal_unstable = attr::allow_internal_unstable(&attrs, &sess.span_diagnostic)
            .map(|features| features.collect::<Vec<Symbol>>().into());

        let mut local_inner_macros = false;
        if let Some(macro_export) = attr::find_by_name(attrs, sym::macro_export) {
            if let Some(l) = macro_export.meta_item_list() {
                local_inner_macros = attr::list_contains_name(&l, sym::local_inner_macros);
            }
        }

        let is_builtin = attr::contains_name(attrs, sym::rustc_builtin_macro);
        let (stability, const_stability) = attr::find_stability(&sess, attrs, span);
        if const_stability.is_some() {
            sess.span_diagnostic.span_err(span, "macros cannot have const stability attributes");
        }

        SyntaxExtension {
            kind,
            span,
            allow_internal_unstable,
            allow_internal_unsafe: attr::contains_name(attrs, sym::allow_internal_unsafe),
            local_inner_macros,
            stability,
            deprecation: attr::find_deprecation(&sess, attrs, span),
            helper_attrs,
            edition,
            is_builtin,
            is_derive_copy: is_builtin && name == sym::Copy,
        }
    }

    pub fn dummy_bang(edition: Edition) -> SyntaxExtension {
        fn expander<'cx>(
            _: &'cx mut ExtCtxt<'_>,
            span: Span,
            _: TokenStream,
        ) -> Box<dyn MacResult + 'cx> {
            DummyResult::any(span)
        }
        SyntaxExtension::default(SyntaxExtensionKind::LegacyBang(Box::new(expander)), edition)
    }

    pub fn dummy_derive(edition: Edition) -> SyntaxExtension {
        fn expander(
            _: &mut ExtCtxt<'_>,
            _: Span,
            _: &ast::MetaItem,
            _: Annotatable,
        ) -> Vec<Annotatable> {
            Vec::new()
        }
        SyntaxExtension::default(SyntaxExtensionKind::Derive(Box::new(expander)), edition)
    }

    pub fn non_macro_attr(mark_used: bool, edition: Edition) -> SyntaxExtension {
        SyntaxExtension::default(SyntaxExtensionKind::NonMacroAttr { mark_used }, edition)
    }

    pub fn expn_data(&self, parent: ExpnId, call_site: Span, descr: Symbol) -> ExpnData {
        ExpnData {
            kind: ExpnKind::Macro(self.macro_kind(), descr),
            parent,
            call_site,
            def_site: self.span,
            allow_internal_unstable: self.allow_internal_unstable.clone(),
            allow_internal_unsafe: self.allow_internal_unsafe,
            local_inner_macros: self.local_inner_macros,
            edition: self.edition,
        }
    }
}

/// Result of resolving a macro invocation.
pub enum InvocationRes {
    Single(Lrc<SyntaxExtension>),
    DeriveContainer(Vec<Lrc<SyntaxExtension>>),
}

/// Error type that denotes indeterminacy.
pub struct Indeterminate;

pub trait Resolver {
    fn next_node_id(&mut self) -> NodeId;

    fn resolve_dollar_crates(&mut self);
    fn visit_ast_fragment_with_placeholders(&mut self, expn_id: ExpnId, fragment: &AstFragment);
    fn register_builtin_macro(&mut self, ident: ast::Ident, ext: SyntaxExtension);

    fn expansion_for_ast_pass(
        &mut self,
        call_site: Span,
        pass: AstPass,
        features: &[Symbol],
        parent_module_id: Option<NodeId>,
    ) -> ExpnId;

    fn resolve_imports(&mut self);

    fn resolve_macro_invocation(
        &mut self,
        invoc: &Invocation,
        eager_expansion_root: ExpnId,
        force: bool,
    ) -> Result<InvocationRes, Indeterminate>;

    fn check_unused_macros(&mut self);

    fn has_derive_copy(&self, expn_id: ExpnId) -> bool;
    fn add_derive_copy(&mut self, expn_id: ExpnId);
    fn cfg_accessible(&mut self, expn_id: ExpnId, path: &ast::Path) -> Result<bool, Indeterminate>;
}

#[derive(Clone)]
pub struct ModuleData {
    pub mod_path: Vec<ast::Ident>,
    pub directory: PathBuf,
}

