Turn `cfg_match` into a builtin #116323

c410-f3r · 2023-10-01T20:13:52Z

Such thing allows stuff that can't be easily done with declarative macros. For example, it is now possible to create arms without brackets.

cfg_match! {
    cfg(unix) => fn foo() -> i32 { 1 },
    _ => fn foo() -> i32 { 2 },
}

// Expands to =>

#[cfg(all(unix, not(any())))]
fn foo() -> i32 { 1 }

#[cfg(all(not(any(unix))))]
fn foo() -> i32 { 2 }

cc @petrochenkov in case you want to say something

rustbot · 2023-10-01T20:13:58Z

r? @compiler-errors

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c410-f3r · 2023-10-01T20:21:52Z

compiler/rustc_ast/src/token.rs

@@ -229,35 +229,61 @@ fn ident_can_begin_type(name: Symbol, span: Span, is_raw: bool) -> bool {
 #[derive(PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
 pub enum TokenKind {
    /* Expression-operator symbols. */
+    /// `=`


I have often wondered about the meaning of some variants, so let's just comment what they actually are.

This change and the // tidy-alphabetical stuff can be submitted separately and I'll approve them right away.

c410-f3r · 2023-10-01T20:24:47Z

compiler/rustc_builtin_macros/src/cfg_match.rs

+struct CfgMatchOutput(Vec<P<Item>>);
+
+impl MacResult for CfgMatchOutput {
+    fn make_impl_items(self: Box<Self>) -> Option<SmallVec<[P<AssocItem>; 1]>> {
+        let mut rslt = SmallVec::with_capacity(self.0.len());
+        rslt.extend(self.0.into_iter().filter_map(|item| {
+            let Item { attrs, id, span, vis, ident, kind, tokens } = item.into_inner();
+            let ItemKind::Fn(fun) = kind else {
+                return None;
+            };
+            Some(P(Item { attrs, id, ident, kind: AssocItemKind::Fn(fun), span, tokens, vis }))
+        }));
+        Some(rslt)
+    }
+
+    fn make_items(self: Box<Self>) -> Option<SmallVec<[P<Item>; 1]>> {
+        Some(<_>::from(self.0))
+    }
+
+    fn make_stmts(self: Box<Self>) -> Option<SmallVec<[Stmt; 1]>> {
+        let mut rslt = SmallVec::with_capacity(self.0.len());
+        rslt.extend(self.0.into_iter().map(|item| {
+            let id = item.id;
+            let span = item.span;
+            Stmt { id, kind: StmtKind::Item(item), span }
+        }));
+        Some(rslt)
+    }
+}


Only items are acceptable as input but the output can be three different things at the current time and I don't know if there are more expected variants. This is a best effort.

c410-f3r · 2023-10-01T21:51:59Z

compiler/rustc_builtin_macros/src/cfg_match.rs

+    let mut parser = cx.new_parser_from_tts(tts);
+    let mut does_not_have_wildcard = true;
+    let mut items = Vec::with_capacity(4);
+    let mut items_offsets = Vec::with_capacity(4);


This auxiliary items_offsets vector is used to avoid creating Vec<Vec<P<Item>>>, i.e., all items are stored in the same contiguous items memory.

`items`: [ Items of first arm, Items of second arm, .. ] |_____0 to 10______| |_____10 to 25_____| | ______________/ | / `items_offsets`: [ 10, 25, ..]

compiler/rustc_ast/src/token.rs

petrochenkov · 2023-10-11T15:48:43Z

compiler/rustc_expand/src/base.rs

@@ -478,6 +478,7 @@ make_MacEager! {
    trait_items: SmallVec<[P<ast::AssocItem>; 1]>,
    foreign_items: SmallVec<[P<ast::ForeignItem>; 1]>,
    stmts: SmallVec<[ast::Stmt; 1]>,
+    token_stream: P<TokenStream>,


Is this necessary?

petrochenkov · 2023-10-11T15:51:17Z

For example, it is now possible to create arms without brackets.

Why is this a goal in the first place?
I don't want to turn library functionality into compiler built-ins without really strong reasons.
No brackets doesn't look like a strong reason to me.

c410-f3r · 2023-10-11T17:38:54Z

For example, it is now possible to create arms without brackets.

Why is this a goal in the first place? I don't want to turn library functionality into compiler built-ins without really strong reasons. No brackets doesn't look like a strong reason to me.

Avoid duplicated arms (Tracking issue for cfg_match #115585 (comment))? Not sure if it is possible but surely not as a declarative macro.
The possibility of creating better diagnostics.
Easily accept expressions and/or statements and/or items as input. Declarative macros will probably require a lot of private @__ ... declarations (I guess).
More tools for future improvements.

Personally, I am just trying to resolve concerns in order to use this feature on stable ASAP so it is not very important if cfg_match is or isn't a builtin. You know... There are several features that have been inactive for years :/

Anyway let me know if this PR isn't really necessary.

Misc improvements cc rust-lang#116323 (comment) r? `@petrochenkov`

Rollup merge of rust-lang#116696 - c410-f3r:in-doc, r=petrochenkov Misc improvements cc rust-lang#116323 (comment) r? `@petrochenkov`

bors · 2023-10-29T04:35:19Z

☔ The latest upstream changes (presumably #117335) made this pull request unmergeable. Please resolve the merge conflicts.

Dylan-DPC · 2023-11-13T06:26:35Z

@c410-f3r any updates on this? thanks

c410-f3r · 2023-11-13T13:48:49Z

Well, the future of this PR depends on the above conversation 👀

JohnCSimon · 2024-04-14T17:10:30Z

@c410-f3r

Ping from triage: I'm closing this due to inactivity, Please reopen when you are ready to continue with this.
Note: if you are going to continue please open the PR BEFORE you push to it, else you won't be able to reopen - this is a quirk of github.
Thanks for your contribution.

@rustbot label: +S-inactive

c410-f3r · 2024-04-14T21:28:00Z

In regards to code, there is no missing feature or incomplete functionality so this PR is technically ready.

But it seems there is no traction so whatever 🤷

Dylan-DPC · 2024-04-15T14:45:20Z

ah, there wasn't much waiting in the original form but there was less/no interest in this so i think closing it would be the better option here.

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c410-f3r commented Oct 1, 2023

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c410-f3r force-pushed the match_cfg branch from 72fa571 to 4ac269d Compare October 1, 2023 21:20

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c410-f3r force-pushed the match_cfg branch from 5581464 to 8f20a6e Compare October 2, 2023 13:51

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Turn cfg_match into a builtin

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c410-f3r force-pushed the match_cfg branch from 8f20a6e to 9423daa Compare October 2, 2023 14:19

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petrochenkov added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 11, 2023

c410-f3r mentioned this pull request Oct 13, 2023

Misc improvements #116696

Merged

matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Oct 14, 2023

Rollup merge of rust-lang#116696 - c410-f3r:in-doc, r=petrochenkov

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Misc improvements cc rust-lang#116323 (comment) r? `@petrochenkov`

rust-timer added a commit to rust-lang-ci/rust that referenced this pull request Oct 14, 2023

Unrolled build for rust-lang#116696

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Rollup merge of rust-lang#116696 - c410-f3r:in-doc, r=petrochenkov Misc improvements cc rust-lang#116323 (comment) r? `@petrochenkov`

JohnCSimon removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Feb 11, 2024

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Turn `cfg_match` into a builtin #116323

Turn `cfg_match` into a builtin #116323

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Turn cfg_match into a builtin #116323

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Turn `cfg_match` into a builtin #116323

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