Rollup merge of #127945 - tgross35:debug-more-non-exhaustive, r=Noratrieb

Implement `debug_more_non_exhaustive`

This implements the ACP at https://github.com/rust-lang/libs-team/issues/248, adding `.finish_non_exhaustive()` for `DebugTuple`, `DebugSet`, `DebugList`, and `DebugMap`.

Also used this as an opportunity to make some documentation and tests more readable by using raw strings instead of escaped quotes.

Tracking issue: https://github.com/rust-lang/rust/issues/127942

3 files changed, 565 insertions, 46 deletions

diff --git a/library/core/src/fmt/builders.rs b/library/core/src/fmt/builders.rs
index 467fa17a6f3..c7c462a4df1 100644
--- a/library/core/src/fmt/builders.rs
+++ b/library/core/src/fmt/builders.rs
@@ -78,7 +78,7 @@ impl fmt::Write for PadAdapter<'_, '_> {
 ///
 /// assert_eq!(
 ///     format!("{:?}", Foo { bar: 10, baz: "Hello World".to_string() }),
-///     "Foo { bar: 10, baz: \"Hello World\" }",
+///     r#"Foo { bar: 10, baz: "Hello World" }"#,
 /// );
 /// ```
 #[must_use = "must eventually call `finish()` on Debug builders"]
@@ -125,7 +125,7 @@ impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
     ///
     /// assert_eq!(
     ///     format!("{:?}", Bar { bar: 10, another: "Hello World".to_string() }),
-    ///     "Bar { bar: 10, another: \"Hello World\", nonexistent_field: 1 }",
+    ///     r#"Bar { bar: 10, another: "Hello World", nonexistent_field: 1 }"#,
     /// );
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
@@ -237,7 +237,7 @@ impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
     ///
     /// assert_eq!(
     ///     format!("{:?}", Bar { bar: 10, baz: "Hello World".to_string() }),
-    ///     "Bar { bar: 10, baz: \"Hello World\" }",
+    ///     r#"Bar { bar: 10, baz: "Hello World" }"#,
     /// );
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
@@ -280,7 +280,7 @@ impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
 ///
 /// assert_eq!(
 ///     format!("{:?}", Foo(10, "Hello World".to_string())),
-///     "Foo(10, \"Hello World\")",
+///     r#"Foo(10, "Hello World")"#,
 /// );
 /// ```
 #[must_use = "must eventually call `finish()` on Debug builders"]
@@ -322,7 +322,7 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
     ///
     /// assert_eq!(
     ///     format!("{:?}", Foo(10, "Hello World".to_string())),
-    ///     "Foo(10, \"Hello World\")",
+    ///     r#"Foo(10, "Hello World")"#,
     /// );
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
@@ -360,6 +360,51 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
         self
     }
 
+    /// Marks the tuple 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_more_non_exhaustive)]
+    ///
+    /// use std::fmt;
+    ///
+    /// struct Foo(i32, String);
+    ///
+    /// impl fmt::Debug for Foo {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         fmt.debug_tuple("Foo")
+    ///            .field(&self.0)
+    ///            .finish_non_exhaustive() // Show that some other field(s) exist.
+    ///     }
+    /// }
+    ///
+    /// assert_eq!(
+    ///     format!("{:?}", Foo(10, "secret!".to_owned())),
+    ///     "Foo(10, ..)",
+    /// );
+    /// ```
+    #[unstable(feature = "debug_more_non_exhaustive", issue = "127942")]
+    pub fn finish_non_exhaustive(&mut self) -> fmt::Result {
+        self.result = self.result.and_then(|_| {
+            if self.fields > 0 {
+                if self.is_pretty() {
+                    let mut slot = None;
+                    let mut state = Default::default();
+                    let mut writer = PadAdapter::wrap(self.fmt, &mut slot, &mut state);
+                    writer.write_str("..\n")?;
+                    self.fmt.write_str(")")
+                } else {
+                    self.fmt.write_str(", ..)")
+                }
+            } else {
+                self.fmt.write_str("(..)")
+            }
+        });
+        self.result
+    }
+
     /// Finishes output and returns any error encountered.
     ///
     /// # Examples
@@ -381,7 +426,7 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
     ///
     /// assert_eq!(
     ///     format!("{:?}", Foo(10, "Hello World".to_string())),
-    ///     "Foo(10, \"Hello World\")",
+    ///     r#"Foo(10, "Hello World")"#,
     /// );
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
@@ -555,6 +600,56 @@ impl<'a, 'b: 'a> DebugSet<'a, 'b> {
         self
     }
 
