Non-terminals, e.g. expr, have diverged between parser and macro matcher #86730

pnkfelix · 2021-06-29T20:50:39Z

Spawned off of PR #84364, based on discussion from lang team meeting (minutes, youtube).

The meaning of the non-terminal "expr" according to the rustc parser has changed (or would like to change) from its meaning according to the current rustc macro matcher.

Concretely:

The inline_const feature has already added a production roughly of the form EXPR ::= const { EXPR }
The let_chains feature may want (in the future) to add a production roughly of the form COND_EXPR ::= let PAT = EXPR, where COND_EXPR can occur as a sub-expression in the condition that drives an if expression or while statement.

In both cases, for backwards compatibility, the expr fragment specifier does not accept these new productions. Examples of the kinds of problems this causes follow:

In other words, today you simply cannot write a macro fragment specifier denoting expressions that will match const { EXPR } or let PAT = EXPR, other than something like $e:tt, which would also match arbitrary token-trees, which won't catch errors as effectively as a more precise fragment specifier.

So, the main question this raises is: How should we resolve this divergence between the parser and macro-rules matcher?

E.g. we could add a new fragment specifier (e.g. expr2024) that covers the new forms and call it a day.
Or we could, at an edition boundary, change the meaning of the expr fragment specifier to cover the new forms. (This would probably need to be coupled with an addition of a new fragment specifier that would capture the old semantics.)
In any case, we need a clear summary of the grammar changes, so that we can understand what expr matches now that it did not before.
I think any change we make here would be well-served to be documented by writing down the actual grammar being implemented, or rather, the changes to the grammar.
We also may need a crater run.

The text was updated successfully, but these errors were encountered:

pnkfelix · 2021-06-29T20:56:05Z

Also, lang team discussion has concluded that we don't have to solve this problem until one of inline_const (#76001) or let_chains (#53667) is being stabilized.

petrochenkov · 2021-06-29T21:46:09Z

It's not only expr, other matchers may be affected too.
The matcher logic needs to be audited and implemented in terms of "reused parser code" + "explicit exceptions for backward compatibility", instead of the inconsistent ad hoc mess that is used now.
UPD: At least expr and ty use can_begin_expr/can_begin_type from the parser now, IIRC it was worse in the past.

The "explicit exceptions for backward compatibility" part can be made conditional on an edition, but that's a technicality.

bstrie · 2022-01-27T03:30:15Z

Another option is to do the maximally backwards-compatible thing with macros 1.0 (either nothing at all, or introduce a unique fragment specifier), and then fix this properly in macros 2.0 while they're still unstable.

bstrie · 2022-01-28T17:45:48Z

Worth noting that this appears to be the one and only remaining issue blocking the stabilization of inline_const, which itself is the only remaining issue blocking the stabilization of asm_const, so getting this sorted out would be very impactful.

memoryruins · 2022-07-15T19:32:47Z

@pnkfelix

Also, lang team discussion has concluded that we don't have to solve this problem until one of inline_const (#76001) or let_chains (#53667) is being stabilized.

The stabilization proposal for let_chains is currently entering its final comment period #94927 (comment). I believe its macro concern in #94927 (comment) was resolved by forbidding let in certain places in #97295 (unless if there was another change that helped resolved it).

Would it be possible to make a similar change to unblock inline_const? and/or now that let_chains is in the process of being stabilized, should this issue be prioritized?

cc @joshtriplett since you opened/resolved the macro concerns on let_chains stabilization, opened an FCP for a different asm feature recently, and are lang team :)

