Auto merge of #23162 - sfackler:debug-builders, r=alexcrichton

I've made some minor changes from the implementation attached to the RFC to try to minimize codegen. The methods now take `&Debug` trait objects rather than being parameterized and there are inlined stub methods that call to non-inlined methods to do the work.

r? @alexcrichton 

cc @huonw for the `derive(Debug)` changes.

src/libcore/fmt/builders.rs

@@ -0,0 +1,301 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use prelude::*;
+use fmt::{self, Write, FlagV1};
+
+struct PadAdapter<'a, 'b: 'a> {
+    fmt: &'a mut fmt::Formatter<'b>,
+    on_newline: bool,
+}
+
+impl<'a, 'b: 'a> PadAdapter<'a, 'b> {
+    fn new(fmt: &'a mut fmt::Formatter<'b>) -> PadAdapter<'a, 'b> {
+        PadAdapter {
+            fmt: fmt,
+            on_newline: false,
+        }
+    }
+}
+
+impl<'a, 'b: 'a> fmt::Write for PadAdapter<'a, 'b> {
+    fn write_str(&mut self, mut s: &str) -> fmt::Result {
+        while !s.is_empty() {
+            if self.on_newline {
+                try!(self.fmt.write_str("    "));
+            }
+
+            let split = match s.find('\n') {
+                Some(pos) => {
+                    self.on_newline = true;
+                    pos + 1
+                }
+                None => {
+                    self.on_newline = false;
+                    s.len()
+                }
+            };
+            try!(self.fmt.write_str(&s[..split]));
+            s = &s[split..];
+        }
+
+        Ok(())
+    }
+}
+
+/// A struct to help with `fmt::Debug` implementations.
+///
+/// Constructed by the `Formatter::debug_struct` method.
+#[must_use]
+pub struct DebugStruct<'a, 'b: 'a> {
+    fmt: &'a mut fmt::Formatter<'b>,
+    result: fmt::Result,
+    has_fields: bool,
+}
+
+pub fn debug_struct_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>, name: &str)
+                                -> DebugStruct<'a, 'b> {
+    let result = fmt.write_str(name);
+    DebugStruct {
+        fmt: fmt,
+        result: result,
+        has_fields: false,
+    }
+}
+
+impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
+    /// Adds a new field to the generated struct output.
+    #[unstable(feature = "debug_builders", reason = "method was just created")]
+    pub fn field(mut self, name: &str, value: &fmt::Debug) -> DebugStruct<'a, 'b> {
+        self.result = self.result.and_then(|_| {
+            let prefix = if self.has_fields {
+                ","
+            } else {
+                " {"
+            };
+
+            if self.is_pretty() {
+                let mut writer = PadAdapter::new(self.fmt);
+                fmt::write(&mut writer, format_args!("{}\n{}: {:#?}", prefix, name, value))
+            } else {
+                write!(self.fmt, "{} {}: {:?}", prefix, name, value)
+            }
+        });
+
+        self.has_fields = true;
+        self
+    }
+
+    /// Consumes the `DebugStruct`, finishing output and returning any error
+    /// encountered.
+    #[unstable(feature = "debug_builders", reason = "method was just created")]
+    pub fn finish(mut self) -> fmt::Result {
+        if self.has_fields {
+            self.result = self.result.and_then(|_| {
+                if self.is_pretty() {
+                    self.fmt.write_str("\n}")
+                } else {
+                    self.fmt.write_str(" }")
+                }
+            });
+        }
+        self.result
+    }
+
+    fn is_pretty(&self) -> bool {
+        self.fmt.flags() & (1 << (FlagV1::Alternate as usize)) != 0
+    }
+}
+
+/// A struct to help with `fmt::Debug` implementations.
+///
+/// Constructed by the `Formatter::debug_tuple` method.
+#[must_use]
+pub struct DebugTuple<'a, 'b: 'a> {
+    fmt: &'a mut fmt::Formatter<'b>,
+    result: fmt::Result,
+    has_fields: bool,
+}
+
+pub fn debug_tuple_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>, name: &str) -> DebugTuple<'a, 'b> {
+    let result = fmt.write_str(name);
+    DebugTuple {
+        fmt: fmt,
+        result: result,
+        has_fields: false,
+    }
+}
+
+impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
+    /// Adds a new field to the generated tuple struct output.
+    #[unstable(feature = "debug_builders", reason = "method was just created")]
+    pub fn field(mut self, value: &fmt::Debug) -> DebugTuple<'a, 'b> {
+        self.result = self.result.and_then(|_| {
+            let (prefix, space) = if self.has_fields {
+                (",", " ")
+            } else {
+                ("(", "")
+            };
+
+            if self.is_pretty() {
+                let mut writer = PadAdapter::new(self.fmt);
+                fmt::write(&mut writer, format_args!("{}\n{:#?}", prefix, value))
