Rollup merge of rust-lang#93759 - dtolnay:usetree, r=nagisa

Pretty print ItemKind::Use in rustfmt style

This PR backports the formatting for `use` items from https://github.com/dtolnay/prettyplease into rustc_ast_pretty.

Before:

```rust
use core::{cmp::{Eq, Ord, PartialEq, PartialOrd},
    convert::{AsMut, AsRef, From, Into},
    iter::{DoubleEndedIterator, ExactSizeIterator, Extend, FromIterator,
    IntoIterator, Iterator},
    marker::{Copy as Copy, Send as Send, Sized as Sized, Sync as Sync, Unpin
    as U}, ops::{*, Drop, Fn, FnMut, FnOnce}};
```

After:

```rust
use core::{
    cmp::{Eq, Ord, PartialEq, PartialOrd},
    convert::{AsMut, AsRef, From, Into},
    iter::{
        DoubleEndedIterator, ExactSizeIterator, Extend, FromIterator,
        IntoIterator, Iterator,
    },
    marker::{
        Copy as Copy, Send as Send, Sized as Sized, Sync as Sync, Unpin as U,
    },
    ops::{*, Drop, Fn, FnMut, FnOnce},
};
```

compiler/rustc_ast_pretty/src/pp/convenience.rs

@@ -75,6 +75,10 @@ impl Printer {
     }
 
     pub fn trailing_comma(&mut self) {
+        self.scan_break(BreakToken { pre_break: Some(','), ..BreakToken::default() });
+    }
+
+    pub fn trailing_comma_or_space(&mut self) {
         self.scan_break(BreakToken {
             blank_space: 1,
             pre_break: Some(','),

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -142,7 +142,7 @@ impl<'a> State<'a> {
             if !field.is_last || has_rest {
                 self.word_space(",");
             } else {
-                self.trailing_comma();
+                self.trailing_comma_or_space();
             }
         }
         if has_rest {

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -1,5 +1,6 @@
 use crate::pp::Breaks::Inconsistent;
-use crate::pprust::state::{AnnNode, PrintState, State};
+use crate::pprust::state::delimited::IterDelimited;
+use crate::pprust::state::{AnnNode, PrintState, State, INDENT_UNIT};
 
 use rustc_ast as ast;
 use rustc_ast::GenericBound;
@@ -138,11 +139,10 @@ impl<'a> State<'a> {
                 self.end(); // end outer head-block
             }
             ast::ItemKind::Use(ref tree) => {
-                self.head(visibility_qualified(&item.vis, "use"));
+                self.print_visibility(&item.vis);
+                self.word_nbsp("use");
                 self.print_use_tree(tree);
                 self.word(";");
-                self.end(); // end inner head-block
-                self.end(); // end outer head-block
             }
             ast::ItemKind::Static(ref ty, mutbl, ref body) => {
                 let def = ast::Defaultness::Final;
@@ -615,8 +615,8 @@ impl<'a> State<'a> {
             ast::UseTreeKind::Simple(rename, ..) => {
                 self.print_path(&tree.prefix, false, 0);
                 if let Some(rename) = rename {
-                    self.space();
-                    self.word_space("as");
+                    self.nbsp();
+                    self.word_nbsp("as");
                     self.print_ident(rename);
                 }
             }
@@ -628,16 +628,36 @@ impl<'a> State<'a> {
                 self.word("*");
             }
             ast::UseTreeKind::Nested(ref items) => {
-                if tree.prefix.segments.is_empty() {
-                    self.word("{");
-                } else {
+                if !tree.prefix.segments.is_empty() {
                     self.print_path(&tree.prefix, false, 0);
-                    self.word("::{");
+                    self.word("::");
+                }
+                if items.is_empty() {
+                    self.word("{}");
+                } else if items.len() == 1 {
+                    self.print_use_tree(&items[0].0);
+                } else {
+                    self.cbox(INDENT_UNIT);
+                    self.word("{");
+                    self.zerobreak();
+                    self.ibox(0);
+                    for use_tree in items.iter().delimited() {
+                        self.print_use_tree(&use_tree.0);
+                        if !use_tree.is_last {
+                            self.word(",");
+                            if let ast::UseTreeKind::Nested(_) = use_tree.0.kind {
+                                self.hardbreak();
+                            } else {
+                                self.space();
+                            }
+                        }
+                    }
+                    self.end();
+                    self.trailing_comma();
+                    self.offset(-INDENT_UNIT);
+                    self.word("}");
+                    self.end();
                 }
-                self.commasep(Inconsistent, &items, |this, &(ref tree, _)| {
-                    this.print_use_tree(tree)
-                });
-                self.word("}");
             }
         }
     }

src/test/pretty/use-tree.rs

@@ -0,0 +1,23 @@
+// pp-exact
+// edition:2021
+
+#![allow(unused_imports)]
+
+use ::std::fmt::{self, Debug, Display, Write as _};
+
+use core::option::Option::*;
+
+use core::{
+    cmp::{Eq, Ord, PartialEq, PartialOrd},
+    convert::{AsMut, AsRef, From, Into},
+    iter::{
+        DoubleEndedIterator, ExactSizeIterator, Extend, FromIterator,
+        IntoIterator, Iterator,
+    },
+    marker::{
+        Copy as Copy, Send as Send, Sized as Sized, Sync as Sync, Unpin as U,
+    },
+    ops::{*, Drop, Fn, FnMut, FnOnce},
+};
+
+fn main() {}