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Turn cfg_match into a builtin #116323

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@c410-f3r c410-f3r commented Oct 1, 2023

cc #115585

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

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rustbot commented Oct 1, 2023

r? @compiler-errors

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@rustbot rustbot added A-testsuite Area: The testsuite used to check the correctness of rustc S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 1, 2023
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@@ -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. */
/// `=`
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I have often wondered about the meaning of some variants, so let's just comment what they actually are.

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

Comment on lines +236 to +293
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)
}
}
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@c410-f3r c410-f3r Oct 1, 2023

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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.

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);
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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, ..]

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@@ -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>,
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Is this necessary?

@petrochenkov
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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.

@petrochenkov 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
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c410-f3r commented Oct 11, 2023

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.

@c410-f3r c410-f3r mentioned this pull request Oct 13, 2023
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Oct 14, 2023
rust-timer added a commit to rust-lang-ci/rust that referenced this pull request Oct 14, 2023
Rollup merge of rust-lang#116696 - c410-f3r:in-doc, r=petrochenkov

Misc improvements

cc rust-lang#116323 (comment)

r? `@petrochenkov`
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bors commented Oct 29, 2023

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

@Dylan-DPC
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@c410-f3r any updates on this? thanks

@c410-f3r
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Well, the future of this PR depends on the above conversation 👀

@JohnCSimon 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
@JohnCSimon JohnCSimon added 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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@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

@JohnCSimon JohnCSimon closed this Apr 14, 2024
@rustbot rustbot added the S-inactive Status: Inactive and waiting on the author. This is often applied to closed PRs. label Apr 14, 2024
@c410-f3r
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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 🤷

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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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9 participants