rust/src/libsyntax/ext/base.rs

// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

pub use self::SyntaxExtension::*;

use ast::{self, Attribute, Name, PatKind};
use attr::HasAttrs;
use codemap::{self, CodeMap, ExpnInfo, Spanned, respan};
use syntax_pos::{Span, ExpnId, NO_EXPANSION};
use errors::DiagnosticBuilder;
use ext::expand::{self, Invocation, Expansion};
use ext::hygiene::Mark;
use ext::tt::macro_rules;
use parse;
use parse::parser;
use parse::token;
use parse::token::{InternedString, str_to_ident};
use ptr::P;
use std_inject;
use util::small_vector::SmallVector;
use fold::Folder;
use feature_gate;

use std::collections::HashMap;
use std::path::PathBuf;
use std::rc::Rc;
use tokenstream;


#[derive(Debug,Clone)]
pub enum Annotatable {
    Item(P<ast::Item>),
    TraitItem(P<ast::TraitItem>),
    ImplItem(P<ast::ImplItem>),
}

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,
        }
    }

    fn map_attrs<F: FnOnce(Vec<Attribute>) -> Vec<Attribute>>(self, f: F) -> Self {
        match self {
            Annotatable::Item(item) => Annotatable::Item(item.map_attrs(f)),
            Annotatable::TraitItem(trait_item) => Annotatable::TraitItem(trait_item.map_attrs(f)),
            Annotatable::ImplItem(impl_item) => Annotatable::ImplItem(impl_item.map_attrs(f)),
        }
    }
}

impl Annotatable {
    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) -> ast::TraitItem {
        match self {
            Annotatable::TraitItem(i) => i.unwrap(),
            _ => panic!("expected Item")
        }
    }

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

// A more flexible ItemDecorator.
pub trait MultiItemDecorator {
    fn expand(&self,
              ecx: &mut ExtCtxt,
              sp: Span,
              meta_item: &ast::MetaItem,
              item: &Annotatable,
              push: &mut FnMut(Annotatable));
}

impl<F> MultiItemDecorator for F
    where F : Fn(&mut ExtCtxt, Span, &ast::MetaItem, &Annotatable, &mut FnMut(Annotatable))
{
    fn expand(&self,
              ecx: &mut ExtCtxt,
              sp: Span,
              meta_item: &ast::MetaItem,
              item: &Annotatable,
              push: &mut FnMut(Annotatable)) {
        (*self)(ecx, sp, meta_item, item, push)
    }
}

// `meta_item` is the annotation, and `item` is the item being modified.
// FIXME Decorators should follow the same pattern too.
pub trait MultiItemModifier {
    fn expand(&self,
              ecx: &mut ExtCtxt,
              span: Span,
              meta_item: &ast::MetaItem,
              item: Annotatable)
              -> Vec<Annotatable>;
}

impl<F, T> MultiItemModifier for F
    where F: Fn(&mut ExtCtxt, Span, &ast::MetaItem, Annotatable) -> T,
          T: Into<Vec<Annotatable>>,
{
    fn expand(&self,
              ecx: &mut ExtCtxt,
              span: Span,
              meta_item: &ast::MetaItem,
              item: Annotatable)
              -> Vec<Annotatable> {
        (*self)(ecx, span, meta_item, item).into()
    }
}

impl Into<Vec<Annotatable>> for Annotatable {
    fn into(self) -> Vec<Annotatable> {
        vec![self]
    }
}

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

pub type MacroExpanderFn =
    for<'cx> fn(&'cx mut ExtCtxt, Span, &[tokenstream::TokenTree])
                -> Box<MacResult+'cx>;

impl<F> TTMacroExpander for F
    where F : for<'cx> Fn(&'cx mut ExtCtxt, Span, &[tokenstream::TokenTree])
                          -> Box<MacResult+'cx>
{
    fn expand<'cx>(&self,
                   ecx: &'cx mut ExtCtxt,
                   span: Span,
                   token_tree: &[tokenstream::TokenTree])
                   -> Box<MacResult+'cx> {
        (*self)(ecx, span, token_tree)
    }
}