+            } else {
+                write!(self.fmt, "{}{}{:?}", prefix, space, value)
+            }
+        });
+
+        self.has_fields = true;
+        self
+    }
+
+    /// Consumes the `DebugTuple`, finishing output and returning any error
+    /// encountered.
+    #[unstable(feature = "debug_builders", reason = "method was just created")]
+    pub fn finish(mut self) -> fmt::Result {
+        if self.has_fields {
+            self.result = self.result.and_then(|_| {
+                if self.is_pretty() {
+                    self.fmt.write_str("\n)")
+                } else {
+                    self.fmt.write_str(")")
+                }
+            });
+        }
+        self.result
+    }
+
+    fn is_pretty(&self) -> bool {
+        self.fmt.flags() & (1 << (FlagV1::Alternate as usize)) != 0
+    }
+}
+
+/// A struct to help with `fmt::Debug` implementations.
+///
+/// Constructed by the `Formatter::debug_set` method.
+#[must_use]
+pub struct DebugSet<'a, 'b: 'a> {
+    fmt: &'a mut fmt::Formatter<'b>,
+    result: fmt::Result,
+    has_fields: bool,
+}
+
+pub fn debug_set_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>, name: &str) -> DebugSet<'a, 'b> {
+    let result = write!(fmt, "{} {{", name);
+    DebugSet {
+        fmt: fmt,
+        result: result,
+        has_fields: false,
+    }
+}
+
+impl<'a, 'b: 'a> DebugSet<'a, 'b> {
+    /// Adds a new entry to the set output.
+    #[unstable(feature = "debug_builders", reason = "method was just created")]
+    pub fn entry(mut self, entry: &fmt::Debug) -> DebugSet<'a, 'b> {
+        self.result = self.result.and_then(|_| {
+            let prefix = if self.has_fields {
+                ","
+            } else {
+                ""
+            };
+
+            if self.is_pretty() {
+                let mut writer = PadAdapter::new(self.fmt);
+                fmt::write(&mut writer, format_args!("{}\n{:#?}", prefix, entry))
+            } else {
+                write!(self.fmt, "{} {:?}", prefix, entry)
+            }
+        });
+
+        self.has_fields = true;
+        self
+    }
+
+    /// Consumes the `DebugSet`, finishing output and returning any error
+    /// encountered.
+    #[unstable(feature = "debug_builders", reason = "method was just created")]
+    pub fn finish(self) -> fmt::Result {
+        self.result.and_then(|_| {
+            let end = match (self.has_fields, self.is_pretty()) {
+                (false, _) => "}",
+                (true, false) => " }",
+                (true, true) => "\n}",
+            };
+            self.fmt.write_str(end)
+        })
+    }
+
+    fn is_pretty(&self) -> bool {
+        self.fmt.flags() & (1 << (FlagV1::Alternate as usize)) != 0
+    }
+}
+
+/// A struct to help with `fmt::Debug` implementations.
+///
+/// Constructed by the `Formatter::debug_map` method.
+#[must_use]
+pub struct DebugMap<'a, 'b: 'a> {
+    fmt: &'a mut fmt::Formatter<'b>,
+    result: fmt::Result,
+    has_fields: bool,
+}
+
+pub fn debug_map_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>, name: &str) -> DebugMap<'a, 'b> {
+    let result = write!(fmt, "{} {{", name);
+    DebugMap {
+        fmt: fmt,
+        result: result,
+        has_fields: false,
+    }
+}
+
+impl<'a, 'b: 'a> DebugMap<'a, 'b> {
+    /// Adds a new entry to the map output.
+    #[unstable(feature = "debug_builders", reason = "method was just created")]
+    pub fn entry(mut self, key: &fmt::Debug, value: &fmt::Debug) -> DebugMap<'a, 'b> {
+        self.result = self.result.and_then(|_| {
+            let prefix = if self.has_fields {
+                ","
+            } else {
+                ""
+            };
+
+            if self.is_pretty() {
+                let mut writer = PadAdapter::new(self.fmt);
+                fmt::write(&mut writer, format_args!("{}\n{:#?}: {:#?}", prefix, key, value))
+            } else {
+                write!(self.fmt, "{} {:?}: {:?}", prefix, key, value)
+            }
+        });
+
+        self.has_fields = true;
+
+        self
+    }
+
+    /// Consumes the `DebugMap`, finishing output and returning any error
+    /// encountered.
+    #[unstable(feature = "debug_builders", reason = "method was just created")]
+    pub fn finish(self) -> fmt::Result {
+        self.result.and_then(|_| {
+            let end = match (self.has_fields, self.is_pretty()) {
+                (false, _) => "}",
+                (true, false) => " }",
+                (true, true) => "\n}",
+            };
+            self.fmt.write_str(end)
+        })
+    }
+
+    fn is_pretty(&self) -> bool {
+        self.fmt.flags() & (1 << (FlagV1::Alternate as usize)) != 0
+    }
+}

