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Generate better code for "large" vec![] macro invocations #17857

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Generate better code for "large" vec![] macro invocations #17857

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27 changes: 27 additions & 0 deletions src/libstd/macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -321,7 +321,22 @@ macro_rules! try(
($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
)

/// A macro to count the number of expressions passed to it
#[macro_export]
macro_rules! count(
($x:expr, $($xs:expr),+) => {
1 + count!($($xs),+)
};
($x:expr) => {
1u
};
($($x:expr),*) => {
0u
};
)

/// Create a `std::vec::Vec` containing the arguments.
#[cfg(stage0)]
#[macro_export]
macro_rules! vec(
($($e:expr),*) => ({
Expand All @@ -333,6 +348,18 @@ macro_rules! vec(
($($e:expr),+,) => (vec!($($e),+))
)

#[not(cfg(stage0))]
#[macro_export]
macro_rules! vec(
($($e:expr),*) => ({
// leading _ to allow empty construction without a warning.
let mut _temp = ::std::vec::Vec::with_capacity(count!($($e),*));
$(_temp.push($e);)*
_temp
});
($($e:expr),+,) => (vec!($($e),+))
)


/// A macro to select an event from a number of receivers.
///
Expand Down