pub trait IdentMacroExpander {
    fn expand<'cx>(&self,
                   cx: &'cx mut ExtCtxt,
                   sp: Span,
                   ident: ast::Ident,
                   token_tree: Vec<tokenstream::TokenTree>,
                   attrs: Vec<ast::Attribute>)
                   -> Box<MacResult+'cx>;
}

pub type IdentMacroExpanderFn =
    for<'cx> fn(&'cx mut ExtCtxt, Span, ast::Ident, Vec<tokenstream::TokenTree>)
                -> Box<MacResult+'cx>;

impl<F> IdentMacroExpander for F
    where F : for<'cx> Fn(&'cx mut ExtCtxt, Span, ast::Ident,
                          Vec<tokenstream::TokenTree>) -> Box<MacResult+'cx>
{
    fn expand<'cx>(&self,
                   cx: &'cx mut ExtCtxt,
                   sp: Span,
                   ident: ast::Ident,
                   token_tree: Vec<tokenstream::TokenTree>,
                   _attrs: Vec<ast::Attribute>)
                   -> Box<MacResult+'cx>
    {
        (*self)(cx, sp, ident, token_tree)
    }
}

// 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| SmallVector::one(ast::Stmt {
            id: ast::DUMMY_NODE_ID,
            span: e.span,
            node: 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 {
    /// Create an expression.
    fn make_expr(self: Box<Self>) -> Option<P<ast::Expr>> {
        None
    }
    /// Create zero or more items.
    fn make_items(self: Box<Self>) -> Option<SmallVector<P<ast::Item>>> {
        None
    }

    /// Create zero or more impl items.
    fn make_impl_items(self: Box<Self>) -> Option<SmallVector<ast::ImplItem>> {
        None
    }

    /// Create zero or more trait items.
    fn make_trait_items(self: Box<Self>) -> Option<SmallVector<ast::TraitItem>> {
        None
    }

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

    /// Create 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<SmallVector<ast::Stmt>> {
        make_stmts_default!(self)
    }

    fn make_ty(self: Box<Self>) -> Option<P<ast::Ty>> {
        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<MacResult> {
                    Box::new(MacEager {
                        $fld: Some(v),
                        ..Default::default()
                    })
                }
            )*
        }
    }
}

make_MacEager! {
    expr: P<ast::Expr>,
    pat: P<ast::Pat>,
    items: SmallVector<P<ast::Item>>,
    impl_items: SmallVector<ast::ImplItem>,
    trait_items: SmallVector<ast::TraitItem>,
    stmts: SmallVector<ast::Stmt>,
    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<SmallVector<P<ast::Item>>> {
        self.items
    }

    fn make_impl_items(self: Box<Self>) -> Option<SmallVector<ast::ImplItem>> {
        self.impl_items
    }

    fn make_trait_items(self: Box<Self>) -> Option<SmallVector<ast::TraitItem>> {
        self.trait_items
    }

    fn make_stmts(self: Box<Self>) -> Option<SmallVector<ast::Stmt>> {
        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.node {
                return Some(P(ast::Pat {
                    id: ast::DUMMY_NODE_ID,
                    span: e.span,
                    node: 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 {
    expr_only: bool,
    span: Span
}

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

    /// Create a default MacResult that can only be an expression.
    ///
    /// Use this for macros that must expand to an expression, so even
    /// if an error is encountered internally, the user will receive
    /// an error that they also used it in the wrong place.
    pub fn expr(sp: Span) -> Box<MacResult+'static> {
        Box::new(DummyResult { expr_only: true, span: sp })
    }

    /// A plain dummy expression.
    pub fn raw_expr(sp: Span) -> P<ast::Expr> {
        P(ast::Expr {
            id: ast::DUMMY_NODE_ID,
            node: ast::ExprKind::Lit(P(codemap::respan(sp, ast::LitKind::Bool(false)))),
            span: sp,
            attrs: ast::ThinVec::new(),
        })
    }

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

    pub fn raw_ty(sp: Span) -> P<ast::Ty> {
        P(ast::Ty {
            id: ast::DUMMY_NODE_ID,
            node: ast::TyKind::Infer,
            span: sp
        })
    }
}

