Summary

Permit

許す
nested
入れ子
 {} groups in imports.
Permit
許す
 * in {} groups in imports.


#![allow(unused)]
fn main() {
use syntax::{
    tokenstream::TokenTree, // >1 segments
    ext::base::{ExtCtxt, MacResult, DummyResult, MacEager}, // nested braces
    ext::build::AstBuilder,
    ext::quote::rt::Span,
};

use syntax::ast::{self, *}; // * in braces

use rustc::mir::{*, transform::{MirPass, MirSource}}; // both * and nested braces
}

Motivation

The motivation is ergonomics. Prefixes

接頭辞
are often shared among imports, especially if many imports import names from the same crate. With this nested
入れ子
 grouping it's more often possible to merge common import prefixes
接頭辞
 and write them once instead of writing them multiple
複数の
 times.

Guide-level explanation

Several use items with common prefix

接頭辞
can be merged into one use item, in which the prefix
接頭辞
 is written once and all the suffixes are listed
リスト、列挙する
 inside curly braces
括弧
 {}.
All kinds of suffixes can be listed
リスト、列挙する
 inside curly braces,
括弧
 including globs * and "subtrees" with their own curly braces.
括弧


#![allow(unused)]
fn main() {
// BEFORE
use syntax::tokenstream::TokenTree;
use syntax::ext::base::{ExtCtxt, MacResult, DummyResult, MacEager};
use syntax::ext::build::AstBuilder,
use syntax::ext::quote::rt::Span,

use syntax::ast;
use syntax::ast::*;

use rustc::mir::*;
use rustc::mir::transform::{MirPass, MirSource};

// AFTER
use syntax::{
    // paths with >1 segments are permitted inside braces
    tokenstream::TokenTree,
    // nested braces are permitted as well
    ext::base::{ExtCtxt, MacResult, DummyResult, MacEager},
    ext::build::AstBuilder,
    ext::quote::rt::Span,
};

// `*` can be listed in braces too
use syntax::ast::{self, *};

// both `*` and nested braces
use rustc::mir::{*, transform::{MirPass, MirSource}};

// the prefix can be empty
use {
    syntax::ast::*;
    rustc::mir::*;
};

// `pub` imports can use this syntax as well
pub use self::Visibility::{self, Public, Inherited};
}

A use item with merged prefixes

接頭辞
behaves identically to several use items with all the prefixes
接頭辞
 "unmerged".

Reference-level explanation

Syntax:

文法

IMPORT = ATTRS VISIBILITY `use` [`::`] IMPORT_TREE `;`

IMPORT_TREE = `*` |
              REL_MOD_PATH `::` `*` |
              `{` IMPORT_TREE_LIST `}` |
              REL_MOD_PATH `::` `{` IMPORT_TREE_LIST `}` |
              REL_MOD_PATH [`as` IDENT]

IMPORT_TREE_LIST = Ø | (IMPORT_TREE `,`)* IMPORT_TREE [`,`]

REL_MOD_PATH = (IDENT `::`)* IDENT

Resolution:
First the import tree is prefixed

接頭辞
with ::, unless it already starts with ::, self or super.
Then resolution is performed as if the whole import tree were flattened, except that {self}/{self as name} are processed specially because a::b::self is illegal.
文法違反


#![allow(unused)]
fn main() {
use a::{
    b::{self as s, c, d as e},
    f::*,
    g::h as i,
    *,
};

=>

use ::a::b as s;
use ::a::b::c;
use ::a::b::d as e;
use ::a::f::*;
use ::a::g::h as i;
use ::a::*;
}

Various

さまざまな
corner cases are resolved naturally through desugaring


#![allow(unused)]
fn main() {
use an::{*, *}; // Use an owl!

=>

use an::*;
use an::*; // Legal, but reported as unused by `unused_imports` lint.
}

Relationships with other proposal

This RFC is an incremental improvement largely independent from other import-related proposals, but it can have effect on some other RFCs.

Some RFCs propose new syntaxes

文法
for absolute paths in the current crate and paths from other crates. Some arguments
引数
 in those proposals are based
基となる、基底(の)
 on usage statistics - "imports from other crates are more common" or "imports from the current crate are more common". More common imports are supposed to get less verbose syntax.
文法

This RFC removes the these statistics from the equation by reducing verbosity for all imports with common prefix.

接頭辞

For example, the difference in verbosity between A, B and C is minimal and doesn't depend on the number of imports.


#![allow(unused)]
fn main() {
// A
use extern::{
    a::b::c,
    d::e::f,
    g::h::i,
};
// B
use crate::{
    a::b::c,
    d::e::f,
    g::h::i,
};
// C
use {
    a::b::c,
    d::e::f,
    g::h::i,
};
}

Drawbacks

The feature encourages (but not requires) multi-line formatting of a single

単一の
import


#![allow(unused)]
fn main() {
use prefix::{
    MyName,
    x::YourName,
    y::Surname,
};
}

With this formatting it becomes harder to grep for use.*MyName.

Rationale and Alternatives

Status quo is always an alternative.

代わりのもの、選択肢

Unresolved questions

None so far.