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Merged
ehuss merged 11 commits into from
Feb 24, 2026
Merged

cfg_select! macro #2103

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b4ef15d
`cfg_select!` macro
folkertdev Dec 8, 2025
8a3349a
more detail
folkertdev Dec 17, 2025
adda479
Add more correct grammar for `cfg_select!`
traviscross Jan 17, 2026
4cdb225
Update src/conditional-compilation.md
folkertdev Feb 17, 2026
e3fb956
Rework the cfg_select intro
ehuss Feb 19, 2026
1637914
Add specific rules for cfg_select behavior
ehuss Feb 19, 2026
6712d5d
Simplify wording of cfg.cfg_select.wildcard
ehuss Feb 19, 2026
032f9ab
Remove cfg.cfg_select.positions
ehuss Feb 19, 2026
a824e61
Simplify wording of cfg.cfg_select.fallthrough
ehuss Feb 19, 2026
11e5d04
Improve wording of cfg.cfg_select.well-formed
ehuss Feb 19, 2026
5c1d0c5
Fix sorting of cfg_select! link definition
ehuss Feb 19, 2026
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63 changes: 61 additions & 2 deletions src/conditional-compilation.md
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Expand Up@@ -31,7 +31,7 @@ r[cfg.intro]
*Conditionally compiled source code* is source code that is compiled only under certain conditions.

r[cfg.attributes-macro]
Source code can be made conditionally compiled using the [`cfg`] and [`cfg_attr`] [attributes] and the built-in [`cfg` macro].
Source code can be made conditionally compiled using the [`cfg`] and [`cfg_attr`] [attributes] and the built-in [`cfg!`] and [`cfg_select!`] [macros].

r[cfg.conditional]
Whether to compile can depend on the target architecture of the compiled crate, arbitrary values passed to the compiler, and other things further described below.
Expand DownExpand Up@@ -436,17 +436,76 @@ let machine_kind = if cfg!(unix) {
println!("I'm running on a {} machine!", machine_kind);
```

r[cfg.cfg_select]
### The `cfg_select` macro

r[cfg.cfg_select.intro]
The built-in [`cfg_select!`][std::cfg_select] macro can be used to select code at compile-time based on multiple configuration predicates.

> [!EXAMPLE]
> ```rust
> cfg_select! {
> unix => {
> fn foo() { /* unix specific functionality */ }
> }
> target_pointer_width = "32" => {
> fn foo() { /* non-unix, 32-bit functionality */ }
> }
> _ => {
> fn foo() { /* fallback implementation */ }
> }
> }
>
> let is_unix_str = cfg_select! {
> unix => "unix",
> _ => "not unix",
> };
> ```

r[cfg.cfg_select.syntax]
```grammar,configuration
@root CfgSelect -> CfgSelectArms?

CfgSelectArms ->
CfgSelectConfigurationPredicate `=>`
(
`{` ^ TokenTree `}` `,`? CfgSelectArms?
| ExpressionWithBlockNoAttrs `,`? CfgSelectArms?
| ExpressionWithoutBlockNoAttrs ( `,` CfgSelectArms? )?
)

