Rollup merge of #91437 - dtolnay:emptybrace, r=nagisa

Pretty print empty blocks as {}

**Example:**

```rust
macro_rules! p {
    ($e:expr) => {
        println!("{}", stringify!($e));
    };
    ($i:item) => {
        println!("{}", stringify!($i));
    };
}

fn main() {
    p!(if true {});
    p!(struct S {});
}
```

**Before:**

```console
if true { }
struct S {
}
```

**After:**

```console
if true {}
struct S {}
```

This affects [`dbg!`](https://doc.rust-lang.org/std/macro.dbg.html), as well as ecosystem uses of stringify such as in [`anyhow::ensure!`](https://docs.rs/anyhow/1/anyhow/macro.ensure.html). Printing a `{ }` in today's heavily rustfmt'd world comes out looking jarring/sloppy.

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -263,14 +263,17 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         self.strsep(",", false, b, elts, op)
     }
 
-    fn maybe_print_comment(&mut self, pos: BytePos) {
+    fn maybe_print_comment(&mut self, pos: BytePos) -> bool {
+        let mut has_comment = false;
         while let Some(ref cmnt) = self.next_comment() {
             if cmnt.pos < pos {
+                has_comment = true;
                 self.print_comment(cmnt);
             } else {
                 break;
             }
         }
+        has_comment
     }
 
     fn print_comment(&mut self, cmnt: &Comment) {
@@ -570,7 +573,10 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         self.print_tts(tts, convert_dollar_crate);
         self.end();
         match delim {
-            DelimToken::Brace => self.bclose(span),
+            DelimToken::Brace => {
+                let empty = tts.is_empty();
+                self.bclose(span, empty);
+            }
             _ => {
                 let token_str = self.token_kind_to_string(&token::CloseDelim(delim));
                 self.word(token_str)
@@ -642,17 +648,20 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         self.end(); // Close the head-box.
     }
 
-    fn bclose_maybe_open(&mut self, span: rustc_span::Span, close_box: bool) {
-        self.maybe_print_comment(span.hi());
-        self.break_offset_if_not_bol(1, -(INDENT_UNIT as isize));
+    fn bclose_maybe_open(&mut self, span: rustc_span::Span, empty: bool, close_box: bool) {
+        let has_comment = self.maybe_print_comment(span.hi());
+        if !empty || has_comment {
+            self.break_offset_if_not_bol(1, -(INDENT_UNIT as isize));
+        }
         self.word("}");
         if close_box {
             self.end(); // Close the outer-box.
         }
     }
 
-    fn bclose(&mut self, span: rustc_span::Span) {
-        self.bclose_maybe_open(span, true)
+    fn bclose(&mut self, span: rustc_span::Span, empty: bool) {
+        let close_box = true;
+        self.bclose_maybe_open(span, empty, close_box)
     }
 
     fn break_offset_if_not_bol(&mut self, n: usize, off: isize) {
@@ -1196,7 +1205,8 @@ impl<'a> State<'a> {
                         for item in items {
                             self.print_item(item);
                         }
-                        self.bclose(item.span);
+                        let empty = item.attrs.is_empty() && items.is_empty();
+                        self.bclose(item.span, empty);
                     }
                     ModKind::Unloaded => {
                         self.s.word(";");
@@ -1216,7 +1226,8 @@ impl<'a> State<'a> {
                 }
                 self.bopen();
                 self.print_foreign_mod(nmod, &item.attrs);
-                self.bclose(item.span);
+                let empty = item.attrs.is_empty() && nmod.items.is_empty();
+                self.bclose(item.span, empty);
             }
             ast::ItemKind::GlobalAsm(ref asm) => {
                 self.head(visibility_qualified(&item.vis, "global_asm!"));
@@ -1291,7 +1302,8 @@ impl<'a> State<'a> {
                 for impl_item in items {
                     self.print_assoc_item(impl_item);
                 }
-                self.bclose(item.span);
+                let empty = item.attrs.is_empty() && items.is_empty();
+                self.bclose(item.span, empty);
             }
             ast::ItemKind::Trait(box ast::Trait {
                 is_auto,
@@ -1326,7 +1338,8 @@ impl<'a> State<'a> {
                 for trait_item in items {
                     self.print_assoc_item(trait_item);
                 }
-                self.bclose(item.span);
+                let empty = item.attrs.is_empty() && items.is_empty();
+                self.bclose(item.span, empty);
             }
             ast::ItemKind::TraitAlias(ref generics, ref bounds) => {
                 self.head("");
@@ -1410,7 +1423,8 @@ impl<'a> State<'a> {
             self.end();
             self.maybe_print_trailing_comment(v.span, None);
         }
-        self.bclose(span)
+        let empty = variants.is_empty();
+        self.bclose(span, empty)
     }
 
     crate fn print_visibility(&mut self, vis: &ast::Visibility) {
@@ -1441,20 +1455,24 @@ impl<'a> State<'a> {
     crate fn print_record_struct_body(&mut self, fields: &[ast::FieldDef], span: rustc_span::Span) {
         self.nbsp();
         self.bopen();
-        self.hardbreak_if_not_bol();
 
