Auto merge of #59076 - dtolnay:comma, r=alexcrichton

Include trailing comma in multiline Debug representation

This PR changes the behavior of [`Formatter::debug_struct`](https://doc.rust-lang.org/std/fmt/struct.Formatter.html#method.debug_struct), [`debug_tuple`](https://doc.rust-lang.org/std/fmt/struct.Formatter.html#method.debug_tuple), [`debug_list`](https://doc.rust-lang.org/std/fmt/struct.Formatter.html#method.debug_list), [`debug_set`](https://doc.rust-lang.org/std/fmt/struct.Formatter.html#method.debug_set), and [`debug_map`](https://doc.rust-lang.org/std/fmt/struct.Formatter.html#method.debug_map) to render trailing commas in `{:#?}` mode, which is the dominant style in modern Rust code.

#### Before:

```console
Language {
    name: "Rust",
    trailing_commas: false
}
```

#### After:

```console
Language {
    name: "Rust",
    trailing_commas: true,
}
```

2 files changed, 58 insertions, 84 deletions

diff --git a/src/libcore/fmt/builders.rs b/src/libcore/fmt/builders.rs
index 45994c2b4f0..df3852973b8 100644
--- a/src/libcore/fmt/builders.rs
+++ b/src/libcore/fmt/builders.rs
@@ -11,7 +11,7 @@ impl<'a> PadAdapter<'a> {
         fmt.wrap_buf(move |buf| {
             *slot = Some(PadAdapter {
                 buf,
-                on_newline: false,
+                on_newline: true,
             });
             slot.as_mut().unwrap()
         })
@@ -128,22 +128,21 @@ impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
     #[stable(feature = "debug_builders", since = "1.2.0")]
     pub fn field(&mut self, name: &str, value: &dyn fmt::Debug) -> &mut DebugStruct<'a, 'b> {
         self.result = self.result.and_then(|_| {
-            let prefix = if self.has_fields {
-                ","
-            } else {
-                " {"
-            };
-
             if self.is_pretty() {
+                if !self.has_fields {
+                    self.fmt.write_str(" {\n")?;
+                }
                 let mut slot = None;
                 let mut writer = PadAdapter::wrap(&mut self.fmt, &mut slot);
-                writer.write_str(prefix)?;
-                writer.write_str("\n")?;
                 writer.write_str(name)?;
                 writer.write_str(": ")?;
-                value.fmt(&mut writer)
+                value.fmt(&mut writer)?;
+                writer.write_str(",\n")
             } else {
-                write!(self.fmt, "{} {}: ", prefix, name)?;
+                let prefix = if self.has_fields { ", " } else { " { " };
+                self.fmt.write_str(prefix)?;
+                self.fmt.write_str(name)?;
+                self.fmt.write_str(": ")?;
                 value.fmt(self.fmt)
             }
         });
@@ -184,7 +183,7 @@ impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
         if self.has_fields {
             self.result = self.result.and_then(|_| {
                 if self.is_pretty() {
-                    self.fmt.write_str("\n}")
+                    self.fmt.write_str("}")
                 } else {
                     self.fmt.write_str(" }")
                 }
@@ -275,21 +274,17 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
     #[stable(feature = "debug_builders", since = "1.2.0")]
     pub fn field(&mut self, value: &dyn fmt::Debug) -> &mut DebugTuple<'a, 'b> {
         self.result = self.result.and_then(|_| {
-            let (prefix, space) = if self.fields > 0 {
-                (",", " ")
-            } else {
-                ("(", "")
-            };
-
             if self.is_pretty() {
+                if self.fields == 0 {
+                    self.fmt.write_str("(\n")?;
+                }
                 let mut slot = None;
                 let mut writer = PadAdapter::wrap(&mut self.fmt, &mut slot);
-                writer.write_str(prefix)?;
-                writer.write_str("\n")?;
-                value.fmt(&mut writer)
+                value.fmt(&mut writer)?;
+                writer.write_str(",\n")
             } else {
+                let prefix = if self.fields == 0 { "(" } else { ", " };
                 self.fmt.write_str(prefix)?;
-                self.fmt.write_str(space)?;
                 value.fmt(self.fmt)
             }
         });
@@ -326,10 +321,7 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
     pub fn finish(&mut self) -> fmt::Result {
         if self.fields > 0 {
             self.result = self.result.and_then(|_| {
-                if self.is_pretty() {
-                    self.fmt.write_str("\n")?;
-                }
-                if self.fields == 1 && self.empty_name {
+                if self.fields == 1 && self.empty_name && !self.is_pretty() {
                     self.fmt.write_str(",")?;
                 }
                 self.fmt.write_str(")")
@@ -353,14 +345,13 @@ impl<'a, 'b: 'a> DebugInner<'a, 'b> {
     fn entry(&mut self, entry: &dyn fmt::Debug) {
         self.result = self.result.and_then(|_| {
             if self.is_pretty() {
+                if !self.has_fields {
+                    self.fmt.write_str("\n")?;
+                }
                 let mut slot = None;
                 let mut writer = PadAdapter::wrap(&mut self.fmt, &mut slot);
-                writer.write_str(if self.has_fields {
-                    ",\n"
-                } else {
-                    "\n"
-                })?;
-                entry.fmt(&mut writer)
+                entry.fmt(&mut writer)?;
+                writer.write_str(",\n")
             } else {
                 if self.has_fields {
                     self.fmt.write_str(", ")?
@@ -372,15 +363,6 @@ impl<'a, 'b: 'a> DebugInner<'a, 'b> {
         self.has_fields = true;
     }
 