#[derive(Clone)]
pub struct ExpansionData {
    pub id: ExpnId,
    pub depth: usize,
    pub module: Rc<ModuleData>,
    pub directory_ownership: DirectoryOwnership,
    pub prior_type_ascription: Option<(Span, bool)>,
}

/// One of these is made during expansion and incrementally updated as we go;
/// when a macro expansion occurs, the resulting nodes have the `backtrace()
/// -> expn_data` of their expansion context stored into their span.
pub struct ExtCtxt<'a> {
    pub parse_sess: &'a ParseSess,
    pub ecfg: expand::ExpansionConfig<'a>,
    pub reduced_recursion_limit: Option<usize>,
    pub root_path: PathBuf,
    pub resolver: &'a mut dyn Resolver,
    pub current_expansion: ExpansionData,
    pub expansions: FxHashMap<Span, Vec<String>>,
    /// Called directly after having parsed an external `mod foo;` in expansion.
    pub(super) extern_mod_loaded: Option<&'a dyn Fn(&ast::Crate)>,
}

impl<'a> ExtCtxt<'a> {
    pub fn new(
        parse_sess: &'a ParseSess,
        ecfg: expand::ExpansionConfig<'a>,
        resolver: &'a mut dyn Resolver,
        extern_mod_loaded: Option<&'a dyn Fn(&ast::Crate)>,
    ) -> ExtCtxt<'a> {
        ExtCtxt {
            parse_sess,
            ecfg,
            reduced_recursion_limit: None,
            resolver,
            extern_mod_loaded,
            root_path: PathBuf::new(),
            current_expansion: ExpansionData {
                id: ExpnId::root(),
                depth: 0,
                module: Rc::new(ModuleData { mod_path: Vec::new(), directory: PathBuf::new() }),
                directory_ownership: DirectoryOwnership::Owned { relative: None },
                prior_type_ascription: None,
            },
            expansions: FxHashMap::default(),
        }
    }

    /// Returns a `Folder` for deeply expanding all macros in an AST node.
    pub fn expander<'b>(&'b mut self) -> expand::MacroExpander<'b, 'a> {
        expand::MacroExpander::new(self, false)
    }

    /// Returns a `Folder` that deeply expands all macros and assigns all `NodeId`s in an AST node.
    /// Once `NodeId`s are assigned, the node may not be expanded, removed, or otherwise modified.
    pub fn monotonic_expander<'b>(&'b mut self) -> expand::MacroExpander<'b, 'a> {
        expand::MacroExpander::new(self, true)
    }
    pub fn new_parser_from_tts(&self, stream: TokenStream) -> parser::Parser<'a> {
        rustc_parse::stream_to_parser(self.parse_sess, stream, MACRO_ARGUMENTS)
    }
    pub fn source_map(&self) -> &'a SourceMap {
        self.parse_sess.source_map()
    }
    pub fn parse_sess(&self) -> &'a ParseSess {
        self.parse_sess
    }
    pub fn call_site(&self) -> Span {
        self.current_expansion.id.expn_data().call_site
    }

    /// Equivalent of `Span::def_site` from the proc macro API,
    /// except that the location is taken from the span passed as an argument.
    pub fn with_def_site_ctxt(&self, span: Span) -> Span {
        span.with_def_site_ctxt(self.current_expansion.id)
    }

    /// Equivalent of `Span::call_site` from the proc macro API,
    /// except that the location is taken from the span passed as an argument.
    pub fn with_call_site_ctxt(&self, span: Span) -> Span {
        span.with_call_site_ctxt(self.current_expansion.id)
    }

    /// Equivalent of `Span::mixed_site` from the proc macro API,
    /// except that the location is taken from the span passed as an argument.
    pub fn with_mixed_site_ctxt(&self, span: Span) -> Span {
        span.with_mixed_site_ctxt(self.current_expansion.id)
    }