src/libcore/fmt/mod.rs

@@ -32,8 +32,11 @@ pub use self::num::radix;
 pub use self::num::Radix;
 pub use self::num::RadixFmt;
 
+pub use self::builders::{DebugStruct, DebugTuple, DebugSet, DebugMap};
+
 mod num;
 mod float;
+mod builders;
 
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(hidden)]
@@ -425,7 +428,7 @@ impl<'a> Formatter<'a> {
     /// # Arguments
     ///
     /// * is_positive - whether the original integer was positive or not.
-    /// * prefix - if the '#' character (FlagAlternate) is provided, this
+    /// * prefix - if the '#' character (Alternate) is provided, this
     ///   is the prefix to put in front of the number.
     /// * buf - the byte array that the number has been formatted into
     ///
@@ -614,6 +617,123 @@ impl<'a> Formatter<'a> {
     /// Optionally specified precision for numeric types
     #[unstable(feature = "core", reason = "method was just created")]
     pub fn precision(&self) -> Option<usize> { self.precision }
+
+    /// Creates a `DebugStruct` builder designed to assist with creation of
+    /// `fmt::Debug` implementations for structs.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::fmt;
+    ///
+    /// struct Foo {
+    ///     bar: i32,
+    ///     baz: String,
+    /// }
+    ///
+    /// impl fmt::Debug for Foo {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+    ///         fmt.debug_struct("Foo")
+    ///             .field("bar", &self.bar)
+    ///             .field("baz", &self.baz)
+    ///             .finish()
+    ///     }
+    /// }
+    ///
+    /// // prints "Foo { bar: 10, baz: "Hello World" }"
+    /// println!("{:?}", Foo { bar: 10, baz: "Hello World".to_string() });
+    /// ```
+    #[unstable(feature = "core", reason = "method was just created")]
+    #[inline]
+    pub fn debug_struct<'b>(&'b mut self, name: &str) -> DebugStruct<'b, 'a> {
+        builders::debug_struct_new(self, name)
+    }
+
+    /// Creates a `DebugTuple` builder designed to assist with creation of
+    /// `fmt::Debug` implementations for tuple structs.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// 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)
+    ///             .field(&self.1)
+    ///             .finish()
+    ///     }
+    /// }
+    ///
+    /// // prints "Foo(10, "Hello World")"
+    /// println!("{:?}", Foo(10, "Hello World".to_string()));
+    /// ```
+    #[unstable(feature = "core", reason = "method was just created")]
+    #[inline]
+    pub fn debug_tuple<'b>(&'b mut self, name: &str) -> DebugTuple<'b, 'a> {
+        builders::debug_tuple_new(self, name)
+    }
+
+    /// Creates a `DebugSet` builder designed to assist with creation of
+    /// `fmt::Debug` implementations for set-like structures.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::fmt;
+    ///
+    /// struct Foo(Vec<i32>);
+    ///
+    /// impl fmt::Debug for Foo {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+    ///         let mut builder = fmt.debug_set("Foo");
+    ///         for i in &self.0 {
+    ///             builder = builder.entry(i);
+    ///         }
+    ///         builder.finish()
+    ///     }
+    /// }
+    ///
+    /// // prints "Foo { 10, 11 }"
+    /// println!("{:?}", Foo(vec![10, 11]));
+    /// ```
+    #[unstable(feature = "core", reason = "method was just created")]
+    #[inline]
+    pub fn debug_set<'b>(&'b mut self, name: &str) -> DebugSet<'b, 'a> {
+        builders::debug_set_new(self, name)
+    }
+
+    /// Creates a `DebugMap` builder designed to assist with creation of
+    /// `fmt::Debug` implementations for map-like structures.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::fmt;
+    ///
+    /// struct Foo(Vec<(String, i32)>);
+    ///
+    /// impl fmt::Debug for Foo {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+    ///         let mut builder = fmt.debug_map("Foo");
+    ///         for &(ref key, ref value) in &self.0 {
+    ///             builder = builder.entry(key, value);
+    ///         }
+    ///         builder.finish()
+    ///     }
+    /// }
+    ///
+    /// // prints "Foo { "A": 10, "B": 11 }"
+    /// println!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)]));
+    /// ```
+    #[unstable(feature = "core", reason = "method was just created")]
+    #[inline]
+    pub fn debug_map<'b>(&'b mut self, name: &str) -> DebugMap<'b, 'a> {
+        builders::debug_map_new(self, name)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/libcoretest/fmt/builders.rs