-    pub fn finish(&mut self) {
-        let prefix = if self.is_pretty() && self.has_fields {
-            "\n"
-        } else {
-            ""
-        };
-        self.result = self.result.and_then(|_| self.fmt.write_str(prefix));
-    }
-
     fn is_pretty(&self) -> bool {
         self.fmt.alternate()
     }
@@ -421,7 +403,7 @@ pub struct DebugSet<'a, 'b: 'a> {
 }
 
 pub fn debug_set_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>) -> DebugSet<'a, 'b> {
-    let result = write!(fmt, "{{");
+    let result = fmt.write_str("{");
     DebugSet {
         inner: DebugInner {
             fmt,
@@ -519,7 +501,6 @@ impl<'a, 'b: 'a> DebugSet<'a, 'b> {
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
     pub fn finish(&mut self) -> fmt::Result {
-        self.inner.finish();
         self.inner.result.and_then(|_| self.inner.fmt.write_str("}"))
     }
 }
@@ -559,7 +540,7 @@ pub struct DebugList<'a, 'b: 'a> {
 }
 
 pub fn debug_list_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>) -> DebugList<'a, 'b> {
-    let result = write!(fmt, "[");
+    let result = fmt.write_str("[");
     DebugList {
         inner: DebugInner {
             fmt,
@@ -657,7 +638,6 @@ impl<'a, 'b: 'a> DebugList<'a, 'b> {
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
     pub fn finish(&mut self) -> fmt::Result {
-        self.inner.finish();
         self.inner.result.and_then(|_| self.inner.fmt.write_str("]"))
     }
 }
@@ -699,7 +679,7 @@ pub struct DebugMap<'a, 'b: 'a> {
 }
 
 pub fn debug_map_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>) -> DebugMap<'a, 'b> {
-    let result = write!(fmt, "{{");
+    let result = fmt.write_str("{");
     DebugMap {
         fmt,
         result,
@@ -734,16 +714,15 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
     pub fn entry(&mut self, key: &dyn fmt::Debug, value: &dyn fmt::Debug) -> &mut DebugMap<'a, 'b> {
         self.result = self.result.and_then(|_| {
             if self.is_pretty() {
+                if !self.has_fields {
+                    self.fmt.write_str("\n")?;
+                }
                 let mut slot = None;
                 let mut writer = PadAdapter::wrap(&mut self.fmt, &mut slot);
-                writer.write_str(if self.has_fields {
-                    ",\n"
-                } else {
-                    "\n"
-                })?;
                 key.fmt(&mut writer)?;
                 writer.write_str(": ")?;
-                value.fmt(&mut writer)
+                value.fmt(&mut writer)?;
+                writer.write_str(",\n")
             } else {
                 if self.has_fields {
                     self.fmt.write_str(", ")?
@@ -818,12 +797,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
     pub fn finish(&mut self) -> fmt::Result {
-        let prefix = if self.is_pretty() && self.has_fields {
-            "\n"
-        } else {
-            ""
-        };
-        self.result.and_then(|_| write!(self.fmt, "{}}}", prefix))
+        self.result.and_then(|_| self.fmt.write_str("}"))
     }
 