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

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

    fn make_items(self: Box<DummyResult>) -> Option<SmallVector<P<ast::Item>>> {
        // this code needs a comment... why not always just return the Some() ?
        if self.expr_only {
            None
        } else {
            Some(SmallVector::zero())
        }
    }

    fn make_impl_items(self: Box<DummyResult>) -> Option<SmallVector<ast::ImplItem>> {
        if self.expr_only {
            None
        } else {
            Some(SmallVector::zero())
        }
    }

    fn make_trait_items(self: Box<DummyResult>) -> Option<SmallVector<ast::TraitItem>> {
        if self.expr_only {
            None
        } else {
            Some(SmallVector::zero())
        }
    }

    fn make_stmts(self: Box<DummyResult>) -> Option<SmallVector<ast::Stmt>> {
        Some(SmallVector::one(ast::Stmt {
            id: ast::DUMMY_NODE_ID,
            node: ast::StmtKind::Expr(DummyResult::raw_expr(self.span)),
            span: self.span,
        }))
    }

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

/// An enum representing the different kinds of syntax extensions.
pub enum SyntaxExtension {
    /// A syntax extension that is attached to an item and creates new items
    /// based upon it.
    ///
    /// `#[derive(...)]` is a `MultiItemDecorator`.
    MultiDecorator(Box<MultiItemDecorator + 'static>),

    /// A syntax extension that is attached to an item and modifies it
    /// in-place. More flexible version than Modifier.
    MultiModifier(Box<MultiItemModifier + 'static>),

    /// A normal, function-like syntax extension.
    ///
    /// `bytes!` is a `NormalTT`.
    ///
    /// The `bool` dictates whether the contents of the macro can
    /// directly use `#[unstable]` things (true == yes).
    NormalTT(Box<TTMacroExpander + 'static>, Option<Span>, bool),

    /// A function-like syntax extension that has an extra ident before
    /// the block.
    ///
    IdentTT(Box<IdentMacroExpander + 'static>, Option<Span>, bool),
}

pub type NamedSyntaxExtension = (Name, SyntaxExtension);

pub trait Resolver {
    fn load_crate(&mut self, extern_crate: &ast::Item, allows_macros: bool) -> Vec<LoadedMacro>;
    fn next_node_id(&mut self) -> ast::NodeId;

    fn visit_expansion(&mut self, mark: Mark, expansion: &Expansion);
    fn add_macro(&mut self, scope: Mark, ident: ast::Ident, ext: Rc<SyntaxExtension>);
    fn add_expansions_at_stmt(&mut self, id: ast::NodeId, macros: Vec<Mark>);

    fn find_attr_invoc(&mut self, attrs: &mut Vec<Attribute>) -> Option<Attribute>;
    fn resolve_invoc(&mut self, invoc: &Invocation) -> Option<Rc<SyntaxExtension>>;
}

pub enum LoadedMacro {
    Def(ast::MacroDef),
    CustomDerive(String, Box<MultiItemModifier>),
}

pub struct DummyResolver;

impl Resolver for DummyResolver {
    fn load_crate(&mut self, _extern_crate: &ast::Item, _allows_macros: bool) -> Vec<LoadedMacro> {
        Vec::new()
    }
    fn next_node_id(&mut self) -> ast::NodeId { ast::DUMMY_NODE_ID }

    fn visit_expansion(&mut self, _invoc: Mark, _expansion: &Expansion) {}
    fn add_macro(&mut self, _scope: Mark, _ident: ast::Ident, _ext: Rc<SyntaxExtension>) {}
    fn add_expansions_at_stmt(&mut self, _id: ast::NodeId, _macros: Vec<Mark>) {}

    fn find_attr_invoc(&mut self, _attrs: &mut Vec<Attribute>) -> Option<Attribute> { None }
    fn resolve_invoc(&mut self, _invoc: &Invocation) -> Option<Rc<SyntaxExtension>> { None }
}