@@ -0,0 +1,401 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+mod debug_struct {
+    use std::fmt;
+
+    #[test]
+    fn test_empty() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_struct("Foo").finish()
+            }
+        }
+
+        assert_eq!("Foo", format!("{:?}", Foo));
+        assert_eq!("Foo", format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_single() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_struct("Foo")
+                    .field("bar", &true)
+                    .finish()
+            }
+        }
+
+        assert_eq!("Foo { bar: true }", format!("{:?}", Foo));
+        assert_eq!(
+"Foo {
+    bar: true
+}",
+                   format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_multiple() {
+        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!("{}/{}", 10i32, 20i32))
+                    .finish()
+            }
+        }
+
+        assert_eq!("Foo { bar: true, baz: 10/20 }", format!("{:?}", Foo));
+        assert_eq!(
+"Foo {
+    bar: true,
+    baz: 10/20
+}",
+                   format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_nested() {
+        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!("{}/{}", 10i32, 20i32))
+                    .finish()
+            }
+        }
+
+        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()
+            }
+        }
+
+        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 {
+    use std::fmt;
+
+    #[test]
+    fn test_empty() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_tuple("Foo").finish()
+            }
+        }
+
+        assert_eq!("Foo", format!("{:?}", Foo));
+        assert_eq!("Foo", format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_single() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_tuple("Foo")
+                    .field(&true)
+                    .finish()
+            }
+        }
+
+        assert_eq!("Foo(true)", format!("{:?}", Foo));
+        assert_eq!(
+"Foo(
+    true
+)",
+                   format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_multiple() {
+        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!("{}/{}", 10i32, 20i32))
+                    .finish()
+            }
+        }
+
+        assert_eq!("Foo(true, 10/20)", format!("{:?}", Foo));
+        assert_eq!(
+"Foo(
+    true,
+    10/20
+)",
+                   format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_nested() {
+        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!("{}/{}", 10i32, 20i32))
+                    .finish()
+            }
+        }
+
+        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()
+            }
+        }
+
+        assert_eq!("Bar(Foo(true, 10/20), \"world\")",
+                   format!("{:?}", Bar));
+        assert_eq!(
+"Bar(
+    Foo(
+        true,
+        10/20
+    ),
+    \"world\"
+)",
+                   format!("{:#?}", Bar));
+    }
+}
+
+mod debug_map {
+    use std::fmt;
+
+    #[test]
+    fn test_empty() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_map("Foo").finish()
+            }
+        }
+
+        assert_eq!("Foo {}", format!("{:?}", Foo));
+        assert_eq!("Foo {}", format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_single() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_map("Foo")
+                    .entry(&"bar", &true)
+                    .finish()
+            }
+        }
+
+        assert_eq!("Foo { \"bar\": true }", format!("{:?}", Foo));
+        assert_eq!(
+"Foo {
+    \"bar\": true
+}",
+                   format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_multiple() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_map("Foo")
+                    .entry(&"bar", &true)
+                    .entry(&10i32, &format_args!("{}/{}", 10i32, 20i32))
+                    .finish()
+            }
+        }
+
+        assert_eq!("Foo { \"bar\": true, 10: 10/20 }", format!("{:?}", Foo));
+        assert_eq!(
+"Foo {
+    \"bar\": true,
+    10: 10/20
+}",
+                   format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_nested() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_map("Foo")
+                    .entry(&"bar", &true)
+                    .entry(&10i32, &format_args!("{}/{}", 10i32, 20i32))
+                    .finish()
+            }
+        }
+
+        struct Bar;
+
+        impl fmt::Debug for Bar {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_map("Bar")
+                    .entry(&"foo", &Foo)
+                    .entry(&Foo, &"world")
+                    .finish()
+            }
+        }
+
+        assert_eq!("Bar { \"foo\": Foo { \"bar\": true, 10: 10/20 }, \
+                    Foo { \"bar\": true, 10: 10/20 }: \"world\" }",
+                   format!("{:?}", Bar));
+        assert_eq!(
+"Bar {
+    \"foo\": Foo {
+        \"bar\": true,
+        10: 10/20
+    },
+    Foo {
+        \"bar\": true,
+        10: 10/20
+    }: \"world\"
+}",
+                   format!("{:#?}", Bar));
+    }
+}
+
+mod debug_set {
+    use std::fmt;
+
+    #[test]
+    fn test_empty() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_set("Foo").finish()
+            }
+        }
+
+        assert_eq!("Foo {}", format!("{:?}", Foo));
+        assert_eq!("Foo {}", format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_single() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_set("Foo")
+                    .entry(&true)
+                    .finish()
+            }
+        }
+
+        assert_eq!("Foo { true }", format!("{:?}", Foo));
+        assert_eq!(
+"Foo {
+    true
+}",
+                   format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_multiple() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_set("Foo")
+                    .entry(&true)
+                    .entry(&format_args!("{}/{}", 10i32, 20i32))
+                    .finish()
+            }
+        }
+
+        assert_eq!("Foo { true, 10/20 }", format!("{:?}", Foo));
+        assert_eq!(
+"Foo {
+    true,
+    10/20
+}",
+                   format!("{:#?}", Foo));
+    }
+
+    #[test]
+    fn test_nested() {
+        struct Foo;
+
+        impl fmt::Debug for Foo {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_set("Foo")
+                    .entry(&true)
+                    .entry(&format_args!("{}/{}", 10i32, 20i32))
+                    .finish()
+            }
+        }
+
+        struct Bar;
+
+        impl fmt::Debug for Bar {
+            fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+                fmt.debug_set("Bar")
+                    .entry(&Foo)
+                    .entry(&"world")
+                    .finish()
+            }
+        }
+
+        assert_eq!("Bar { Foo { true, 10/20 }, \"world\" }",
+                   format!("{:?}", Bar));
+        assert_eq!(
+"Bar {
+    Foo {
+        true,
+        10/20
+    },
+    \"world\"
+}",
+                   format!("{:#?}", Bar));
+    }
+}