-        for field in fields {
+        let empty = fields.is_empty();
+        if !empty {
             self.hardbreak_if_not_bol();
-            self.maybe_print_comment(field.span.lo());
-            self.print_outer_attributes(&field.attrs);
-            self.print_visibility(&field.vis);
-            self.print_ident(field.ident.unwrap());
-            self.word_nbsp(":");
-            self.print_type(&field.ty);
-            self.s.word(",");
+
+            for field in fields {
+                self.hardbreak_if_not_bol();
+                self.maybe_print_comment(field.span.lo());
+                self.print_outer_attributes(&field.attrs);
+                self.print_visibility(&field.vis);
+                self.print_ident(field.ident.unwrap());
+                self.word_nbsp(":");
+                self.print_type(&field.ty);
+                self.s.word(",");
+            }
         }
 
-        self.bclose(span)
+        self.bclose(span, empty);
     }
 
     crate fn print_struct(
@@ -1633,7 +1651,8 @@ impl<'a> State<'a> {
             }
         }
 
-        self.bclose_maybe_open(blk.span, close_box);
+        let empty = attrs.is_empty() && blk.stmts.is_empty();
+        self.bclose_maybe_open(blk.span, empty, close_box);
         self.ann.post(self, AnnNode::Block(blk))
     }
 
@@ -2010,7 +2029,8 @@ impl<'a> State<'a> {
                 for arm in arms {
                     self.print_arm(arm);
                 }
-                self.bclose(expr.span);
+                let empty = attrs.is_empty() && arms.is_empty();
+                self.bclose(expr.span, empty);
             }
             ast::ExprKind::Closure(
                 capture_clause,

compiler/rustc_hir_pretty/src/lib.rs

@@ -2174,7 +2174,7 @@ impl<'a> State<'a> {
         match decl.output {
             hir::FnRetTy::Return(ref ty) => {
                 self.print_type(&ty);
-                self.maybe_print_comment(ty.span.lo())
+                self.maybe_print_comment(ty.span.lo());
             }
             hir::FnRetTy::DefaultReturn(..) => unreachable!(),
         }
@@ -2368,7 +2368,7 @@ impl<'a> State<'a> {
         self.end();
 
         if let hir::FnRetTy::Return(ref output) = decl.output {
-            self.maybe_print_comment(output.span.lo())
+            self.maybe_print_comment(output.span.lo());
         }
     }
 

src/test/pretty/ast-stmt-expr-attr.rs

@@ -1,6 +1,6 @@
 // pp-exact
 
-fn main() { }
+fn main() {}
 
 #[cfg(FALSE)]
 fn syntax() {
@@ -117,7 +117,7 @@ fn syntax() {
     let _ = #[attr] foo!(#! [attr]);
     let _ = #[attr] foo![];
     let _ = #[attr] foo![#! [attr]];
-    let _ = #[attr] foo! { };
+    let _ = #[attr] foo! {};
     let _ = #[attr] foo! { #! [attr] };
     let _ = #[attr] Foo{bar: baz,};
     let _ = #[attr] Foo{..foo};
@@ -135,7 +135,7 @@ fn syntax() {
         foo!();
 
         #[attr]
-        foo! { }
+        foo! {}
 
         #[attr]
         foo![];
@@ -170,6 +170,6 @@ fn syntax() {
     {
 
         #[attr]
-        foo! { }
+        foo! {}
     }
 }

src/test/pretty/attr-derive.rs

@@ -29,4 +29,4 @@ enum Enum {
     Qwerty,
 }
 
-fn main() { }
+fn main() {}

src/test/pretty/auto-trait.rs

@@ -2,8 +2,8 @@
 
 // pp-exact
 
-auto trait MyTrait { }
+auto trait MyTrait {}
 
-unsafe auto trait UnsafeMyTrait { }
+unsafe auto trait UnsafeMyTrait {}
 
-pub fn main() { }
+pub fn main() {}

src/test/pretty/block-comment-trailing-whitespace2.rs

@@ -1,9 +1,8 @@
 // compile-flags: --crate-type=lib
 
 // pp-exact
-fn f() {
-} /*
-  The next line should not be indented.
+fn f() {} /*
+          The next line should not be indented.
 
