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lib.rs
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// TODO
// - eliminate the closure hole by recognizing nested calls and shadowed labels, then maintaining a whitelist?
// - inline @up rule to reduce recursion depth
// the tests need more recursion to parse all the code
#![cfg_attr(test, recursion_limit = "1000")]
// on nightly, we can re-export static_cond!
#![cfg_attr(feature = "nightly", feature(macro_reexport))]
#[cfg(all(test, // for testing ...
not(feature = "nightly")))] // ... on beta/stable ...
#[macro_use] // ... we use the macros ...
#[no_link] // ... but no code ...
extern crate static_cond; // ... from static-cond
#[cfg(feature = "nightly")] // on nightly ...
#[macro_use] // ... we use the macros ...
#[no_link] // ... but no code ...
#[macro_reexport(static_cond)] // ... and re-export static-cond! ...
extern crate static_cond; // ... from static-cond
/// Provides the "early exit from any block" control-flow primitive that was mentioned in [RFC 243][link].
///
/// If not using the "nightly" Cargo feature, you must depend on `static-cond` and put `#[macro_use] extern crate static_cond;` at the crate root.
///
/// See README.md for more details.
///
/// [link]: https://github.com/rust-lang/rfcs/blob/master/text/0243-trait-based-exception-handling.md#early-exit-from-any-block
///
/// Examples
/// ========
///
/// ```
/// # #[macro_use] extern crate named_block;
/// # #[macro_use] extern crate static_cond;
/// # fn main() {
/// let x = block!('a: {
/// break 'a 0;
/// 1
/// });
/// assert_eq!(x, 0);
/// # }
/// ```
///
/// ```
/// # #[macro_use] extern crate named_block;
/// # #[macro_use] extern crate static_cond;
/// # fn main() {
/// assert_eq!(
/// 42,
/// block!('a: {
/// enum Foo { Bar(i32) }
/// let closure = #[block(ignore)] {
/// move |Foo::Bar(x): Foo| -> i32 {
/// x + block!('a: {
/// break 'a 41;
/// })
/// }
/// };
///
/// closure(Foo::Bar(1))
/// })
/// );
/// # }
/// ```
#[macro_export]
macro_rules! block {
// =======================================================================================
// PRIVATE RULES
// =======================================================================================
// ======================================================
// UTILITY RULES
// ======================================================
// utility: coerce an AST fragment with interpolated TTs into an expression
// (see https://danielkeep.github.io/tlborm/book/blk-ast-coercion.html)
(@as_expr $e:expr) => { $e };
// utility: deliberately cause a compile error with a CamelCaseMessage
(@error $err:ident) => {{
struct $err;
let _: () = $err;
}};
// ======================================================
// OUTPUT STAGE
// ======================================================
// This is called from the scanner when everything has
// been processed and we are ready to write out the
// final expansion.
// final output from the top level of the macro
(@wrap $life:tt () $ret:ident ($($init:tt)*) $out:expr) => {
block!(@as_expr
{
let $ret $($init)*;
$life: loop {
$ret = $out;
break $life;
}
$ret
})
};
(@wrap $life:tt (loop) $ret:ident ($($init:tt)*) $out:expr) => {
block!(@as_expr
{
let $ret $($init)*;
$life: loop {
$out;
}
$ret
})
};
// ======================================================
// SCANNER STAGE
// ======================================================
// This is the meat of the macro. It transfers code from
// the input (left of ->) to the output (right of ->)
// while transforming break statements or triggering
// errors as needed.
// The next four rules handle the end of the input code -- either the macro
// is done, or we need to pop the stack and keep walking. We can tell which
// it is by checking the context stack. If it's empty, we can go to output.
// Otherwise, we take the current output, surround it by the brace type,
// and move up the context stack.
// no context: we're done!
(@scan {} $life:tt $ret:ident () -> ($($out:tt)*) (() $lp:tt $init:tt)) => {
block!(@wrap $life $lp $ret $init { $($out)* })
};
// pop stack and surround with {}
(@scan {} $life:tt $ret:ident () -> ($($out:tt)*) $stack:tt) => {
block!(@up $life $ret { $($out)* } $stack)
};
// pop stack and surround with ()
(@scan () $life:tt $ret:ident () -> ($($out:tt)*) $stack:tt) => {
block!(@up $life $ret ( $($out)* ) $stack)
};
// pop stack and surround with []
(@scan [] $life:tt $ret:ident () -> ($($out:tt)*) $stack:tt) => {
block!(@up $life $ret [ $($out)* ] $stack)
};
// The next nine rules are triggered when the tree walker encounters a
// break/continue statement.
// bare "break" and "continue" statements are errors (TODO allow bare break?)
