RFC: Nested groups in imports

text/0000-use-nested-groups.md

@@ -0,0 +1,177 @@
+- Feature Name: use_nested_groups
+- Start Date: 2017-08-25
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Permit nested `{}` groups in imports.  
+Permit `*` in `{}` groups in imports.
+
+```rust
+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
+[motivation]: #motivation
+
+The motivation is ergonomics.
+Prefixes are often shared among imports, especially if many imports
+import names from the same crate. With this nested grouping it's more often
+possible to merge common import prefixes and write them once instead of writing
+them multiple times.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Several `use` items with common prefix can be merged into one `use` item,
+in which the prefix is written once and all the suffixes are listed inside
+curly braces `{}`.  
+All kinds of suffixes can be listed inside curly braces, including globs `*` and
+"subtrees" with their own curly braces.
+
+```rust
+// 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::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}}; 
+```
+
+A `use` item with merged prefixes behaves identically to several `use` items
+with all the prefixes "unmerged".
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Syntax:
+```
+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 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.
+
+```rust
+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::*;
+```
+
+# 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 for absolute paths in the current crate
+and paths from other crates. Some arguments in those proposals are based 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.
+
+This RFC removes the these statistics from the equation by reducing verbosity
+for all imports with common prefix.  
+For example, the difference in verbosity between `A`, `B` and
+`C` is minimal and doesn't depend on the number of imports.
+```rust
+// 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
+[drawbacks]: #drawbacks
+
+The feature encourages (but not requires) multi-line formatting of a single
+import
+```rust
+use prefix::{
+    MyName,
+    x::YourName,
+    y::Surname,
+};
+```
+With this formatting it becomes harder to grep for `use.*MyName`.
+
+# Rationale and Alternatives
+[alternatives]: #alternatives
+
+Status quo is always an alternative.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None so far.