Auto merge of #66716 - derekdreery:debug_non_exhaustive, r=dtolnay

Implement `DebugStruct::non_exhaustive`.

This patch adds a function (finish_non_exhaustive) to add ellipsis before the closing brace when formatting using `DebugStruct`.

 ## Example

 ```rust
 #![feature(debug_non_exhaustive)]
 use std::fmt;

 struct Bar {
     bar: i32,
     hidden: f32,
 }

 impl fmt::Debug for Bar {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt.debug_struct("Bar")
            .field("bar", &self.bar)
            .non_exhaustive(true) // Show that some other field(s) exist.
            .finish()
     }
 }

 assert_eq!(
     format!("{:?}", Bar { bar: 10, hidden: 1.0 }),
     "Bar { bar: 10, .. }",
 );
 ```

3 files changed, 142 insertions, 0 deletions

diff --git a/src/libcore/fmt/builders.rs b/src/libcore/fmt/builders.rs
index 626eb1e862d..dd0f3ccb158 100644
--- a/src/libcore/fmt/builders.rs
+++ b/src/libcore/fmt/builders.rs
@@ -159,6 +159,62 @@ impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
         self
     }
 
+    /// Marks the struct as non-exhaustive, indicating to the reader that there are some other
+    /// fields that are not shown in the debug representation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(debug_non_exhaustive)]
+    /// use std::fmt;
+    ///
+    /// struct Bar {
+    ///     bar: i32,
+    ///     hidden: f32,
+    /// }
+    ///
+    /// impl fmt::Debug for Bar {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         fmt.debug_struct("Bar")
+    ///            .field("bar", &self.bar)
+    ///            .finish_non_exhaustive() // Show that some other field(s) exist.
+    ///     }
+    /// }
+    ///
+    /// assert_eq!(
+    ///     format!("{:?}", Bar { bar: 10, hidden: 1.0 }),
+    ///     "Bar { bar: 10, .. }",
+    /// );
+    /// ```
+    #[unstable(feature = "debug_non_exhaustive", issue = "67364")]
+    pub fn finish_non_exhaustive(&mut self) -> fmt::Result {
+        self.result = self.result.and_then(|_| {
+            // Draw non-exhaustive dots (`..`), and open brace if necessary (no fields).
+            if self.is_pretty() {
+                if !self.has_fields {
+                    self.fmt.write_str(" {\n")?;
+                }
+                let mut slot = None;
+                let mut state = Default::default();
+                let mut writer = PadAdapter::wrap(&mut self.fmt, &mut slot, &mut state);
+                writer.write_str("..\n")?;
+            } else {
+                if self.has_fields {
+                    self.fmt.write_str(", ..")?;
+                } else {
+                    self.fmt.write_str(" { ..")?;
+                }
+            }
+            if self.is_pretty() {
+                self.fmt.write_str("}")?
+            } else {
+                self.fmt.write_str(" }")?;
+            }
+            Ok(())
+        });
+        self.result
+    }
+
     /// Finishes output and returns any error encountered.
     ///
     /// # Examples
diff --git a/src/libcore/tests/fmt/builders.rs b/src/libcore/tests/fmt/builders.rs
index 90a9bccda15..129c121e8ce 100644
--- a/src/libcore/tests/fmt/builders.rs
+++ b/src/libcore/tests/fmt/builders.rs
@@ -93,6 +93,91 @@ mod debug_struct {
             format!("{:#?}", Bar)
         );
     }
+
+    #[test]
+    fn test_only_non_exhaustive() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_struct("Foo").finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!("Foo { .. }", format!("{:?}", Foo));
+        assert_eq!(
+            "Foo {
+    ..
+}",
+            format!("{:#?}", Foo)
+        );
+    }
+
+    #[test]
+    fn test_multiple_and_non_exhaustive() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_struct("Foo")
+                    .field("bar", &true)
+                    .field("baz", &format_args!("{}/{}", 10, 20))
+                    .finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!("Foo { bar: true, baz: 10/20, .. }", format!("{:?}", Foo));
+        assert_eq!(
+            "Foo {
+    bar: true,
+    baz: 10/20,
+    ..
+}",
+            format!("{:#?}", Foo)
+        );
+    }
+
+    #[test]
+    fn test_nested_non_exhaustive() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_struct("Foo")
+                    .field("bar", &true)
+                    .field("baz", &format_args!("{}/{}", 10, 20))
+                    .finish_non_exhaustive()
+            }
+        }
+
+        struct Bar;
+
+        impl fmt::Debug for Bar {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_struct("Bar")
+                    .field("foo", &Foo)
+                    .field("hello", &"world")
+                    .finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!(
+            "Bar { foo: Foo { bar: true, baz: 10/20, .. }, hello: \"world\", .. }",
+            format!("{:?}", Bar)
+        );
+        assert_eq!(
+            "Bar {
+    foo: Foo {
+        bar: true,
+        baz: 10/20,
+        ..
+    },
+    hello: \"world\",
+    ..
+}",
+            format!("{:#?}", Bar)
+        );
+    }
 }
 
 mod debug_tuple {
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 86cf6fc104c..8c034938c2b 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -5,6 +5,7 @@
 #![feature(core_private_bignum)]
 #![feature(core_private_diy_float)]
 #![feature(debug_map_key_value)]
+#![feature(debug_non_exhaustive)]
 #![feature(dec2flt)]
 #![feature(exact_size_is_empty)]
 #![feature(fixed_size_array)]