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Update used to use the attribute template #1905

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Update used to use the attribute template #1905

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45d2a26
Unwrap used
ehuss Jun 3, 2025
c61f0e2
Slightly reword the used intro
ehuss Jun 3, 2025
2c0ea0b
Move used example into an example block
ehuss Jun 3, 2025
8f55815
Add attribute template rules for used
ehuss Jun 3, 2025
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104 changes: 56 additions & 48 deletions src/abi.md
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Original file line number Diff line number Diff line change
Expand Up @@ -13,53 +13,62 @@ r[abi.used]
## The `used` attribute

r[abi.used.intro]
The *`used` attribute* can only be applied to [`static` items]. This [attribute] forces the
compiler to keep the variable in the output object file (.o, .rlib, etc. excluding final binaries)
even if the variable is not used, or referenced, by any other item in the crate.
However, the linker is still free to remove such an item.

Below is an example that shows under what conditions the compiler keeps a `static` item in the
output object file.

``` rust
// foo.rs

// This is kept because of `#[used]`:
#[used]
static FOO: u32 = 0;

// This is removable because it is unused:
#[allow(dead_code)]
static BAR: u32 = 0;

// This is kept because it is publicly reachable:
pub static BAZ: u32 = 0;

// This is kept because it is referenced by a public, reachable function:
static QUUX: u32 = 0;

pub fn quux() -> &'static u32 {
&QUUX
}

// This is removable because it is referenced by a private, unused (dead) function:
static CORGE: u32 = 0;

#[allow(dead_code)]
fn corge() -> &'static u32 {
&CORGE
}
```

``` console
$ rustc -O --emit=obj --crate-type=rlib foo.rs

$ nm -C foo.o
0000000000000000 R foo::BAZ
0000000000000000 r foo::FOO
0000000000000000 R foo::QUUX
0000000000000000 T foo::quux
```
The *`used` [attribute]* forces a [`static` item][items.static] to be kept in the output object file (.o, .rlib, etc. excluding final binaries) even if the static is never used, or referenced, by an other item in the crate. However, the linker is still free to remove such an item.

> [!EXAMPLE]
> This example that shows under what conditions the compiler keeps a `static` item in the output object file.
>
> ```rust
> // foo.rs
>
> // This is kept because of `#[used]`:
> #[used]
> static FOO: u32 = 0;
>
> // This is removable because it is unused:
> #[allow(dead_code)]
> static BAR: u32 = 0;
>
> // This is kept because it is publicly reachable:
> pub static BAZ: u32 = 0;
>
> // This is kept because it is referenced by a public, reachable function:
> static QUUX: u32 = 0;
>
> pub fn quux() -> &'static u32 {
> &QUUX
> }
>
> // This is removable because it is referenced by a private, unused (dead) function:
> static CORGE: u32 = 0;
>
> #[allow(dead_code)]
> fn corge() -> &'static u32 {
> &CORGE
> }
> ```
>
> ```console
> $ rustc -O --emit=obj --crate-type=rlib foo.rs
>
> $ nm -C foo.o
> 0000000000000000 R foo::BAZ
> 0000000000000000 r foo::FOO
> 0000000000000000 R foo::QUUX
> 0000000000000000 T foo::quux
> ```

r[abi.used.syntax]
The `used` attribute uses the [MetaWord] syntax and thus does not take any inputs.

r[abi.used.allowed-positions]
The `used` attribute may only be applied to [`static` items][items.static].

r[abi.used.duplicates]
Only the first instance of `used` on an item is honored. Subsequent `used` attributes are ignored.

> [!NOTE]
> `rustc` currently warns on subsequent duplicate `used` attributes.

r[abi.no_mangle]
## The `no_mangle` attribute
Expand Down Expand Up @@ -135,7 +144,6 @@ r[abi.export_name.edition2024]
> [!EDITION-2024]
> Before the 2024 edition it is allowed to use the `export_name` attribute without the `unsafe` qualification.

[`static` items]: items/static-items.md
[attribute]: attributes.md
[extern functions]: items/functions.md#extern-function-qualifier
[external blocks]: items/external-blocks.md
Expand Down