    /// Returns span for the macro which originally caused the current expansion to happen.
    ///
    /// Stops backtracing at include! boundary.
    pub fn expansion_cause(&self) -> Option<Span> {
        self.current_expansion.id.expansion_cause()
    }

    pub fn struct_span_warn<S: Into<MultiSpan>>(&self, sp: S, msg: &str) -> DiagnosticBuilder<'a> {
        self.parse_sess.span_diagnostic.struct_span_warn(sp, msg)
    }
    pub fn struct_span_err<S: Into<MultiSpan>>(&self, sp: S, msg: &str) -> DiagnosticBuilder<'a> {
        self.parse_sess.span_diagnostic.struct_span_err(sp, msg)
    }
    pub fn struct_span_fatal<S: Into<MultiSpan>>(&self, sp: S, msg: &str) -> DiagnosticBuilder<'a> {
        self.parse_sess.span_diagnostic.struct_span_fatal(sp, msg)
    }

    /// Emit `msg` attached to `sp`, and stop compilation immediately.
    ///
    /// `span_err` should be strongly preferred where-ever possible:
    /// this should *only* be used when:
    ///
    /// - continuing has a high risk of flow-on errors (e.g., errors in
    ///   declaring a macro would cause all uses of that macro to
    ///   complain about "undefined macro"), or
    /// - there is literally nothing else that can be done (however,
    ///   in most cases one can construct a dummy expression/item to
    ///   substitute; we never hit resolve/type-checking so the dummy
    ///   value doesn't have to match anything)
    pub fn span_fatal<S: Into<MultiSpan>>(&self, sp: S, msg: &str) -> ! {
        self.parse_sess.span_diagnostic.span_fatal(sp, msg).raise();
    }

    /// Emit `msg` attached to `sp`, without immediately stopping
    /// compilation.
    ///
    /// Compilation will be stopped in the near future (at the end of
    /// the macro expansion phase).
    pub fn span_err<S: Into<MultiSpan>>(&self, sp: S, msg: &str) {
        self.parse_sess.span_diagnostic.span_err(sp, msg);
    }
    pub fn span_err_with_code<S: Into<MultiSpan>>(&self, sp: S, msg: &str, code: DiagnosticId) {
        self.parse_sess.span_diagnostic.span_err_with_code(sp, msg, code);
    }
    pub fn span_warn<S: Into<MultiSpan>>(&self, sp: S, msg: &str) {
        self.parse_sess.span_diagnostic.span_warn(sp, msg);
    }
    pub fn span_bug<S: Into<MultiSpan>>(&self, sp: S, msg: &str) -> ! {
        self.parse_sess.span_diagnostic.span_bug(sp, msg);
    }
    pub fn trace_macros_diag(&mut self) {
        for (sp, notes) in self.expansions.iter() {
            let mut db = self.parse_sess.span_diagnostic.span_note_diag(*sp, "trace_macro");
            for note in notes {
                db.note(note);
            }
            db.emit();
        }
        // Fixme: does this result in errors?
        self.expansions.clear();
    }
    pub fn bug(&self, msg: &str) -> ! {
        self.parse_sess.span_diagnostic.bug(msg);
    }
    pub fn trace_macros(&self) -> bool {
        self.ecfg.trace_mac
    }
    pub fn set_trace_macros(&mut self, x: bool) {
        self.ecfg.trace_mac = x
    }
    pub fn ident_of(&self, st: &str, sp: Span) -> ast::Ident {
        ast::Ident::from_str_and_span(st, sp)
    }
    pub fn std_path(&self, components: &[Symbol]) -> Vec<ast::Ident> {
        let def_site = self.with_def_site_ctxt(DUMMY_SP);
        iter::once(Ident::new(kw::DollarCrate, def_site))
            .chain(components.iter().map(|&s| Ident::with_dummy_span(s)))
            .collect()
    }
    pub fn name_of(&self, st: &str) -> ast::Name {
        Symbol::intern(st)
    }

    pub fn check_unused_macros(&mut self) {
        self.resolver.check_unused_macros();
    }

    /// Resolves a path mentioned inside Rust code.
    ///
    /// This unifies the logic used for resolving `include_X!`, and `#[doc(include)]` file paths.
    ///
    /// Returns an absolute path to the file that `path` refers to.
    pub fn resolve_path(
        &self,
        path: impl Into<PathBuf>,
        span: Span,
    ) -> Result<PathBuf, DiagnosticBuilder<'a>> {
        let path = path.into();