+    /// Marks the set as non-exhaustive, indicating to the reader that there are some other
+    /// elements that are not shown in the debug representation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(debug_more_non_exhaustive)]
+    ///
+    /// use std::fmt;
+    ///
+    /// struct Foo(Vec<i32>);
+    ///
+    /// impl fmt::Debug for Foo {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         // Print at most two elements, abbreviate the rest
+    ///         let mut f = fmt.debug_set();
+    ///         let mut f = f.entries(self.0.iter().take(2));
+    ///         if self.0.len() > 2 {
+    ///             f.finish_non_exhaustive()
+    ///         } else {
+    ///             f.finish()
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// assert_eq!(
+    ///     format!("{:?}", Foo(vec![1, 2, 3, 4])),
+    ///     "{1, 2, ..}",
+    /// );
+    /// ```
+    #[unstable(feature = "debug_more_non_exhaustive", issue = "127942")]
+    pub fn finish_non_exhaustive(&mut self) -> fmt::Result {
+        self.inner.result = self.inner.result.and_then(|_| {
+            if self.inner.has_fields {
+                if self.inner.is_pretty() {
+                    let mut slot = None;
+                    let mut state = Default::default();
+                    let mut writer = PadAdapter::wrap(self.inner.fmt, &mut slot, &mut state);
+                    writer.write_str("..\n")?;
+                    self.inner.fmt.write_str("}")
+                } else {
+                    self.inner.fmt.write_str(", ..}")
+                }
+            } else {
+                self.inner.fmt.write_str("..}")
+            }
+        });
+        self.inner.result
+    }
+
     /// Finishes output and returns any error encountered.
     ///
     /// # Examples
@@ -699,6 +794,55 @@ impl<'a, 'b: 'a> DebugList<'a, 'b> {
         self
     }
 
+    /// Marks the list as non-exhaustive, indicating to the reader that there are some other
+    /// elements that are not shown in the debug representation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(debug_more_non_exhaustive)]
+    ///
+    /// use std::fmt;
+    ///
+    /// struct Foo(Vec<i32>);
+    ///
+    /// impl fmt::Debug for Foo {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         // Print at most two elements, abbreviate the rest
+    ///         let mut f = fmt.debug_list();
+    ///         let mut f = f.entries(self.0.iter().take(2));
+    ///         if self.0.len() > 2 {
+    ///             f.finish_non_exhaustive()
+    ///         } else {
+    ///             f.finish()
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// assert_eq!(
+    ///     format!("{:?}", Foo(vec![1, 2, 3, 4])),
+    ///     "[1, 2, ..]",
+    /// );
+    /// ```
+    #[unstable(feature = "debug_more_non_exhaustive", issue = "127942")]
+    pub fn finish_non_exhaustive(&mut self) -> fmt::Result {
+        self.inner.result.and_then(|_| {
+            if self.inner.has_fields {
+                if self.inner.is_pretty() {
+                    let mut slot = None;
+                    let mut state = Default::default();
+                    let mut writer = PadAdapter::wrap(self.inner.fmt, &mut slot, &mut state);
+                    writer.write_str("..\n")?;
+                    self.inner.fmt.write_str("]")
+                } else {
+                    self.inner.fmt.write_str(", ..]")
+                }
+            } else {
+                self.inner.fmt.write_str("..]")
+            }
+        })
+    }
+
     /// Finishes output and returns any error encountered.
     ///
     /// # Examples
@@ -750,7 +894,7 @@ impl<'a, 'b: 'a> DebugList<'a, 'b> {
 ///
 /// assert_eq!(
 ///     format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
-///     "{\"A\": 10, \"B\": 11}",
+///     r#"{"A": 10, "B": 11}"#,
 /// );
 /// ```
 #[must_use = "must eventually call `finish()` on Debug builders"]
@@ -790,7 +934,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
     ///
     /// assert_eq!(
     ///     format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
-    ///     "{\"whole\": [(\"A\", 10), (\"B\", 11)]}",
+    ///     r#"{"whole": [("A", 10), ("B", 11)]}"#,
     /// );
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
@@ -826,7 +970,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
     ///
     /// assert_eq!(
     ///     format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
-    ///     "{\"whole\": [(\"A\", 10), (\"B\", 11)]}",
+    ///     r#"{"whole": [("A", 10), ("B", 11)]}"#,
     /// );
     /// ```
     #[stable(feature = "debug_map_key_value", since = "1.42.0")]
@@ -902,7 +1046,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
     ///
     /// assert_eq!(
     ///     format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
-    ///     "{\"whole\": [(\"A\", 10), (\"B\", 11)]}",
+    ///     r#"{"whole": [("A", 10), ("B", 11)]}"#,
     /// );
     /// ```
     #[stable(feature = "debug_map_key_value", since = "1.42.0")]
@@ -960,7 +1104,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
     ///
     /// assert_eq!(
     ///     format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
-    ///     "{\"A\": 10, \"B\": 11}",
+    ///     r#"{"A": 10, "B": 11}"#,
     /// );
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
@@ -976,6 +1120,62 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
         self
     }
 