     fn is_pretty(&self) -> bool {
diff --git a/src/libcore/tests/fmt/builders.rs b/src/libcore/tests/fmt/builders.rs
index fd7192cc151..e4b75fe1fa2 100644
--- a/src/libcore/tests/fmt/builders.rs
+++ b/src/libcore/tests/fmt/builders.rs
@@ -30,7 +30,7 @@ mod debug_struct {
         assert_eq!("Foo { bar: true }", format!("{:?}", Foo));
         assert_eq!(
 "Foo {
-    bar: true
+    bar: true,
 }",
                    format!("{:#?}", Foo));
     }
@@ -52,7 +52,7 @@ mod debug_struct {
         assert_eq!(
 "Foo {
     bar: true,
-    baz: 10/20
+    baz: 10/20,
 }",
                    format!("{:#?}", Foo));
     }
@@ -87,9 +87,9 @@ mod debug_struct {
 "Bar {
     foo: Foo {
         bar: true,
-        baz: 10/20
+        baz: 10/20,
     },
-    hello: \"world\"
+    hello: \"world\",
 }",
                    format!("{:#?}", Bar));
     }
@@ -127,7 +127,7 @@ mod debug_tuple {
         assert_eq!("Foo(true)", format!("{:?}", Foo));
         assert_eq!(
 "Foo(
-    true
+    true,
 )",
                    format!("{:#?}", Foo));
     }
@@ -149,7 +149,7 @@ mod debug_tuple {
         assert_eq!(
 "Foo(
     true,
-    10/20
+    10/20,
 )",
                    format!("{:#?}", Foo));
     }
@@ -184,9 +184,9 @@ mod debug_tuple {
 "Bar(
     Foo(
         true,
-        10/20
+        10/20,
     ),
-    \"world\"
+    \"world\",
 )",
                    format!("{:#?}", Bar));
     }
@@ -224,7 +224,7 @@ mod debug_map {
         assert_eq!("{\"bar\": true}", format!("{:?}", Foo));
         assert_eq!(
 "{
-    \"bar\": true
+    \"bar\": true,
 }",
                    format!("{:#?}", Foo));
     }
@@ -246,7 +246,7 @@ mod debug_map {
         assert_eq!(
 "{
     \"bar\": true,
-    10: 10/20
+    10: 10/20,
 }",
                    format!("{:#?}", Foo));
     }
@@ -282,12 +282,12 @@ mod debug_map {
 "{
     \"foo\": {
         \"bar\": true,
-        10: 10/20
+        10: 10/20,
     },
     {
         \"bar\": true,
-        10: 10/20
-    }: \"world\"
+        10: 10/20,
+    }: \"world\",
 }",
                    format!("{:#?}", Bar));
     }
@@ -325,7 +325,7 @@ mod debug_set {
         assert_eq!("{true}", format!("{:?}", Foo));
         assert_eq!(
 "{
-    true
+    true,
 }",
                    format!("{:#?}", Foo));
     }
@@ -347,7 +347,7 @@ mod debug_set {
         assert_eq!(
 "{
     true,
-    10/20
+    10/20,
 }",
                    format!("{:#?}", Foo));
     }
@@ -382,9 +382,9 @@ mod debug_set {
 "{
     {
         true,
-        10/20
+        10/20,
     },
-    \"world\"
+    \"world\",
 }",
                    format!("{:#?}", Bar));
     }
@@ -422,7 +422,7 @@ mod debug_list {
         assert_eq!("[true]", format!("{:?}", Foo));
         assert_eq!(
 "[
-    true
+    true,
 ]",
                    format!("{:#?}", Foo));
     }
@@ -444,7 +444,7 @@ mod debug_list {
         assert_eq!(
 "[
     true,
-    10/20
+    10/20,
 ]",
                    format!("{:#?}", Foo));
     }
@@ -479,9 +479,9 @@ mod debug_list {
 "[
     [
         true,
-        10/20
+        10/20,
     ],
-    \"world\"
+    \"world\",
 ]",
                    format!("{:#?}", Bar));
     }
@@ -513,31 +513,31 @@ fn test_formatting_parameters_are_forwarded() {
     assert_eq!(format!("{:#03?}", struct_), "
 Foo {
     bar: 1024,
-    baz: 007
+    baz: 007,
 }
     ".trim());
     assert_eq!(format!("{:#03?}", tuple), "
 (
     1024,
-    007
+    007,
 )
     ".trim());
     assert_eq!(format!("{:#03?}", list), "
 [
     1024,
-    007
+    007,
 ]
     ".trim());
     assert_eq!(format!("{:#03?}", map), r#"
 {
     "bar": 1024,
-    "baz": 007
+    "baz": 007,
 }
     "#.trim());
     assert_eq!(format!("{:#03?}", set), "
 {
     007,
-    1024
+    1024,
 }
     ".trim());
 }