CfgSelectConfigurationPredicate ->
ConfigurationPredicate | `_`
```
Comment on lines +466 to +479

@ehussehussJan 15, 2026

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This grammar doesn't quite look correct to me.

Some comments:

  • We don't have a precedent for documenting macro inputs. I don't think we can express it like `cfg_select!` `{` … `}` because the braces can also be () or [] (also ! is a separate token), and the macro name can be renamed. I would suggest just documenting what the input to the macro is, and not the surrounding invocation syntax.
  • This doesn't handle the difference in behavior of the last branch.
  • This doesn't specify that the non-braced value needs to be an expression of a certain kind.
  • This doesn't handle the optional , behavior.

I'm thinking the grammar would be something closer to:

@root CfgSelect ->
CfgSelectBranch*
LastCfgSelectBranch?
CfgSelectBranch ->
CfgSelectConfigurationPredicate `=>` `{` TokenTree `}`
| CfgSelectConfigurationPredicate `=>` ExpressionWithBlockX `,`?
| CfgSelectConfigurationPredicate `=>` ExpressionWithoutBlockX `,`
ExpressionWithBlockX ->
BlockExpression
| ConstBlockExpression
| UnsafeBlockExpression
| LoopExpression
| IfExpression
| MatchExpression
ExpressionWithoutBlockX ->
LiteralExpression
| PathExpression
| OperatorExpression
| GroupedExpression
| ArrayExpression
| AwaitExpression
| IndexExpression
| TupleExpression
| TupleIndexingExpression
| StructExpression
| CallExpression
| MethodCallExpression
| FieldExpression
| ClosureExpression
| AsyncBlockExpression
| ContinueExpression
| BreakExpression
| RangeExpression
| ReturnExpression
| UnderscoreExpression
| MacroInvocation
LastCfgSelectBranch ->
CfgSelectConfigurationPredicate => ExpressionX `,`?
ExpressionX -> ExpressionWithBlockX | ExpressionWithoutBlockX
CfgSelectConfigurationPredicate ->
ConfigurationPredicate | `_`

Where the X expressions are from the Expression grammar, but without the outer attributes. If this is correct, we'll need to rework Expression to support that.

Does that make sense?

@ehussehussJan 15, 2026•
edited
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Though looking again, my suggestion above won't work because we are moving towards a grammar that does not have infinite lookahead for disambiguation. So the LastCfgSelectBranch won't work. I offhand can't think of a way to actually express that...

(Maybe lookahead is required?)

@traviscrosstraviscrossJan 17, 2026

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Pushed something that might be closer.


r[cfg.cfg_select.first-arm]
`cfg_select` expands to the payload of the first arm whose configuration predicate evaluates to true.

r[cfg.cfg_select.braces]
If the entire payload is wrapped in curly braces, the braces are removed during expansion.

r[cfg.cfg_select.wildcard]
The configuration predicate `_` always evaluates to true.

r[cfg.cfg_select.fallthrough]
It is a compile error if none of the predicates evaluate to true.

r[cfg.cfg_select.well-formed]
Each right-hand side must be a syntactically valid expansion for the position in which the macro is invoked.

[Testing]: attributes/testing.md
[`--cfg`]: ../rustc/command-line-arguments.html#--cfg-configure-the-compilation-environment
[`--test`]: ../rustc/command-line-arguments.html#--test-build-a-test-harness
[`cfg`]: #the-cfg-attribute
[`cfg` macro]: #the-cfg-macro
[`cfg!`]: #the-cfg-macro
[`cfg_attr`]: #the-cfg_attr-attribute
[`cfg_select!`]: #the-cfg_select-macro
[`crate_name`]: crates-and-source-files.md#the-crate_name-attribute
[`crate_type`]: linkage.md
[`target_feature` attribute]: attributes/codegen.md#the-target_feature-attribute
[attribute]: attributes.md
[attributes]: attributes.md
[cargo-feature]: ../cargo/reference/features.html
[crate type]: linkage.md
[macros]: macros.md
[static C runtime]: linkage.md#static-and-dynamic-c-runtimes
66 changes: 33 additions & 33 deletions src/expressions.md
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Expand Up@@ -8,41 +8,41 @@ Expression ->
| ExpressionWithBlock

ExpressionWithoutBlock ->
OuterAttribute*
(
LiteralExpression
| PathExpression
| OperatorExpression
| GroupedExpression
| ArrayExpression
| AwaitExpression
| IndexExpression
| TupleExpression
| TupleIndexingExpression
| StructExpression
| CallExpression
| MethodCallExpression
| FieldExpression
| ClosureExpression
| AsyncBlockExpression
| ContinueExpression
| BreakExpression
| RangeExpression
| ReturnExpression
| UnderscoreExpression
| MacroInvocation
)
OuterAttribute* ExpressionWithoutBlockNoAttrs

ExpressionWithoutBlockNoAttrs ->
LiteralExpression
| PathExpression
| OperatorExpression
| GroupedExpression
| ArrayExpression
| AwaitExpression
| IndexExpression
| TupleExpression
| TupleIndexingExpression
| StructExpression
| CallExpression
| MethodCallExpression
| FieldExpression
| ClosureExpression
| AsyncBlockExpression
| ContinueExpression
| BreakExpression
| RangeExpression
| ReturnExpression
| UnderscoreExpression
| MacroInvocation

ExpressionWithBlock ->
OuterAttribute*
(
BlockExpression
| ConstBlockExpression
| UnsafeBlockExpression
| LoopExpression
| IfExpression
| MatchExpression
)
OuterAttribute* ExpressionWithBlockNoAttrs

ExpressionWithBlockNoAttrs ->
BlockExpression
| ConstBlockExpression
| UnsafeBlockExpression
| LoopExpression
| IfExpression
| MatchExpression
```

r[expr.intro]
Expand Down