Rollup merge of rust-lang#143941 - folkertdev:cfg-select-docs, r=traviscross

update `cfg_select!` documentation

tracking issue: rust-lang#115585

After rust-lang#143461, and with an eye on a soon(ish) stabilization, I think the docs need some work.

The existing text read more like a motivation for the feature existing to me, so I've tried to now be a bit more descriptive. Still, suggestions are very welcome.

I also added a test for an empty `select! {}` because it's just the sort of thing that might break.

r? ``@traviscross``

Author: samueltardieu

compiler/rustc_builtin_macros/messages.ftl

@@ -81,11 +81,11 @@ builtin_macros_cfg_accessible_literal_path = `cfg_accessible` path cannot be a l
 builtin_macros_cfg_accessible_multiple_paths = multiple `cfg_accessible` paths are specified
 builtin_macros_cfg_accessible_unspecified_path = `cfg_accessible` path is not specified
 
-builtin_macros_cfg_select_no_matches = none of the rules in this `cfg_select` evaluated to true
+builtin_macros_cfg_select_no_matches = none of the predicates in this `cfg_select` evaluated to true
 
-builtin_macros_cfg_select_unreachable = unreachable rule
+builtin_macros_cfg_select_unreachable = unreachable predicate
     .label = always matches
-    .label2 = this rules is never reached
+    .label2 = this predicate is never reached
 
 builtin_macros_coerce_pointee_requires_maybe_sized = `derive(CoercePointee)` requires `{$name}` to be marked `?Sized`
 

compiler/rustc_builtin_macros/src/cfg_select.rs

@@ -1,12 +1,12 @@
 use rustc_ast::tokenstream::TokenStream;
 use rustc_attr_parsing as attr;
 use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacroExpanderResult};
-use rustc_parse::parser::cfg_select::{CfgSelectBranches, CfgSelectRule, parse_cfg_select};
+use rustc_parse::parser::cfg_select::{CfgSelectBranches, CfgSelectPredicate, parse_cfg_select};
 use rustc_span::{Ident, Span, sym};
 
 use crate::errors::{CfgSelectNoMatches, CfgSelectUnreachable};
 
-/// Selects the first arm whose rule evaluates to true.
+/// Selects the first arm whose predicate evaluates to true.
 fn select_arm(ecx: &ExtCtxt<'_>, branches: CfgSelectBranches) -> Option<(TokenStream, Span)> {
     for (cfg, tt, arm_span) in branches.reachable {
         if attr::cfg_matches(
@@ -30,11 +30,11 @@ pub(super) fn expand_cfg_select<'cx>(
     ExpandResult::Ready(match parse_cfg_select(&mut ecx.new_parser_from_tts(tts)) {
         Ok(branches) => {
             if let Some((underscore, _, _)) = branches.wildcard {
-                // Warn for every unreachable rule. We store the fully parsed branch for rustfmt.
-                for (rule, _, _) in &branches.unreachable {
-                    let span = match rule {
-                        CfgSelectRule::Wildcard(underscore) => underscore.span,
-                        CfgSelectRule::Cfg(cfg) => cfg.span(),
+                // Warn for every unreachable predicate. We store the fully parsed branch for rustfmt.
+                for (predicate, _, _) in &branches.unreachable {
+                    let span = match predicate {
+                        CfgSelectPredicate::Wildcard(underscore) => underscore.span,
+                        CfgSelectPredicate::Cfg(cfg) => cfg.span(),
                     };
                     let err = CfgSelectUnreachable { span, wildcard_span: underscore.span };
                     ecx.dcx().emit_warn(err);
@@ -50,7 +50,7 @@ pub(super) fn expand_cfg_select<'cx>(
                     Ident::with_dummy_span(sym::cfg_select),
                 );
             } else {
-                // Emit a compiler error when none of the rules matched.
+                // Emit a compiler error when none of the predicates matched.
                 let guar = ecx.dcx().emit_err(CfgSelectNoMatches { span: sp });
                 DummyResult::any(sp, guar)
             }

compiler/rustc_parse/src/parser/cfg_select.rs

@@ -7,7 +7,7 @@ use rustc_span::Span;
 use crate::exp;
 use crate::parser::Parser;
 
