diff --git a/src/libcore/benches/fmt.rs b/src/libcore/benches/fmt.rs
new file mode 100644
index 00000000000..92f10c760c6
--- /dev/null
+++ b/src/libcore/benches/fmt.rs
@@ -0,0 +1,110 @@
+use std::io::{self, Write as IoWrite};
+use std::fmt::{self, Write as FmtWrite};
+use test::Bencher;
+
+#[bench]
+fn write_vec_value(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        for _ in 0..1000 {
+            mem.write_all("abc".as_bytes()).unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_vec_ref(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        let wr = &mut mem as &mut dyn io::Write;
+        for _ in 0..1000 {
+            wr.write_all("abc".as_bytes()).unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_vec_macro1(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        let wr = &mut mem as &mut dyn io::Write;
+        for _ in 0..1000 {
+            write!(wr, "abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_vec_macro2(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        let wr = &mut mem as &mut dyn io::Write;
+        for _ in 0..1000 {
+            write!(wr, "{}", "abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_vec_macro_debug(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        let wr = &mut mem as &mut dyn io::Write;
+        for _ in 0..1000 {
+            write!(wr, "{:?}", "☃").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_value(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        for _ in 0..1000 {
+            mem.write_str("abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_ref(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        let wr = &mut mem as &mut dyn fmt::Write;
+        for _ in 0..1000 {
+            wr.write_str("abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_macro1(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        for _ in 0..1000 {
+            write!(mem, "abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_macro2(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        let wr = &mut mem as &mut dyn fmt::Write;
+        for _ in 0..1000 {
+            write!(wr, "{}", "abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_macro_debug(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        let wr = &mut mem as &mut dyn fmt::Write;
+        for _ in 0..1000 {
+            write!(wr, "{:?}", "☃").unwrap();
+        }
+    });
+}
diff --git a/src/libcore/benches/lib.rs b/src/libcore/benches/lib.rs
index 5b4971c81dd..48572af611a 100644
--- a/src/libcore/benches/lib.rs
+++ b/src/libcore/benches/lib.rs
@@ -11,3 +11,4 @@ mod iter;
 mod num;
 mod ops;
 mod slice;
+mod fmt;
diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 5221d6f17c2..935579f4943 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -1006,28 +1006,30 @@ pub fn write(output: &mut dyn Write, args: Arguments) -> Result {
         curarg: args.args.iter(),
     };
 
-    let mut pieces = args.pieces.iter();
+    let mut idx = 0;
 
     match args.fmt {
         None => {
             // We can use default formatting parameters for all arguments.
-            for (arg, piece) in args.args.iter().zip(pieces.by_ref()) {
+            for (arg, piece) in args.args.iter().zip(args.pieces.iter()) {
                 formatter.buf.write_str(*piece)?;
                 (arg.formatter)(arg.value, &mut formatter)?;
+                idx += 1;
             }
         }
         Some(fmt) => {
             // Every spec has a corresponding argument that is preceded by
             // a string piece.
-            for (arg, piece) in fmt.iter().zip(pieces.by_ref()) {
+            for (arg, piece) in fmt.iter().zip(args.pieces.iter()) {
                 formatter.buf.write_str(*piece)?;
                 formatter.run(arg)?;
+                idx += 1;
             }
         }
     }
 
     // There can be only one trailing string piece left.
-    if let Some(piece) = pieces.next() {
+    if let Some(piece) = args.pieces.get(idx) {
         formatter.buf.write_str(*piece)?;
     }
 
diff --git a/src/libfmt_macros/lib.rs b/src/libfmt_macros/lib.rs
index 32ae878909f..da440cdd72f 100644
--- a/src/libfmt_macros/lib.rs
+++ b/src/libfmt_macros/lib.rs
@@ -72,6 +72,15 @@ pub enum Position<'a> {
     ArgumentNamed(&'a str),
 }
 
+impl Position<'_> {
+    pub fn index(&self) -> Option<usize> {
+        match self {
+            ArgumentIs(i) | ArgumentImplicitlyIs(i) => Some(*i),
+            _ => None,
+        }
+    }
+}
+
 /// Enum of alignments which are supported.
 #[derive(Copy, Clone, PartialEq)]
 pub enum Alignment {
diff --git a/src/libsyntax_ext/format.rs b/src/libsyntax_ext/format.rs
index 3e3bca7080f..215df4224c5 100644
--- a/src/libsyntax_ext/format.rs
+++ b/src/libsyntax_ext/format.rs
@@ -493,7 +493,10 @@ impl<'a, 'b> Context<'a, 'b> {
 
                 let fill = arg.format.fill.unwrap_or(' ');
 
-                if *arg != simple_arg || fill != ' ' {
+                let pos_simple =
+                    arg.position.index() == simple_arg.position.index();
+
+                if !pos_simple || arg.format != simple_arg.format || fill != ' ' {
                     self.all_pieces_simple = false;
                 }