Auto merge of #39356 - krdln:format-with-capacity, r=aturon

Use `String::with_capacity` in `format!`

Add an `Arguments::estimated_capacity` to estimate the length of formatted text and use it in `std::fmt::format` as the initial capacity of the buffer.

The capacity is calculated based on the literal parts of format string, see the details in the implementation.

Some benches:
```rust
empty:       format!("{}", black_box(""))
literal:     format!("Literal")
long:        format!("Hello Hello Hello Hello, {}!", black_box("world"))
long_rev:    format!("{}, hello hello hello hello!", black_box("world"))
long_rev_2:  format!("{}{}, hello hello hello hello!", 1, black_box("world"))
short:       format!("Hello, {}!", black_box("world"))
short_rev:   format!("{}, hello!", black_box("world"))
short_rev_2: format!("{}{}, hello!", 1, black_box("world"))
surround:    format!("aaaaa{}ccccc{}eeeee", black_box("bbbbb"), black_box("eeeee"))
two_spaced:  format!("{} {}", black_box("bbbbb"), black_box("eeeee"))
worst_case:  format!("{} a long piece...", black_box("and even longer argument. not sure why it has to be so long"))
```
```
 empty        25            28                      3   12.00%
 literal      35            29                     -6  -17.14%
 long         80            46                    -34  -42.50%
 long_rev     79            45                    -34  -43.04%
 long_rev_2   111           66                    -45  -40.54%
 short        73            46                    -27  -36.99%
 short_rev    74            76                      2    2.70%
 short_rev_2  107           108                     1    0.93%
 surround     142           65                    -77  -54.23%
 two_spaced   111           115                     4    3.60%
 worst_case   89            101                    12   13.48%
```

1 files changed, 26 insertions, 0 deletions

diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 2ba7d6e8bd1..a989f914db6 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -265,6 +265,32 @@ impl<'a> Arguments<'a> {
             args: args
         }
     }
+
+    /// Estimates the length of the formatted text.
+    ///
+    /// This is intended to be used for setting initial `String` capacity
+    /// when using `format!`. Note: this is neither the lower nor upper bound.
+    #[doc(hidden)] #[inline]
+    #[unstable(feature = "fmt_internals", reason = "internal to format_args!",
+               issue = "0")]
+    pub fn estimated_capacity(&self) -> usize {
+        let pieces_length: usize = self.pieces.iter()
+            .map(|x| x.len()).sum();
+
+        if self.args.is_empty() {
+            pieces_length
+        } else if self.pieces[0] == "" && pieces_length < 16 {
+            // If the format string starts with an argument,
+            // don't preallocate anything, unless length
+            // of pieces is significant.
+            0
+        } else {
+            // There are some arguments, so any additional push
+            // will reallocate the string. To avoid that,
+            // we're "pre-doubling" the capacity here.
+            pieces_length.checked_mul(2).unwrap_or(0)
+        }
+    }
 }
 
 /// This structure represents a safely precompiled version of a format string