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

#[derive(Clone)]
pub struct ExpansionData {
    pub mark: Mark,
    pub depth: usize,
    pub backtrace: ExpnId,
    pub module: Rc<ModuleData>,
    pub in_block: 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_info of their expansion context stored into their span.
pub struct ExtCtxt<'a> {
    pub parse_sess: &'a parse::ParseSess,
    pub cfg: ast::CrateConfig,
    pub ecfg: expand::ExpansionConfig<'a>,
    pub crate_root: Option<&'static str>,
    pub resolver: &'a mut Resolver,
    pub exported_macros: Vec<ast::MacroDef>,
    pub derive_modes: HashMap<InternedString, Box<MultiItemModifier>>,
    pub current_expansion: ExpansionData,
}

impl<'a> ExtCtxt<'a> {
    pub fn new(parse_sess: &'a parse::ParseSess, cfg: ast::CrateConfig,
               ecfg: expand::ExpansionConfig<'a>,
               resolver: &'a mut Resolver)
               -> ExtCtxt<'a> {
        ExtCtxt {
            parse_sess: parse_sess,
            cfg: cfg,
            ecfg: ecfg,
            crate_root: None,
            exported_macros: Vec::new(),
            resolver: resolver,
            derive_modes: HashMap::new(),
            current_expansion: ExpansionData {
                mark: Mark::root(),
                depth: 0,
                backtrace: NO_EXPANSION,
                module: Rc::new(ModuleData { mod_path: Vec::new(), directory: PathBuf::new() }),
                in_block: false,
            },
        }
    }

    /// 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 node ids in an AST node.
    /// Once node ids 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, tts: &[tokenstream::TokenTree])
        -> parser::Parser<'a> {
        parse::tts_to_parser(self.parse_sess, tts.to_vec(), self.cfg())
    }
    pub fn codemap(&self) -> &'a CodeMap { self.parse_sess.codemap() }
    pub fn parse_sess(&self) -> &'a parse::ParseSess { self.parse_sess }
    pub fn cfg(&self) -> ast::CrateConfig { self.cfg.clone() }
    pub fn call_site(&self) -> Span {
        self.codemap().with_expn_info(self.backtrace(), |ei| match ei {
            Some(expn_info) => expn_info.call_site,
            None => self.bug("missing top span")
        })
    }
    pub fn backtrace(&self) -> ExpnId { self.current_expansion.backtrace }

    /// Returns span for the macro which originally caused the current expansion to happen.
    ///
    /// Stops backtracing at include! boundary.
    pub fn expansion_cause(&self) -> Span {
        let mut expn_id = self.backtrace();
        let mut last_macro = None;
        loop {
            if self.codemap().with_expn_info(expn_id, |info| {
                info.map_or(None, |i| {
                    if i.callee.name().as_str() == "include" {
                        // Stop going up the backtrace once include! is encountered
                        return None;
                    }
                    expn_id = i.call_site.expn_id;
                    last_macro = Some(i.call_site);
                    return Some(());
                })
            }).is_none() {
                break
            }
        }
        last_macro.expect("missing expansion backtrace")
    }

    pub fn bt_push(&mut self, ei: ExpnInfo) {
        if self.current_expansion.depth > self.ecfg.recursion_limit {
            self.span_fatal(ei.call_site,
                            &format!("recursion limit reached while expanding the macro `{}`",
                                    ei.callee.name()));
        }

        let mut call_site = ei.call_site;
        call_site.expn_id = self.backtrace();
        self.current_expansion.backtrace = self.codemap().record_expansion(ExpnInfo {
            call_site: call_site,
            callee: ei.callee
        });
    }
    pub fn bt_pop(&mut self) {}

    pub fn insert_macro(&mut self, def: ast::MacroDef) {
        if def.export {
            self.exported_macros.push(def.clone());
        }
        if def.use_locally {
            let ext = macro_rules::compile(self, &def);
            self.resolver.add_macro(self.current_expansion.mark, def.ident, Rc::new(ext));
        }
    }

    pub fn insert_custom_derive(&mut self, name: &str, ext: Box<MultiItemModifier>, sp: Span) {
        if !self.ecfg.enable_rustc_macro() {
            feature_gate::emit_feature_err(&self.parse_sess.span_diagnostic,
                                           "rustc_macro",
                                           sp,
                                           feature_gate::GateIssue::Language,
                                           "loading custom derive macro crates \
                                            is experimentally supported");
        }
        let name = token::intern_and_get_ident(name);
        if self.derive_modes.insert(name.clone(), ext).is_some() {
            self.span_err(sp, &format!("cannot shadow existing derive mode `{}`", name));
        }
    }

    pub fn struct_span_warn(&self,
                            sp: Span,
                            msg: &str)
                            -> DiagnosticBuilder<'a> {
        self.parse_sess.span_diagnostic.struct_span_warn(sp, msg)
    }
    pub fn struct_span_err(&self,
                           sp: Span,
                           msg: &str)
                           -> DiagnosticBuilder<'a> {
        self.parse_sess.span_diagnostic.struct_span_err(sp, msg)
    }
    pub fn struct_span_fatal(&self,
                             sp: Span,
                             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(&self, sp: Span, msg: &str) -> ! {
        panic!(self.parse_sess.span_diagnostic.span_fatal(sp, msg));
    }