-  That one. It shouldn't have been indented.
-  */
+          That one. It shouldn't have been indented.
+          */

src/test/pretty/closure-reform-pretty.rs

@@ -3,14 +3,14 @@
 
 // pp-exact
 
-fn call_it(f: Box<FnMut(String) -> String>) { }
+fn call_it(f: Box<FnMut(String) -> String>) {}
 
-fn call_this<F>(f: F) where F: Fn(&str) + Send { }
+fn call_this<F>(f: F) where F: Fn(&str) + Send {}
 
-fn call_that<F>(f: F) where F: for<'a> Fn(&'a isize, &'a isize) -> isize { }
+fn call_that<F>(f: F) where F: for<'a> Fn(&'a isize, &'a isize) -> isize {}
 
-fn call_extern(f: fn() -> isize) { }
+fn call_extern(f: fn() -> isize) {}
 
-fn call_abid_extern(f: extern "C" fn() -> isize) { }
+fn call_abid_extern(f: extern "C" fn() -> isize) {}
 
-pub fn main() { }
+pub fn main() {}

src/test/pretty/disamb-stmt-expr.rs

@@ -7,4 +7,4 @@
 fn id<F>(f: F) -> isize where F: Fn() -> isize { f() }
 
 fn wsucc(_n: isize) -> isize { id(|| { 1 }) - 0 }
-fn main() { }
+fn main() {}

src/test/pretty/enum-variant-vis.rs

@@ -2,7 +2,7 @@
 
 // Check that the visibility is printed on an enum variant.
 
-fn main() { }
+fn main() {}
 
 #[cfg(FALSE)]
 enum Foo { pub V, }

src/test/pretty/example1.rs

@@ -1,3 +1,3 @@
 // pp-exact
 
-fn main() { }
+fn main() {}

src/test/pretty/example2.pp

@@ -1,3 +1,3 @@
 // pp-exact:example2.pp
 
-fn main() { }
+fn main() {}

src/test/pretty/example2.rs

@@ -1,3 +1,3 @@
 // pp-exact:example2.pp
 
-fn main() { }
+fn main() {}

src/test/pretty/expanded-and-path-remap-80832.pp

@@ -10,4 +10,4 @@
 // pp-exact:expanded-and-path-remap-80832.pp
 // compile-flags: --remap-path-prefix {{src-base}}=the/src
 
-fn main() { }
+fn main() {}

src/test/pretty/fn-return.rs

@@ -1,7 +1,7 @@
 // pp-exact
 
-// Check that `fn f() -> () { }` does not print as `fn f() { }`.
+// Check that `fn f() -> () {}` does not print as `fn f() {}`.
 
-fn f() -> () { }
+fn f() -> () {}
 
-fn main() { }
+fn main() {}

src/test/pretty/fn-types.rs

@@ -1,5 +1,5 @@
 // pp-exact
 
-fn from_foreign_fn(_x: fn()) { }
-fn from_stack_closure<F>(_x: F) where F: Fn() { }
-fn main() { }
+fn from_foreign_fn(_x: fn()) {}
+fn from_stack_closure<F>(_x: F) where F: Fn() {}
+fn main() {}

src/test/pretty/fn-variadic.rs

@@ -12,4 +12,4 @@ pub unsafe extern "C" fn bar(_: i32, mut ap: ...) -> usize {
     ap.arg::<usize>()
 }
 
-fn main() { }
+fn main() {}

src/test/pretty/if-attr.rs

@@ -4,34 +4,34 @@
 fn simple_attr() {
 
     #[attr]
-    if true { }
+    if true {}
 
     #[allow_warnings]
-    if true { }
+    if true {}
 }
 
 #[cfg(FALSE)]
 fn if_else_chain() {
 
     #[first_attr]
-    if true { } else if false { } else { }
+    if true {} else if false {} else {}
 }
 
 #[cfg(FALSE)]
 fn if_let() {
 
     #[attr]
-    if let Some(_) = Some(true) { }
+    if let Some(_) = Some(true) {}
 }
 
 #[cfg(FALSE)]
 fn let_attr_if() {
-    let _ = #[attr] if let _ = 0 { };
-    let _ = #[attr] if true { };
+    let _ = #[attr] if let _ = 0 {};
+    let _ = #[attr] if true {};
 
-    let _ = #[attr] if let _ = 0 { } else { };
-    let _ = #[attr] if true { } else { };
+    let _ = #[attr] if let _ = 0 {} else {};
+    let _ = #[attr] if true {} else {};
 }
 
 
-fn main() { }
+fn main() {}

src/test/pretty/issue-12590-a.rs

@@ -6,4 +6,4 @@
 #[path = "issue-12590-b.rs"]
 mod issue_12590_b;
 
-fn main() { }
+fn main() {}

src/test/pretty/issue-12590-c.pp

@@ -13,7 +13,7 @@
 #[path = "issue-12590-b.rs"]
 mod issue_12590_b {
 
-    fn b() { }
-    fn main() { }
+    fn b() {}
+    fn main() {}
 }
-fn main() { }
+fn main() {}

src/test/pretty/issue-12590-c.rs

@@ -7,4 +7,4 @@
 #[path = "issue-12590-b.rs"]
 mod issue_12590_b;
 
-fn main() { }
+fn main() {}