+    /// Marks the map as non-exhaustive, indicating to the reader that there are some other
+    /// entries that are not shown in the debug representation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(debug_more_non_exhaustive)]
+    ///
+    /// use std::fmt;
+    ///
+    /// struct Foo(Vec<(String, i32)>);
+    ///
+    /// impl fmt::Debug for Foo {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         // Print at most two elements, abbreviate the rest
+    ///         let mut f = fmt.debug_map();
+    ///         let mut f = f.entries(self.0.iter().take(2).map(|&(ref k, ref v)| (k, v)));
+    ///         if self.0.len() > 2 {
+    ///             f.finish_non_exhaustive()
+    ///         } else {
+    ///             f.finish()
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// assert_eq!(
+    ///     format!("{:?}", Foo(vec![
+    ///         ("A".to_string(), 10),
+    ///         ("B".to_string(), 11),
+    ///         ("C".to_string(), 12),
+    ///     ])),
+    ///     r#"{"A": 10, "B": 11, ..}"#,
+    /// );
+    /// ```
+    #[unstable(feature = "debug_more_non_exhaustive", issue = "127942")]
+    pub fn finish_non_exhaustive(&mut self) -> fmt::Result {
+        self.result = self.result.and_then(|_| {
+            assert!(!self.has_key, "attempted to finish a map with a partial entry");
+
+            if self.has_fields {
+                if self.is_pretty() {
+                    let mut slot = None;
+                    let mut state = Default::default();
+                    let mut writer = PadAdapter::wrap(self.fmt, &mut slot, &mut state);
+                    writer.write_str("..\n")?;
+                    self.fmt.write_str("}")
+                } else {
+                    self.fmt.write_str(", ..}")
+                }
+            } else {
+                self.fmt.write_str("..}")
+            }
+        });
+        self.result
+    }
+
     /// Finishes output and returns any error encountered.
     ///
     /// # Panics
@@ -1000,7 +1200,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
     ///
     /// assert_eq!(
     ///     format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
-    ///     "{\"A\": 10, \"B\": 11}",
+    ///     r#"{"A": 10, "B": 11}"#,
     /// );
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
diff --git a/library/core/tests/fmt/builders.rs b/library/core/tests/fmt/builders.rs
index 2bdc334b7c0..ba4801f5912 100644
--- a/library/core/tests/fmt/builders.rs
+++ b/library/core/tests/fmt/builders.rs
@@ -79,23 +79,23 @@ mod debug_struct {
         }
 
         assert_eq!(
-            "Bar { foo: Foo { bar: true, baz: 10/20 }, hello: \"world\" }",
+            r#"Bar { foo: Foo { bar: true, baz: 10/20 }, hello: "world" }"#,
             format!("{Bar:?}")
         );
         assert_eq!(
-            "Bar {
+            r#"Bar {
     foo: Foo {
         bar: true,
         baz: 10/20,
     },
-    hello: \"world\",
-}",
+    hello: "world",
+}"#,
             format!("{Bar:#?}")
         );
     }
 
     #[test]
-    fn test_only_non_exhaustive() {
+    fn test_empty_non_exhaustive() {
         struct Foo;
 
         impl fmt::Debug for Foo {
@@ -157,19 +157,19 @@ mod debug_struct {
         }
 
         assert_eq!(
-            "Bar { foo: Foo { bar: true, baz: 10/20, .. }, hello: \"world\", .. }",
+            r#"Bar { foo: Foo { bar: true, baz: 10/20, .. }, hello: "world", .. }"#,
             format!("{Bar:?}")
         );
         assert_eq!(
-            "Bar {
+            r#"Bar {
     foo: Foo {
         bar: true,
         baz: 10/20,
         ..
     },
-    hello: \"world\",
+    hello: "world",
     ..
-}",
+}"#,
             format!("{Bar:#?}")
         );
     }
@@ -249,15 +249,89 @@ mod debug_tuple {
             }
         }
 