…shtriplett Stabilize `let_chains` in Rust 1.64 # Stabilization proposal This PR proposes the stabilization of `#![feature(let_chains)]` in a future-compatibility way that will allow the **possible** addition of the `EXPR is PAT` syntax. Tracking issue: rust-lang#53667 Version: 1.64 (beta => 2022-08-11, stable => 2022-10-22). ## What is stabilized The ability to chain let expressions along side local variable declarations or ordinary conditional expressions. For example: ```rust pub enum Color { Blue, Red, Violet, } pub enum Flower { Rose, Tulip, Violet, } pub fn roses_are_red_violets_are_blue_printer( (first_flower, first_flower_color): (Flower, Color), (second_flower, second_flower_color): (Flower, Color), pick_up_lines: &[&str], ) { if let Flower::Rose = first_flower && let Color::Red = first_flower_color && let Flower::Violet = second_flower && let Color::Blue = second_flower_color && let &[first_pick_up_line, ..] = pick_up_lines { println!("Roses are red, violets are blue, {}", first_pick_up_line); } } fn main() { roses_are_red_violets_are_blue_printer( (Flower::Rose, Color::Red), (Flower::Violet, Color::Blue), &["sugar is sweet and so are you"], ); } ``` ## Motivation The main motivation for this feature is improving readability, ergonomics and reducing paper cuts. For more examples, see the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/2497-if-let-chains.md). ## What isn't stabilized * Let chains in match guards (`if_let_guard`) * Resolution of divergent non-terminal matchers * The `EXPR is PAT` syntax ## History * On 2017-12-24, [RFC: if- and while-let-chains](rust-lang/rfcs#2260) * On 2018-07-12, [eRFC: if- and while-let-chains, take 2](rust-lang/rfcs#2497) * On 2018-08-24, [Tracking issue for eRFC 2497, "if- and while-let-chains, take 2](rust-lang#53667) * On 2019-03-19, [Run branch cleanup after copy prop](rust-lang#59290) * On 2019-03-26, [Generalize diagnostic for x = y where bool is the expected type](rust-lang#59439) * On 2019-04-24, [Introduce hir::ExprKind::Use and employ in for loop desugaring](rust-lang#60225) * On 2019-03-19, [[let_chains, 1/6] Remove hir::ExprKind::If](rust-lang#59288) * On 2019-05-15, [[let_chains, 2/6] Introduce Let(..) in AST, remove IfLet + WhileLet and parse let chains](rust-lang#60861) * On 2019-06-20, [[let_chains, 3/6] And then there was only Loop](rust-lang#61988) * On 2020-11-22, [Reintroduce hir::ExprKind::If](rust-lang#79328) * On 2020-12-24, [Introduce hir::ExprKind::Let - Take 2](rust-lang#80357) * On 2021-02-19, [Lower condition of if expression before it's "then" block](rust-lang#82308) * On 2021-09-01, [Fix drop handling for `if let` expressions](rust-lang#88572) * On 2021-09-04, [Formally implement let chains](rust-lang#88642) * On 2022-01-19, [Add tests to ensure that let_chains works with if_let_guard](rust-lang#93086) * On 2022-01-18, [Introduce `enhanced_binary_op` feature](rust-lang#93049) * On 2022-01-22, [Fix `let_chains` and `if_let_guard` feature flags](rust-lang#93213) * On 2022-02-25, [Initiate the inner usage of `let_chains`](rust-lang#94376) * On 2022-01-28, [[WIP] Introduce ast::StmtKind::LetElse to allow the usage of `let_else` with `let_chains`](rust-lang#93437) * On 2022-02-26, [1 - Make more use of `let_chains`](rust-lang#94396) * On 2022-02-26, [2 - Make more use of `let_chains`](rust-lang#94400) * On 2022-02-27, [3 - Make more use of `let_chains`](rust-lang#94420) * On 2022-02-28, [4 - Make more use of `let_chains`](rust-lang#94445) * On 2022-02-28, [5 - Make more use of `let_chains`](rust-lang#94448) * On 2022-02-28, [6 - Make more use of `let_chains`](rust-lang#94465) * On 2022-03-01, [7 - Make more use of `let_chains`](rust-lang#94476) * On 2022-03-01, [8 - Make more use of `let_chains`](rust-lang#94484) * On 2022-03-01, [9 - Make more use of `let_chains`](rust-lang#94498) * On 2022-03-08, [Warn users about `||` in let chain expressions](rust-lang#94754) From the first RFC (2017-12-24) to the theoretical future stabilization day (2022-10-22), it can be said that this feature took 4 years, 9 months and 28 days of research, development, discussions, agreements and headaches to be settled. ## Divergent non-terminal matchers More specifically, rust-lang#86730. ```rust macro_rules! mac { ($e:expr) => { if $e { true } else { false } }; } fn main() { // OK! assert_eq!