Block expressions
Syntax
BlockExpression :
{
InnerAttribute*
Statements?
}
Statements :
Statement+
| Statement+ ExpressionWithoutBlock
| ExpressionWithoutBlock
A block expression, or block, is a control flow expression and anonymous
namespace scope for items and variable declarations. As a control flow
expression, a block sequentially executes its component non-item declaration
statements and then its final optional expression. As an anonymous namespace
scope, item declarations are only in scope inside the block itself and variables
declared by let
statements are in scope from the next statement until the end
of the block.
Blocks are written as {
, then any inner attributes, then statements,
then an optional expression, and finally a }
. Statements are usually required
to be followed a semicolon, with two exceptions. Item declaration statements do
not need to be followed by a semicolon. Expression statements usually require
a following semicolon except if its outer expression is a flow control
expression. Furthermore, extra semicolons between statements are allowed, but
these semicolons do not affect semantics.
Note: The semicolon following a statement is not a part of the statement itself. They are invalid when using the
stmt
macro matcher.
When evaluating a block expression, each statement, except for item declaration statements, is executed sequentially. Then the final expression is executed, if given.
The type of a block is the type of the final expression, or ()
if the final
expression is omitted.
# #![allow(unused_variables)] #fn main() { # fn fn_call() {} let _: () = { fn_call(); }; let five: i32 = { fn_call(); 5 }; assert_eq!(5, five); #}
Note: As a control flow expression, if a block expression is the outer expression of an expression statement, the expected type is
()
unless it is followed immediately by a semicolon.
Blocks are always value expressions and evaluate the last expression in
value expression context. This can be used to force moving a value if really
needed. For example, the following example fails on the call to consume_self
because the struct was moved out of s
in the block expression.
# #![allow(unused_variables)] #fn main() { struct Struct; impl Struct { fn consume_self(self) {} fn borrow_self(&self) {} } fn move_by_block_expression() { let s = Struct; // Move the value out of `s` in the block expression. (&{ s }).borrow_self(); // Fails to execute because `s` is moved out of. s.consume_self(); } #}
unsafe
blocks
Syntax
UnsafeBlockExpression :
unsafe
BlockExpression
See unsafe
block for more information on when to use unsafe
A block of code can be prefixed with the unsafe
keyword to permit unsafe
operations. Examples:
# #![allow(unused_variables)] #fn main() { unsafe { let b = [13u8, 17u8]; let a = &b[0] as *const u8; assert_eq!(*a, 13); assert_eq!(*a.offset(1), 17); } # unsafe fn an_unsafe_fn() -> i32 { 10 } let a = unsafe { an_unsafe_fn() }; #}
Attributes on block expressions
Inner attributes are allowed directly after the opening brace of a block expression in the following situations:
- Function and method bodies.
- Loop bodies (
loop
,while
,while let
, andfor
). - Block expressions used as a statement.
- Block expressions as elements of array expressions, tuple expressions, call expressions, tuple-style struct and enum variant expressions.
- A block expression as the tail expression of another block expression.
The attributes that have meaning on a block expression are cfg
and the
lint check attributes.
For example, this function returns true
on unix platforms and false
on other
platforms.
# #![allow(unused_variables)] #fn main() { fn is_unix_platform() -> bool { #[cfg(unix)] { true } #[cfg(not(unix))] { false } } #}