(@scan $paren:tt $life:tt $ret:ident (break) -> ($($out:tt)*) $stack:tt) => {
block!(@scan $paren $life $ret () -> ($($out)* block!(@error NoBareBreakInNamedBlock);) $stack)
};
(@scan $paren:tt $life:tt $ret:ident (break; $($tail:tt)*) -> ($($out:tt)*) $stack:tt) => {
block!(@scan $paren $life $ret ($($tail)*) -> ($($out)* block!(@error NoBareBreakInNamedBlock);) $stack)
};
(@scan $paren:tt $life:tt $ret:ident (continue) -> ($($out:tt)*) $stack:tt) => {
block!(@scan $paren $life $ret () -> ($($out)* block!(@error NoBareContinueInNamedBlock);) $stack)
};
(@scan $paren:tt $life:tt $ret:ident (continue; $($tail:tt)*) -> ($($out:tt)*) $stack:tt) => {
block!(@scan $paren $life $ret ($($tail)*) -> ($($out)* block!(@error NoBareContinueInNamedBlock);) $stack)
};
// "break LIFETIME;" (no EXPR)
(@scan $paren:tt $life1:tt $ret:ident (break $life2:tt; $($tail:tt)*) -> ($($out:tt)*) $stack:tt) => {
block!(@scan $paren $life1 $ret ($($tail)*) -> ($($out)* break $life2;) $stack)
};
// "break LIFETIME EXPR": compare the lifetimes, if they match then transform the statement, otherwise leave it alone
(@scan $paren:tt $life1:tt $ret:ident (break $life2:tt $e:expr; $($tail:tt)*) -> ($($out:tt)*) ($stack:tt $lp:tt $init:tt)) => {
static_cond! {
if $life1 == $life2 {
block!(@scan $paren $life1 $ret ($($tail)*) -> ($($out)* { $ret = $e; break $life2; }) ($stack $lp ()))
} else {
block!(@scan $paren $life1 $ret ($($tail)*) -> ($($out)* break $life2 $e;) ($stack $lp ()))
}
}
};
(@scan $paren:tt $life1:tt $ret:ident (break $life2:tt $e:expr) -> ($($out:tt)*) ($stack:tt $lp:tt $init:tt)) => {
static_cond! {
if $life1 == $life2 {
block!(@scan $paren $life1 $ret () -> ($($out)* { $ret = $e; break $life2 }) ($stack $lp ()))
// TODO make sure this isn't adding too many semicolons
} else {
block!(@scan $paren $life1 $ret () -> ($($out)* break $life2 $e;) ($stack $lp ()))
}
}
};
// "continue LIFETIME": compare the lifetimes, if they match then error, otherwise leave it alone
// (this only applies to bare blocks)
(@scan $paren:tt $life1:tt $ret:ident (continue $life2:tt; $($tail:tt)*) -> ($($out:tt)*) ($stack:tt () $init:tt)) => {
static_cond! {
if $life1 == $life2 {
block!(@scan $paren $life1 $ret ($($tail)*) -> ($($out)* block!(@error NoMatchedContinueInNamedBlock);) ($stack () $init))
} else {
block!(@scan $paren $life1 $ret ($($tail)*) -> ($($out)* continue $life2;) ($stack () $init))
}
}
};
(@scan $paren:tt $life1:tt $ret:ident (continue $life2:tt) -> ($($out:tt)*) $stack:tt) => {
static_cond! {
if $life1 == $life2 {
block!(@scan $paren $life1 $ret () -> ($($out)* block!(@error NoMatchedContinueInNamedBlock);) $stack)
} else {
block!(@scan $paren $life1 $ret () -> ($($out)* continue $life2;) $stack)
}
}
};
// tree walker ignores #[block(ignore)] tts, closures, and items
(@scan_item $paren:tt $life:tt $ret:ident ($ignore:item $($tail:tt)*) -> ($($out:tt)*) $stack:tt) => {
block!(@scan $paren $life $ret ($($tail)*) -> ($($out)* $ignore) $stack)
};
// #[block(ignore)] attribute is ignored
(@scan $paren:tt $life:tt $ret:ident (#[block(ignore)] $ignore:tt $($tail:tt)*) -> ($($out:tt)*) $stack:tt) => {
block!(@scan $paren $life $ret ($($tail)*) -> ($($out)* $ignore) $stack)
};
// other attributes pass through
(@scan $paren:tt $life:tt $ret:ident (#[$attr:meta] $($tail:tt)*) -> ($($out:tt)*) $stack:tt) => {
block!(@scan $paren $life $ret ($($tail)*) -> ($($out)* #[$attr]) $stack)
};
// ignore items: use, extern, static, const, unsafe trait/impl/fn, fn, mod, type, enum, trait, impl, struct
(@scan $paren:tt $life:tt $ret:ident (pub $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (pub $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (use $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (use $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (extern $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (extern $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (mod $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (mod $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (static $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (static $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (const $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (const $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (trait $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (trait $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (unsafe trait $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (unsafe trait $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (impl $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (impl $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (unsafe impl $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (unsafe impl $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (fn $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (fn $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (unsafe fn $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (unsafe fn $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (type $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (type $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (enum $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (enum $($tail)*) -> $out $stack)
};
(@scan $paren:tt $life:tt $ret:ident (struct $($tail:tt)*) -> $out:tt $stack:tt) => {
block!(@scan_item $paren $life $ret (struct $($tail)*) -> $out $stack)
};
// tree walker descends into token trees
(@scan $paren:tt $life:tt $ret:ident ({ $($inner:tt)* } $($tail:tt)*) -> $out:tt ($stack:tt $lp:tt $init:tt)) => {
block!(@scan {} $life $ret ($($inner)*) -> ()
(($paren ($($tail)*) -> $out $stack) $lp $init))
};
(@scan $paren:tt $life:tt $ret:ident (( $($inner:tt)* ) $($tail:tt)*) -> $out:tt ($stack:tt $lp:tt $init:tt)) => {
block!(@scan () $life $ret ($($inner)*) -> ()
(($paren ($($tail)*) -> $out $stack) $lp $init))
};
(@scan $paren:tt $life:tt $ret:ident ([ $($inner:tt)* ] $($tail:tt)*) -> $out:tt ($stack:tt $lp:tt $init:tt)) => {
block!(@scan [] $life $ret ($($inner)*) -> ()
(($paren ($($tail)*) -> $out $stack) $lp $init))
};
// fall-through case for tree walker: transfer over a token
(@scan $paren:tt $life:tt $ret:ident ($head:tt $($tail:tt)*) -> ($($out:tt)*) $stack:tt) => {
block!(@scan $paren $life $ret ($($tail)*) -> ($($out)* $head) $stack)
};
// reformats arguments when popping a context off the tree walker stack
// TODO this could be folded into the @scan rules that call it, to reduce recursion depth
(@up $life:tt $ret:ident $thing:tt (($paren:tt $tail:tt -> ($($out:tt)*) $stack:tt) $lp:tt $init:tt)) => {
block!(@scan $paren $life $ret $tail -> ($($out)* $thing) ($stack $lp $init))
};
// entry point for bare block
($life:tt: { $($body:tt)* }) => {
block!(@scan {} $life _ret ($($body)*) -> () (() () ()))
// | | | | | | || | |
// | | | | | | || | ^ initialization
// | | | | | | || ^ loop type
// | | | | | | |^ tree walker stack
// | | | | | | ^ passed-through context
// | | | | | ^ transformed code
// | | | | ^ code to be transformed
// | | | ^ block exit variable name (gensym)
// | | ^ block label
// | ^ surrounding bracket type
// ^ start the tree walker!
};
// entry point for loop
($life:tt: loop { $($body:tt)* }) => {
block!(@scan {} $life _ret ($($body)*) -> () (() (loop) (= ())))
};
}
#[cfg(test)]
mod tests {
#[test]
fn it_works() {
let flag = true;
let x = block!('a: {
if flag { break 'a "early exit"; }
"normal exit"
});
assert_eq!(x, "early exit");
}
#[test]
fn shadowing() {
let flag = false;
let x = block!('b: {
if flag { break 'b "early exit"; }
let _y = block!('c: {
if flag { break 'b "inner early exit"; };
String::from("inner normal exit")
});
#[block(ignore)]
{
#[allow(dead_code)]
fn f() -> i32 {
block!('b: {
break 'b 42;
})
}
}
#[allow(dead_code)]
fn g() {
block!('b: {
break 'b 42;
});
while false {
break;
}
while false {
continue;
}
'b: while false {
continue 'b;
}
}
enum Foo { Bar(i32) }
let closure = move |Foo::Bar(x): Foo| -> i32 {
x + block!('d: {
break 'd 42;
})
};
assert_eq!(closure(Foo::Bar(0)), 42);
"normal exit"
});
assert_eq!(x, "normal exit");
'e: for i in 1..5 {
assert!(i >= 1 && i < 5);
block!('d: {
//continue; //~ERROR NoBareContinueInNamedBlock
//continue 'd; //~ERROR NoMatchedContinueInNamedBlock
continue 'e;
});
}
}
#[test]
fn loops() {
assert_eq!(42, block!('a: loop { break 'a 42 }));
assert_eq!((), block!('a: {})); // make sure it works with no breaks
let mut v = vec![];
let mut i = 0;
block!('a: loop {
i += 1;
if i == 5 {
continue 'a;
} else if i == 10 {
break 'a;
}
v.push(i);
});
assert_eq!(&*v, &[1, 2, 3, 4, 6, 7, 8, 9]);
}
}