(mac!(true && let 1 = 1), true); // ERROR! Anything starting with `let` is not considered an expression assert_eq!(mac!(let 1 = 1 && true), true); } ``` To the best of my knowledge, such error or divergence is orthogonal, does not prevent stabilization and can be tackled independently in the near future or effectively in the next Rust 2024 edition. If not, then https://github.com/c410-f3r/rust/tree/let-macro-blah contains a set of changes that will consider `let` an expression. It is possible that none of the solutions above satisfies all applicable constraints but I personally don't know of any other plausible answers. ## Alternative syntax Taking into account the usefulness of this feature and the overwhelming desire to use both now and in the past, `let PAT = EXPR` will be utilized for stabilization but it doesn't or shall create any obstacle for a **possible** future addition of `EXPR is PAT`. The introductory snippet would then be written as the following. ```rust if first_flower is Flower::Rose && first_flower_color is Color::Red && second_flower is Flower::Violet && second_flower_color is Color::Blue && pick_up_lines is &[first_pick_up_line, ..] { println!("Roses are red, violets are blue, {}", first_pick_up_line); } ``` Just to reinforce, this PR only unblocks a **possible** future road for `EXPR is PAT` and does emphasize what is better or what is worse. ## Tests * [Verifies the drop order of let chains and ensures it won't change in the future in an unpredictable way](https://github.com/rust-lang/rust/blob/master/src/test/ui/mir/mir_let_chains_drop_order.rs) * [AST lowering does not wrap let chains in an `DropTemps` expression](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/ast-lowering-does-not-wrap-let-chains.rs) * [Checks pretty printing output](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs) * [Verifies uninitialized variables due to MIR modifications](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/chains-without-let.rs) * [A collection of statements where `let` expressions are forbidden](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs) * [All or at least most of the places where let chains are allowed](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs) * [Ensures that irrefutable lets are allowed in let chains](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/irrefutable-lets.rs) * [issue-88498.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-88498.rs), [issue-90722.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-90722.rs), [issue-92145.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-92145.rs) and [issue-93150.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-93150.rs) were bugs found by third parties and fixed overtime. * [Indexing was triggering a ICE due to a wrongly constructed MIR graph](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/no-double-assigments.rs) * [Protects the precedence of `&&` in relation to other things](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs) * [`let_chains`, as well as `if_let_guard`, has a valid MIR graph that evaluates conditional expressions correctly](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/then-else-blocks.rs) Most of the infra-structure used by let chains is also used by `if` expressions in stable compiler versions since rust-lang#80357 and rust-lang#88572. As a result, no bugs were found since the integration of rust-lang#88642. ## Possible future work * Let chains in match guards is implemented and working but stabilization is blocked by `if_let_guard`. * The usage of `let_chains` with `let_else` is possible but not implemented. Regardless, one attempt was introduced and closed in rust-lang#93437. Thanks `@Centril` for creating the RFC and huge thanks (again) to `@matthewjasper` for all the reviews, mentoring and MIR implementations. Fixes rust-lang#53667