-        assert_eq!("Bar(Foo(true, 10/20), \"world\")", format!("{Bar:?}"));
+        assert_eq!(r#"Bar(Foo(true, 10/20), "world")"#, format!("{Bar:?}"));
         assert_eq!(
-            "Bar(
+            r#"Bar(
     Foo(
         true,
         10/20,
     ),
-    \"world\",
+    "world",
+)"#,
+            format!("{Bar:#?}")
+        );
+    }
+
+    #[test]
+    fn test_empty_non_exhaustive() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_tuple("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_tuple("Foo")
+                    .field(&true)
+                    .field(&format_args!("{}/{}", 10, 20))
+                    .finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!("Foo(true, 10/20, ..)", format!("{Foo:?}"));
+        assert_eq!(
+            "Foo(
+    true,
+    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_tuple("Foo")
+                    .field(&true)
+                    .field(&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_tuple("Bar").field(&Foo).field(&"world").finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!(r#"Bar(Foo(true, 10/20, ..), "world", ..)"#, format!("{Bar:?}"));
+        assert_eq!(
+            r#"Bar(
+    Foo(
+        true,
+        10/20,
+        ..
+    ),
+    "world",
+    ..
+)"#,
             format!("{Bar:#?}")
         );
     }
@@ -301,11 +375,11 @@ mod debug_map {
         assert_eq!(format!("{Entry:?}"), format!("{KeyValue:?}"));
         assert_eq!(format!("{Entry:#?}"), format!("{KeyValue:#?}"));
 
-        assert_eq!("{\"bar\": true}", format!("{Entry:?}"));
+        assert_eq!(r#"{"bar": true}"#, format!("{Entry:?}"));
         assert_eq!(
-            "{
-    \"bar\": true,
-}",
+            r#"{
+    "bar": true,
+}"#,
             format!("{Entry:#?}")
         );
     }
@@ -339,12 +413,12 @@ mod debug_map {
         assert_eq!(format!("{Entry:?}"), format!("{KeyValue:?}"));
         assert_eq!(format!("{Entry:#?}"), format!("{KeyValue:#?}"));
 
-        assert_eq!("{\"bar\": true, 10: 10/20}", format!("{Entry:?}"));
+        assert_eq!(r#"{"bar": true, 10: 10/20}"#, format!("{Entry:?}"));
         assert_eq!(
-            "{
-    \"bar\": true,
+            r#"{
+    "bar": true,
     10: 10/20,
-}",
+}"#,
             format!("{Entry:#?}")
         );
     }
@@ -371,21 +445,20 @@ mod debug_map {
         }
 
         assert_eq!(
-            "{\"foo\": {\"bar\": true, 10: 10/20}, \
-                    {\"bar\": true, 10: 10/20}: \"world\"}",
+            r#"{"foo": {"bar": true, 10: 10/20}, {"bar": true, 10: 10/20}: "world"}"#,
             format!("{Bar:?}")
         );
         assert_eq!(
-            "{
-    \"foo\": {
-        \"bar\": true,
+            r#"{
+    "foo": {
+        "bar": true,
         10: 10/20,
     },
     {
-        \"bar\": true,
+        "bar": true,
         10: 10/20,
-    }: \"world\",
-}",
+    }: "world",
+}"#,
             format!("{Bar:#?}")
         );
     }
@@ -471,6 +544,103 @@ mod debug_map {
 