src/libcoretest/fmt/mod.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 mod num;
+mod builders;
 
 #[test]
 fn test_format_flags() {

src/libcoretest/lib.rs

@@ -23,6 +23,7 @@
 #![feature(hash)]
 #![feature(io)]
 #![feature(collections)]
+#![feature(debug_builders)]
 #![allow(deprecated)] // rand
 
 extern crate core;

src/libsyntax/codemap.rs

@@ -848,7 +848,7 @@ impl CodeMap {
 
                     let span_comes_from_this_expansion =
                         info.callee.span.map_or(span == info.call_site, |mac_span| {
-                            mac_span.lo <= span.lo && span.hi < mac_span.hi
+                            mac_span.lo <= span.lo && span.hi <= mac_span.hi
                         });
 
                     debug!("span_allows_unstable: from this expansion? {}, allows unstable? {}",

src/libsyntax/ext/deriving/show.rs

@@ -11,7 +11,6 @@
 use ast;
 use ast::{MetaItem, Item, Expr,};
 use codemap::Span;
-use ext::format;
 use ext::base::ExtCtxt;
 use ext::build::AstBuilder;
 use ext::deriving::generic::*;
@@ -19,8 +18,6 @@ use ext::deriving::generic::ty::*;
 use parse::token;
 use ptr::P;
 
-use std::collections::HashMap;
-
 pub fn expand_deriving_show<F>(cx: &mut ExtCtxt,
                                span: Span,
                                mitem: &MetaItem,
@@ -56,14 +53,12 @@ pub fn expand_deriving_show<F>(cx: &mut ExtCtxt,
     trait_def.expand(cx, mitem, item, push)
 }
 
