Character between U+0001 ~ U+001F in string should be escaped when serialized

[ak1t0]

Fixes servo/servo#15947

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This change is 

[nox]

r? @SimonSapin

[SimonSapin]

Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, 2 unresolved discussions.

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src/serializer.rs, line 260 at r1 (raw file):

                b'\r' => "\\D ",
                b'\0' => "\u{FFFD}",
                x if (x >= b'\x01' && x <= b'\x1F') || x == b'\x7F' => {

A range pattern here would look nicer than a guard:b'\x01'...b'\x1F' | b'\x7F'

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src/serializer.rs, line 261 at r1 (raw file):

                b'\0' => "\u{FFFD}",
                x if (x >= b'\x01' && x <= b'\x1F') || x == b'\x7F' => {
                    string = format!("\\{:x} ", x);

format! does memory allocation in order to return a string, which can be costly. To avoid it, change the type of this match expression from &str to Option<&str> (so wrapping str literals in Some( ) in other branches) where None indicates that the byte needs to be hexadecimal-escaped. Below, use try!(write!(self.inner, "\\{:x}", b)); in that case to write directly to the output stream without an intermediate String.

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Comments from Reviewable

[ak1t0]

I fixed that as it was pointed out to me.

[SimonSapin]

Thank you! Looks good.

@bors-servo r+

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Reviewed 1 of 1 files at r2, 1 of 1 files at r3.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable

[bors-servo]

📌 Commit 8214af3 has been approved by SimonSapin

[bors-servo]

⌛ Testing commit 8214af3 with merge 20b509a...

[bors-servo]

☀️ Test successful - status-travis
Approved by: SimonSapin
Pushing 20b509a to master...