Small perf improvement for fmt

[sinkuu]

Added benchmark is based on #10761

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

Thanks for this! The first commit looks good to me (although somewhat requiring wizardry to fully understand, specializing iterators does make for varying performance...)

For the second commit though the code here is way out of cache for me (that optimization was added in 2014). Can you explain a bit what's going on there to help understand it?

[sinkuu]

For formats without non-default formatting params (like positions not in order ("{2} {0} {1}") or specs ("{:02X}")), Arguments::new_v1 constructor can be used instead of Arguments::new_v1_formatted. new_v1 constructs Arguments with fmt field being None, which activates the fast path in fmt::write.

This optimization should apply to "{}" (which equals to "{0}"), but there seems to be a overlook in #45807 (added ArgumentImplicitlyIs variant without updating the optimization logic).

Example

fn foo(s: &mut String) {
    // parsed as `ArgumentIs(0)`
    write!(s, "{0}", "foo");
    // parsed as `ArgumentImplicitlyIs(0)`
    write!(s, "{}", "foo");
}

Expansion before this PR:

fn foo(s: &mut String) {
    // parsed as `ArgumentIs(0)`
    s.write_fmt($crate::fmt::Arguments::new_v1(
        &[""],
        &match (&"foo",) {
            (arg0,) => [$crate::fmt::ArgumentV1::new(arg0, $crate::fmt::Display::fmt)],
        },
    ));
    // parsed as `ArgumentImplicitlyIs(0)`
    s.write_fmt($crate::fmt::Arguments::new_v1_formatted(
        &[""],
        &match (&"foo",) {
            (arg0,) => [$crate::fmt::ArgumentV1::new(arg0, $crate::fmt::Display::fmt)],
        },
        &[$crate::fmt::rt::v1::Argument {
            position: $crate::fmt::rt::v1::Position::At(0usize),
            format: $crate::fmt::rt::v1::FormatSpec {
                fill: ' ',
                align: $crate::fmt::rt::v1::Alignment::Unknown,
                flags: 0u32,
                precision: $crate::fmt::rt::v1::Count::Implied,
                width: $crate::fmt::rt::v1::Count::Implied,
            },
        }],
    ));
}

Expansion after this PR:

fn foo(s: &mut String) {
    // parsed as `ArgumentIs(0)`
    s.write_fmt($crate::fmt::Arguments::new_v1(
        &[""],
        &match (&"foo",) {
            (arg0,) => [$crate::fmt::ArgumentV1::new(arg0, $crate::fmt::Display::fmt)],
        },
    ));
    // parsed as `ArgumentImplicitlyIs(0)`
    s.write_fmt($crate::fmt::Arguments::new_v1(
        &[""],
        &match (&"foo",) {
            (arg0,) => [$crate::fmt::ArgumentV1::new(arg0, $crate::fmt::Display::fmt)],
        },
    ));
}

[alexcrichton]

@bors: r+

Sounds good to me, thanks for explaining!

[bors]

📌 Commit 038d837 has been approved by alexcrichton

[Centril]

@bors r-

Failed in #57627 due to toolstate change (clippy).
Let's revisit this when we can make toolstate breakages again.

[sinkuu]

This is ready to go. Can you re-"r+"?

[Centril]

@bors r=alexcrichton

[bors]

📌 Commit 038d837 has been approved by alexcrichton

[bors]

⌛ Testing commit 038d837 with merge ad30e9a...