-pub enum CfgSelectRule {
+pub enum CfgSelectPredicate {
     Cfg(MetaItemInner),
     Wildcard(Token),
 }
@@ -20,7 +20,7 @@ pub struct CfgSelectBranches {
     pub wildcard: Option<(Token, TokenStream, Span)>,
     /// All branches after the first wildcard, including further wildcards.
     /// These branches are kept for formatting.
-    pub unreachable: Vec<(CfgSelectRule, TokenStream, Span)>,
+    pub unreachable: Vec<(CfgSelectPredicate, TokenStream, Span)>,
 }
 
 /// Parses a `TokenTree` that must be of the form `{ /* ... */ }`, and returns a `TokenStream` where
@@ -52,7 +52,7 @@ pub fn parse_cfg_select<'a>(p: &mut Parser<'a>) -> PResult<'a, CfgSelectBranches
             match branches.wildcard {
                 None => branches.wildcard = Some((underscore, tts, span)),
                 Some(_) => {
-                    branches.unreachable.push((CfgSelectRule::Wildcard(underscore), tts, span))
+                    branches.unreachable.push((CfgSelectPredicate::Wildcard(underscore), tts, span))
                 }
             }
         } else {
@@ -64,7 +64,9 @@ pub fn parse_cfg_select<'a>(p: &mut Parser<'a>) -> PResult<'a, CfgSelectBranches
 
             match branches.wildcard {
                 None => branches.reachable.push((meta_item, tts, span)),
-                Some(_) => branches.unreachable.push((CfgSelectRule::Cfg(meta_item), tts, span)),
+                Some(_) => {
+                    branches.unreachable.push((CfgSelectPredicate::Cfg(meta_item), tts, span))
+                }
             }
         }
     }

library/core/src/macros/mod.rs

@@ -196,16 +196,14 @@ pub macro assert_matches {
     },
 }
 
-/// A macro for defining `#[cfg]` match-like statements.
+/// Selects code at compile-time based on `cfg` predicates.
 ///
-/// It is similar to the `if/elif` C preprocessor macro by allowing definition of a cascade of
-/// `#[cfg]` cases, emitting the implementation which matches first.
+/// This macro evaluates, at compile-time, a series of `cfg` predicates,
+/// selects the first that is true, and emits the code guarded by that
+/// predicate. The code guarded by other predicates is not emitted.
 ///
-/// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
-/// without having to rewrite each clause multiple times.
-///
-/// Trailing `_` wildcard match arms are **optional** and they indicate a fallback branch when
-/// all previous declarations do not evaluate to true.
+/// An optional trailing `_` wildcard can be used to specify a fallback. If
+/// none of the predicates are true, a [`compile_error`] is emitted.
 ///
 /// # Example
 ///
@@ -225,7 +223,7 @@ pub macro assert_matches {
 /// }
 /// ```
 ///
-/// If desired, it is possible to return expressions through the use of surrounding braces:
+/// The `cfg_select!` macro can also be used in expression position:
 ///
 /// ```
 /// #![feature(cfg_select)]

tests/ui/macros/cfg_select.rs

@@ -18,10 +18,13 @@ fn arm_rhs_must_be_in_braces() -> i32 {
 cfg_select! {
     _ => {}
     true => {}
-    //~^ WARN unreachable rule
+    //~^ WARN unreachable predicate
 }
 
 cfg_select! {
-    //~^ ERROR none of the rules in this `cfg_select` evaluated to true
+    //~^ ERROR none of the predicates in this `cfg_select` evaluated to true
     false => {}
 }
+
+cfg_select! {}
+//~^ ERROR none of the predicates in this `cfg_select` evaluated to true

tests/ui/macros/cfg_select.stderr

@@ -4,15 +4,15 @@ error: expected `{`, found `1`
 LL |         true => 1
    |                 ^ expected `{`
 
-warning: unreachable rule
+warning: unreachable predicate
   --> $DIR/cfg_select.rs:20:5
    |
 LL |     _ => {}
    |     - always matches
 LL |     true => {}
-   |     ^^^^ this rules is never reached
+   |     ^^^^ this predicate is never reached
 
-error: none of the rules in this `cfg_select` evaluated to true
+error: none of the predicates in this `cfg_select` evaluated to true
   --> $DIR/cfg_select.rs:24:1
    |
 LL | / cfg_select! {
@@ -21,5 +21,11 @@ LL | |     false => {}
 LL | | }
    | |_^
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: none of the predicates in this `cfg_select` evaluated to true
+  --> $DIR/cfg_select.rs:29:1
+   |
+LL | cfg_select! {}
+   | ^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors; 1 warning emitted