-/// We construct a format string and then defer to std::fmt, since that
+/// We use the debug builders to do the heavy lifting here
-/// knows what's up with formatting and so on.
 fn show_substructure(cx: &mut ExtCtxt, span: Span,
                      substr: &Substructure) -> P<Expr> {
-    // build `<name>`, `<name>({}, {}, ...)` or `<name> { <field>: {},
+    // build fmt.debug_struct(<name>).field(<fieldname>, &<fieldval>)....build()
-    // <field>: {}, ... }` based on the "shape".
+    // or fmt.debug_tuple(<name>).field(&<fieldval>)....build()
-    //
+    // based on the "shape".
-    // Easy start: they all start with the name.
     let name = match *substr.fields {
         Struct(_) => substr.type_ident,
         EnumMatching(_, v, _) => v.node.name,
@@ -72,70 +67,53 @@ fn show_substructure(cx: &mut ExtCtxt, span: Span,
         }
     };
 
-    let mut format_string = String::from_str(&token::get_ident(name));
+    // We want to make sure we have the expn_id set so that we can use unstable methods
-    // the internal fields we're actually formatting
+    let span = Span { expn_id: cx.backtrace(), .. span };
-    let mut exprs = Vec::new();
+    let name = cx.expr_lit(span, ast::Lit_::LitStr(token::get_ident(name),
+                                                   ast::StrStyle::CookedStr));
+    let mut expr = substr.nonself_args[0].clone();
 
-    // Getting harder... making the format string:
     match *substr.fields {
-        // unit struct/nullary variant: no work necessary!
-        Struct(ref fields) if fields.len() == 0 => {}
-        EnumMatching(_, _, ref fields) if fields.len() == 0 => {}
-
         Struct(ref fields) | EnumMatching(_, _, ref fields) => {
-            if fields[0].name.is_none() {
+            if fields.is_empty() || fields[0].name.is_none() {
                 // tuple struct/"normal" variant
+                expr = cx.expr_method_call(span,
+                                           expr,
+                                           token::str_to_ident("debug_tuple"),
+                                           vec![name]);
 
-                format_string.push_str("(");
+                for field in fields {
-
+                    expr = cx.expr_method_call(span,
-                for (i, field) in fields.iter().enumerate() {
+                                               expr,
-                    if i != 0 { format_string.push_str(", "); }
+                                               token::str_to_ident("field"),
-
+                                               vec![cx.expr_addr_of(field.span,
-                    format_string.push_str("{:?}");
+                                                                    field.self_.clone())]);
-
-                    exprs.push(field.self_.clone());
                 }
-
-                format_string.push_str(")");
             } else {
                 // normal struct/struct variant
+                expr = cx.expr_method_call(span,
+                                           expr,
+                                           token::str_to_ident("debug_struct"),
+                                           vec![name]);
 
-                format_string.push_str(" {{");
+                for field in fields {
-
+                    let name = cx.expr_lit(field.span, ast::Lit_::LitStr(
-                for (i, field) in fields.iter().enumerate() {
+                            token::get_ident(field.name.clone().unwrap()),
-                    if i != 0 { format_string.push_str(","); }
+                            ast::StrStyle::CookedStr));
-
+                    expr = cx.expr_method_call(span,
-                    let name = token::get_ident(field.name.unwrap());
+                                               expr,
-                    format_string.push_str(" ");
+                                               token::str_to_ident("field"),
-                    format_string.push_str(&name);
+                                               vec![name,
-                    format_string.push_str(": {:?}");
+                                                    cx.expr_addr_of(field.span,
-
+                                                                    field.self_.clone())]);
-                    exprs.push(field.self_.clone());
                 }
-
-                format_string.push_str(" }}");
             }
         }
         _ => unreachable!()
     }
 
-    // AST construction!
+    cx.expr_method_call(span,
-    // we're basically calling
+                        expr,
-    //
+                        token::str_to_ident("finish"),
-    // format_arg_method!(fmt, write_fmt, "<format_string>", exprs...)
+                        vec![])
-    //
-    // but doing it directly via ext::format.
-    let formatter = substr.nonself_args[0].clone();
-
-    let meth = cx.ident_of("write_fmt");
-    let s = token::intern_and_get_ident(&format_string[..]);
-    let format_string = cx.expr_str(span, s);
-
-    // phew, not our responsibility any more!
-
-    let args = vec![
-        format::expand_preparsed_format_args(cx, span, format_string,
-                                             exprs, vec![], HashMap::new())
-    ];
-    cx.expr_method_call(span, formatter, meth, args)
 }

src/libsyntax/ext/expand.rs

@@ -1121,7 +1121,7 @@ fn expand_annotatable(a: Annotatable,
                         callee: NameAndSpan {
                             name: mname.to_string(),
                             format: MacroAttribute,
-                            span: None,
+                            span: Some(attr.span),
                             // attributes can do whatever they like,
                             // for now.
                             allow_internal_unstable: true,