src/libsyntax_ext/standard_library_imports.rs

use syntax::{ast, attr};
use syntax::edition::Edition;
use syntax::ext::hygiene::MacroKind;
use syntax::ptr::P;
use syntax::source_map::{ExpnData, ExpnKind, dummy_spanned, respan};
use syntax::symbol::{Ident, Symbol, kw, sym};
use syntax_pos::DUMMY_SP;

use std::iter;

pub fn inject(
    mut krate: ast::Crate, alt_std_name: Option<&str>, edition: Edition
) -> (ast::Crate, Option<Symbol>) {
    let rust_2018 = edition >= Edition::Edition2018;

    // the first name in this list is the crate name of the crate with the prelude
    let names: &[&str] = if attr::contains_name(&krate.attrs, sym::no_core) {
        return (krate, None);
    } else if attr::contains_name(&krate.attrs, sym::no_std) {
        if attr::contains_name(&krate.attrs, sym::compiler_builtins) {
            &["core"]
        } else {
            &["core", "compiler_builtins"]
        }
    } else {
        &["std"]
    };

    // .rev() to preserve ordering above in combination with insert(0, ...)
    let alt_std_name = alt_std_name.map(Symbol::intern);
    for orig_name_str in names.iter().rev() {
        // HACK(eddyb) gensym the injected crates on the Rust 2018 edition,
        // so they don't accidentally interfere with the new import paths.
        let orig_name_sym = Symbol::intern(orig_name_str);
        let orig_name_ident = Ident::with_dummy_span(orig_name_sym);
        let (rename, orig_name) = if rust_2018 {
            (orig_name_ident.gensym(), Some(orig_name_sym))
        } else {
            (orig_name_ident, None)
        };
        krate.module.items.insert(0, P(ast::Item {
            attrs: vec![attr::mk_attr_outer(
                attr::mk_word_item(ast::Ident::with_dummy_span(sym::macro_use))
            )],
            vis: dummy_spanned(ast::VisibilityKind::Inherited),
            node: ast::ItemKind::ExternCrate(alt_std_name.or(orig_name)),
            ident: rename,
            id: ast::DUMMY_NODE_ID,
            span: DUMMY_SP,
            tokens: None,
        }));
    }

    // the crates have been injected, the assumption is that the first one is the one with
    // the prelude.
    let name = names[0];

    let span = DUMMY_SP.fresh_expansion(ExpnData::allow_unstable(
        ExpnKind::Macro(MacroKind::Attr, sym::std_inject), DUMMY_SP, edition,
        [sym::prelude_import][..].into(),
    ));

    krate.module.items.insert(0, P(ast::Item {
        attrs: vec![attr::mk_attr_outer(
            attr::mk_word_item(ast::Ident::new(sym::prelude_import, span)))],
        vis: respan(span.shrink_to_lo(), ast::VisibilityKind::Inherited),
        node: ast::ItemKind::Use(P(ast::UseTree {
            prefix: ast::Path {
                segments: iter::once(ast::Ident::with_dummy_span(kw::PathRoot))
                    .chain(
                        [name, "prelude", "v1"].iter().cloned()
                            .map(ast::Ident::from_str)
                    ).map(ast::PathSegment::from_ident).collect(),
                span,
            },
            kind: ast::UseTreeKind::Glob,
            span,
        })),
        id: ast::DUMMY_NODE_ID,
        ident: ast::Ident::invalid(),
        span,
        tokens: None,
    }));

    (krate, Some(Symbol::intern(name)))
}