    /// 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(&self, sp: Span, msg: &str) {
        self.parse_sess.span_diagnostic.span_err(sp, msg);
    }
    pub fn span_warn(&self, sp: Span, msg: &str) {
        self.parse_sess.span_diagnostic.span_warn(sp, msg);
    }
    pub fn span_unimpl(&self, sp: Span, msg: &str) -> ! {
        self.parse_sess.span_diagnostic.span_unimpl(sp, msg);
    }
    pub fn span_bug(&self, sp: Span, msg: &str) -> ! {
        self.parse_sess.span_diagnostic.span_bug(sp, msg);
    }
    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) -> ast::Ident {
        str_to_ident(st)
    }
    pub fn std_path(&self, components: &[&str]) -> Vec<ast::Ident> {
        let mut v = Vec::new();
        if let Some(s) = self.crate_root {
            v.push(self.ident_of(s));
        }
        v.extend(components.iter().map(|s| self.ident_of(s)));
        return v
    }
    pub fn name_of(&self, st: &str) -> ast::Name {
        token::intern(st)
    }

    pub fn initialize(&mut self, user_exts: Vec<NamedSyntaxExtension>, krate: &ast::Crate) {
        if std_inject::no_core(&krate) {
            self.crate_root = None;
        } else if std_inject::no_std(&krate) {
            self.crate_root = Some("core");
        } else {
            self.crate_root = Some("std");
        }

        for (name, extension) in user_exts {
            let ident = ast::Ident::with_empty_ctxt(name);
            self.resolver.add_macro(Mark::root(), ident, Rc::new(extension));
        }

        let mut module = ModuleData {
            mod_path: vec![token::str_to_ident(&self.ecfg.crate_name)],
            directory: PathBuf::from(self.parse_sess.codemap().span_to_filename(krate.span)),
        };
        module.directory.pop();
        self.current_expansion.module = Rc::new(module);
    }
}

/// Extract 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(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
                              -> Option<Spanned<(InternedString, ast::StrStyle)>> {
    // Update `expr.span`'s expn_id now in case expr is an `include!` macro invocation.
    let expr = expr.map(|mut expr| {
        expr.span.expn_id = cx.backtrace();
        expr
    });

    // we want to be able to handle e.g. concat("foo", "bar")
    let expr = cx.expander().fold_expr(expr);
    match expr.node {
        ast::ExprKind::Lit(ref l) => match l.node {
            ast::LitKind::Str(ref s, style) => return Some(respan(expr.span, (s.clone(), style))),
            _ => cx.span_err(l.span, err_msg)
        },
        _ => cx.span_err(expr.span, err_msg)
    }
    None
}

pub fn expr_to_string(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
                      -> Option<(InternedString, ast::StrStyle)> {
    expr_to_spanned_string(cx, expr, err_msg).map(|s| s.node)
}

/// 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::TokenTree],
                      name: &str) {
    if !tts.is_empty() {
        cx.span_err(sp, &format!("{} takes no arguments", name));
    }
}

/// Extract the string literal from the first token of `tts`. If this
/// is not a string literal, emit an error and return None.
pub fn get_single_str_from_tts(cx: &mut ExtCtxt,
                               sp: Span,
                               tts: &[tokenstream::TokenTree],
                               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());
    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()
    })
}

/// Extract 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::TokenTree]) -> Option<Vec<P<ast::Expr>>> {
    let mut p = cx.new_parser_from_tts(tts);
    let mut es = Vec::new();
    while p.token != token::Eof {
        es.push(cx.expander().fold_expr(panictry!(p.parse_expr())));
        if p.eat(&token::Comma) {
            continue;
        }
        if p.token != token::Eof {
            cx.span_err(sp, "expected token: `,`");
            return None;
        }
    }
    Some(es)
}