         let _ = format!("{Foo:?}");
     }
+
+    #[test]
+    fn test_empty_non_exhaustive() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_map().finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!("{..}", format!("{Foo:?}"));
+        assert_eq!("{..}", format!("{Foo:#?}"));
+    }
+
+    #[test]
+    fn test_multiple_and_non_exhaustive() {
+        struct Entry;
+
+        impl fmt::Debug for Entry {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_map()
+                    .entry(&"bar", &true)
+                    .entry(&10, &format_args!("{}/{}", 10, 20))
+                    .finish_non_exhaustive()
+            }
+        }
+
+        struct KeyValue;
+
+        impl fmt::Debug for KeyValue {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_map()
+                    .key(&"bar")
+                    .value(&true)
+                    .key(&10)
+                    .value(&format_args!("{}/{}", 10, 20))
+                    .finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!(format!("{Entry:?}"), format!("{KeyValue:?}"));
+        assert_eq!(format!("{Entry:#?}"), format!("{KeyValue:#?}"));
+
+        assert_eq!(r#"{"bar": true, 10: 10/20, ..}"#, format!("{Entry:?}"));
+        assert_eq!(
+            r#"{
+    "bar": true,
+    10: 10/20,
+    ..
+}"#,
+            format!("{Entry:#?}")
+        );
+    }
+
+    #[test]
+    fn test_nested_non_exhaustive() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_map()
+                    .entry(&"bar", &true)
+                    .entry(&10, &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_map().entry(&"foo", &Foo).entry(&Foo, &"world").finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!(
+            r#"{"foo": {"bar": true, 10: 10/20, ..}, {"bar": true, 10: 10/20, ..}: "world", ..}"#,
+            format!("{Bar:?}")
+        );
+        assert_eq!(
+            r#"{
+    "foo": {
+        "bar": true,
+        10: 10/20,
+        ..
+    },
+    {
+        "bar": true,
+        10: 10/20,
+        ..
+    }: "world",
+    ..
+}"#,
+            format!("{Bar:#?}")
+        );
+    }
 }
 
 mod debug_set {
@@ -547,15 +717,89 @@ mod debug_set {
             }
         }
 
-        assert_eq!("{{true, 10/20}, \"world\"}", format!("{Bar:?}"));
+        assert_eq!(r#"{{true, 10/20}, "world"}"#, format!("{Bar:?}"));
         assert_eq!(
-            "{
+            r#"{
     {
         true,
         10/20,
     },
-    \"world\",
+    "world",
+}"#,
+            format!("{Bar:#?}")
+        );
+    }
+
+    #[test]
+    fn test_empty_non_exhaustive() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_set().finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!("{..}", format!("{Foo:?}"));
+        assert_eq!("{..}", 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_set()
+                    .entry(&true)
+                    .entry(&format_args!("{}/{}", 10, 20))
+                    .finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!("{true, 10/20, ..}", format!("{Foo:?}"));
+        assert_eq!(
+            "{
+    true,
+    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_set()
+                    .entry(&true)
+                    .entry(&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_set().entry(&Foo).entry(&"world").finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!(r#"{{true, 10/20, ..}, "world", ..}"#, format!("{Bar:?}"));
+        assert_eq!(
+            r#"{
+    {
+        true,
+        10/20,
+        ..
+    },
+    "world",
+    ..
+}"#,
             format!("{Bar:#?}")
         );
     }
@@ -635,15 +879,89 @@ mod debug_list {
             }
         }
 
-        assert_eq!("[[true, 10/20], \"world\"]", format!("{Bar:?}"));
+        assert_eq!(r#"[[true, 10/20], "world"]"#, format!("{Bar:?}"));
         assert_eq!(
-            "[
+            r#"[
     [
         true,
         10/20,
     ],
-    \"world\",
+    "world",
+]"#,
+            format!("{Bar:#?}")
+        );
+    }
+
+    #[test]
+    fn test_empty_non_exhaustive() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_list().finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!("[..]", format!("{Foo:?}"));
+        assert_eq!("[..]", format!("{Foo:#?}"));
+    }
+
+    #[test]
+    fn test_multiple_non_exhaustive() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_list()
+                    .entry(&true)
+                    .entry(&format_args!("{}/{}", 10, 20))
+                    .finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!("[true, 10/20, ..]", format!("{Foo:?}"));
+        assert_eq!(
+            "[
+    true,
+    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_list()
+                    .entry(&true)
+                    .entry(&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_list().entry(&Foo).entry(&"world").finish_non_exhaustive()
+            }
+        }
+
+        assert_eq!(r#"[[true, 10/20, ..], "world", ..]"#, format!("{Bar:?}"));
+        assert_eq!(
+            r#"[
+    [
+        true,
+        10/20,
+        ..
+    ],
+    "world",
+    ..
+]"#,
             format!("{Bar:#?}")
         );
     }
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index 2f0d03abed3..073429c7628 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -43,6 +43,7 @@
 #![feature(core_io_borrowed_buf)]
 #![feature(core_private_bignum)]
 #![feature(core_private_diy_float)]
+#![feature(debug_more_non_exhaustive)]
 #![feature(dec2flt)]
 #![feature(duration_constants)]
 #![feature(duration_constructors)]