        // Relative paths are resolved relative to the file in which they are found
        // after macro expansion (that is, they are unhygienic).
        if !path.is_absolute() {
            let callsite = span.source_callsite();
            let mut result = match self.source_map().span_to_unmapped_path(callsite) {
                FileName::Real(path) => path,
                FileName::DocTest(path, _) => path,
                other => {
                    return Err(self.struct_span_err(
                        span,
                        &format!("cannot resolve relative path in non-file source `{}`", other),
                    ));
                }
            };
            result.pop();
            result.push(path);
            Ok(result)
        } else {
            Ok(path)
        }
    }
}

/// Extracts a string literal from the macro expanded version of `expr`,
/// emitting `err_msg` if `expr` is not a string literal. This does not stop
/// compilation on error, merely emits a non-fatal error and returns `None`.
pub fn expr_to_spanned_string<'a>(
    cx: &'a mut ExtCtxt<'_>,
    expr: P<ast::Expr>,
    err_msg: &str,
) -> Result<(Symbol, ast::StrStyle, Span), Option<DiagnosticBuilder<'a>>> {
    // Perform eager expansion on the expression.
    // We want to be able to handle e.g., `concat!("foo", "bar")`.
    let expr = cx.expander().fully_expand_fragment(AstFragment::Expr(expr)).make_expr();

    Err(match expr.kind {
        ast::ExprKind::Lit(ref l) => match l.kind {
            ast::LitKind::Str(s, style) => return Ok((s, style, expr.span)),
            ast::LitKind::Err(_) => None,
            _ => Some(cx.struct_span_err(l.span, err_msg)),
        },
        ast::ExprKind::Err => None,
        _ => Some(cx.struct_span_err(expr.span, err_msg)),
    })
}

pub fn expr_to_string(
    cx: &mut ExtCtxt<'_>,
    expr: P<ast::Expr>,
    err_msg: &str,
) -> Option<(Symbol, ast::StrStyle)> {
    expr_to_spanned_string(cx, expr, err_msg)
        .map_err(|err| {
            err.map(|mut err| {
                err.emit();
            })
        })
        .ok()
        .map(|(symbol, style, _)| (symbol, style))
}

/// Non-fatally assert that `tts` is empty. Note that this function
/// returns even when `tts` is non-empty, macros that *need* to stop
/// compilation should call
/// `cx.parse_sess.span_diagnostic.abort_if_errors()` (this should be
/// done as rarely as possible).
pub fn check_zero_tts(cx: &ExtCtxt<'_>, sp: Span, tts: TokenStream, name: &str) {
    if !tts.is_empty() {
        cx.span_err(sp, &format!("{} takes no arguments", name));
    }
}

/// Interpreting `tts` as a comma-separated sequence of expressions,
/// expect exactly one string literal, or emit an error and return `None`.
pub fn get_single_str_from_tts(
    cx: &mut ExtCtxt<'_>,
    sp: Span,
    tts: TokenStream,
    name: &str,
) -> Option<String> {
    let mut p = cx.new_parser_from_tts(tts);
    if p.token == token::Eof {
        cx.span_err(sp, &format!("{} takes 1 argument", name));
        return None;
    }
    let ret = panictry!(p.parse_expr());
    let _ = p.eat(&token::Comma);

    if p.token != token::Eof {
        cx.span_err(sp, &format!("{} takes 1 argument", name));
    }
    expr_to_string(cx, ret, "argument must be a string literal").map(|(s, _)| s.to_string())
}

/// Extracts comma-separated expressions from `tts`. If there is a
/// parsing error, emit a non-fatal error and return `None`.
pub fn get_exprs_from_tts(
    cx: &mut ExtCtxt<'_>,
    sp: Span,
    tts: TokenStream,
) -> Option<Vec<P<ast::Expr>>> {
    let mut p = cx.new_parser_from_tts(tts);
    let mut es = Vec::new();
    while p.token != token::Eof {
        let expr = panictry!(p.parse_expr());

        // Perform eager expansion on the expression.
        // We want to be able to handle e.g., `concat!("foo", "bar")`.
        let expr = cx.expander().fully_expand_fragment(AstFragment::Expr(expr)).make_expr();

        es.push(expr);
        if p.eat(&token::Comma) {
            continue;
        }
        if p.token != token::Eof {
            cx.span_err(sp, "expected token: `,`");
            return None;
        }
    }
    Some(es)
}