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.. default-domain:: spec

Values

Legality Rules

:dp:`fls_buyaqara7am4` A value is either a literal or the result of a computation, that may be stored in a memory location, and interpreted based on some type.

:dp:`fls_CUJyMj0Sj8NS` An :dt:`allocated object` is a value stored at some memory address.

:dp:`fls_kaomYy0Ml4Nh` The :dt:`base address` of an allocated object is the memory address where the object is stored.

:dp:`fls_B5cmkWfD5GNt` An [allocated object]s :dt:`memory size` is the number of bytes the object spans in memory from its base address.

:dp:`fls_rixdyyc525xp` Two [value]s overlap when

Undefined Behavior

:dp:`fls_6lg0oaaopc26` It is undefined behavior to create a value from uninitialized memory unless the type of the value is a union type.

:dp:`fls_oqhQ62mDLckN` It is undefined behavior to create an allocated object at base address :c:`null`.

:dp:`fls_uhwpuv6cx4ip` It is undefined behavior to create an allocated object with memory size size at a base address base where base + size is greater than the architectures maximum :c:`usize` value.

:dp:`fls_xuuFKmm181bs` It is undefined behavior to create an allocated object with memory size size where size is greater than the architectures maximum :c:`isize` value.

Constants

Syntax

.. syntax::

   ConstantDeclaration ::=
       $$const$$ (Name | $$_$$) TypeAscription ConstantInitializer? $$;$$

   ConstantInitializer ::=
       $$=$$ Expression

Legality Rules

:dp:`fls_5o5iu4j8in4l` A constant is an immutable value expression whose uses are substituted by the value.

:dp:`fls_3mhj0kkupwuz` An unnamed constant is a constant declared with character 0x5F (low line).

:dp:`fls_ka4y2yd100dx` The type specification of a constant shall have 'static lifetime.

:dp:`fls_vt9tlkd676ql` The type of a constant shall implement the :std:`core::marker::Sized` trait.

:dp:`fls_ooOYxhVh8hZo` The type of a constant cannot be a mutable reference type.

:dp:`fls_ndmfqxjpvsqy` A constant initializer is a construct that provides the value of its related constant.

:dp:`fls_6rxwbbhf5tc5` A constant shall have a constant initializer, unless it is an associated trait constant.

:dp:`fls_vnc3ttnid1qr` The expression of a constant initializer shall be a constant expression.

:dp:`fls_deuo1pn8cjd6` The value of a constant is determined by evaluating its constant initializer.

Dynamic Semantics

:dp:`fls_xezt9hl069h4` The elaboration of a constant evaluates its constant initializer.

:dp:`fls_ndobth7s92if` A path that refers to a constant is replaced with the value of the constant.

Examples

const ZERO: u32 = 0;

Statics

Syntax

.. syntax::

   StaticDeclaration ::=
       ItemSafety? $$static$$ $$mut$$? Name TypeAscription StaticInitializer? $$;$$

   StaticInitializer ::=
       $$=$$ Expression

Legality Rules

:dp:`fls_ibrmiwfypldh` A static is a value that is associated with a specific memory location.

:dp:`fls_mt94jvoot9dx` A static defined within a generic function exists once in the output executable or library.

:dp:`fls_k0r2c6uq29tu` The type specification of a static shall have 'static lifetime.

:dp:`fls_b6ods85htuyn` The type of a static shall implement the :std:`core::marker::Sized` trait.

:dp:`fls_WRpcVF1fLEpr` A static shall only be subject to an :s:`ItemSafety` if it is an external static in an unsafe external block.

:dp:`fls_doi4z6u55bi7` A mutable static is a static with keyword mut whose value can be modified.

:dp:`fls_74hp208pto22` Access to a mutable static shall require unsafe context.

:dp:`fls_jfde2vg6mtww` An immutable static is a static whose value cannot be modified.

:dp:`fls_k4tyqb1j6zjo` The type of an immutable static shall implement the :std:`core::marker::Sync` trait.

:dp:`fls_t17h5h6a6v4c` A static initializer is a construct that provides the value of its related static.

:dp:`fls_yq0hpy4jx2qb` A static shall have a static initializer, unless it is an external static.

:dp:`fls_vgidvfwzm4ks` The expression of a static initializer shall be a constant expression.

:dp:`fls_8dcldbvu7lav` A use of a static is a place expression referring to the unique location of the static.

Dynamic Semantics

:dp:`fls_w0nb0mphho7b` The elaboration of a static evaluates its static initializer.

:dp:`fls_eeocxst9vafn` All [path]s that refer to a static refer to the same memory location.

:dp:`fls_47khd5ljsxeq` A static is not dropped during destruction.

:dp:`fls_dowxbphqvk3n` An immutable static whose type is not subject to interior mutability may reside in read-only memory.

Undefined Behavior

:dp:`fls_b5wsmii7vz3v` It is undefined behavior to mutate an immutable static whose type is not subject to interior mutability.

Examples

static mut GLOBAL: u32 = 0;

Temporaries

Legality Rules

:dp:`fls_awpw61yofckz` A temporary is an anonymous variable produced by some intermediate computation.

Variables

Legality Rules

:dp:`fls_hl5tnd9yy252` A variable is a placeholder for a value that is allocated on the stack.

:dp:`fls_vgi0gh5zmoiu` The following [construct]s are [variable]s:

:dp:`fls_r9km9f969bu8` A variable shall be used only after it has been initialized through all [reachable control flow path]s up to the point of its usage.

Dynamic Semantics

:dp:`fls_g8etd5lsgn9j` A variable is not initialized when allocated.

Constant Promotion

Legality Rules

:dp:`fls_udn9lyf3m0z6` Constant promotion is the process of converting a value expression into a constant.

:dp:`fls_yvkdcs4pmxjf` Constant promotion is possible only when

:dp:`fls_m690b8qg9d9r` Constant promotion is always possible for expression &mut [], promoting the produced mutable borrow to have 'static lifetime.

:dp:`fls_uf0sg25awre6` Constant promotion proceeds as follows:

  1. :dp:`fls_o7cqfdnr253y` An anonymous constant is created, whose constant initializer holds the result of the value expression.
  2. :dp:`fls_ap85svxyuhvg` The value of the anonymous constant is borrowed with 'static lifetime.