joshtriplett · 2022-08-03T17:29:51Z

Since let PAT = expr isn't allowed in an arbitrary expression, only in if let and while let, it sholdn't be added to expr. And AFAICT you currently have to match if-let and while-let explicitly in macro matches if you want their contents. So, people would need to add support for let chains in those matches. We could add something like cond to help with that, but I don't think that's a blocker.

Stabilize `let_chains` in Rust 1.64 # Stabilization proposal This PR proposes the stabilization of `#![feature(let_chains)]` in a future-compatibility way that will allow the **possible** addition of the `EXPR is PAT` syntax. Tracking issue: #53667 Version: 1.64 (beta => 2022-08-11, stable => 2022-10-22). ## What is stabilized The ability to chain let expressions along side local variable declarations or ordinary conditional expressions. For example: ```rust pub enum Color { Blue, Red, Violet, } pub enum Flower { Rose, Tulip, Violet, } pub fn roses_are_red_violets_are_blue_printer( (first_flower, first_flower_color): (Flower, Color), (second_flower, second_flower_color): (Flower, Color), pick_up_lines: &[&str], ) { if let Flower::Rose = first_flower && let Color::Red = first_flower_color && let Flower::Violet = second_flower && let Color::Blue = second_flower_color && let &[first_pick_up_line, ..] = pick_up_lines { println!("Roses are red, violets are blue, {}", first_pick_up_line); } } fn main() { roses_are_red_violets_are_blue_printer( (Flower::Rose, Color::Red), (Flower::Violet, Color::Blue), &["sugar is sweet and so are you"], ); } ``` ## Motivation The main motivation for this feature is improving readability, ergonomics and reducing paper cuts. For more examples, see the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/2497-if-let-chains.md). ## What isn't stabilized * Let chains in match guards (`if_let_guard`) * Resolution of divergent non-terminal matchers * The `EXPR is PAT` syntax ## History * On 2017-12-24, [RFC: if- and while-let-chains](rust-lang/rfcs#2260) * On 2018-07-12, [eRFC: if- and while-let-chains, take 2](rust-lang/rfcs#2497) * On 2018-08-24, [Tracking issue for eRFC 2497, "if- and while-let-chains, take 2](rust-lang/rust#53667) * On 2019-03-19, [Run branch cleanup after copy prop](rust-lang/rust#59290) * On 2019-03-26, [Generalize diagnostic for x = y where bool is the expected type](rust-lang/rust#59439) * On 2019-04-24, [Introduce hir::ExprKind::Use and employ in for loop desugaring](rust-lang/rust#60225) * On 2019-03-19, [[let_chains, 1/6] Remove hir::ExprKind::If](rust-lang/rust#59288) * On 2019-05-15, [[let_chains, 2/6] Introduce Let(..) in AST, remove IfLet + WhileLet and parse let chains](rust-lang/rust#60861) * On 2019-06-20, [[let_chains, 3/6] And then there was only Loop](rust-lang/rust#61988) * On 2020-11-22, [Reintroduce hir::ExprKind::If](rust-lang/rust#79328) * On 2020-12-24, [Introduce hir::ExprKind::Let - Take 2](rust-lang/rust#80357) * On 2021-02-19, [Lower condition of if expression before it's "then" block](rust-lang/rust#82308) * On 2021-09-01, [Fix drop handling for `if let` expressions](rust-lang/rust#88572) * On 2021-09-04, [Formally implement let chains](rust-lang/rust#88642) * On 2022-01-19, [Add tests to ensure that let_chains works with if_let_guard](rust-lang/rust#93086) * On 2022-01-18, [Introduce `enhanced_binary_op` feature](rust-lang/rust#93049) * On 2022-01-22, [Fix `let_chains` and `if_let_guard` feature flags](rust-lang/rust#93213) * On 2022-02-25, [Initiate the inner usage of `let_chains`](rust-lang/rust#94376) * On 2022-01-28, [[WIP] Introduce ast::StmtKind::LetElse to allow the usage of `let_else` with `let_chains`](rust-lang/rust#93437) * On 2022-02-26, [1 - Make more use of `let_chains`](rust-lang/rust#94396) * On 2022-02-26, [2 - Make more use of `let_chains`](rust-lang/rust#94400) * On 2022-02-27, [3 - Make more use of `let_chains`](rust-lang/rust#94420) * On 2022-02-28, [4 - Make more use of `let_chains`](rust-lang/rust#94445) * On 2022-02-28, [5 - Make more use of `let_chains`](rust-lang/rust#94448) * On 2022-02-28, [6 - Make more use of `let_chains`](rust-lang/rust#94465) * On 2022-03-01, [7 - Make more use of `let_chains`](rust-lang/rust#94476) * On 2022-03-01, [8 - Make more use of `let_chains`](rust-lang/rust#94484) * On 2022-03-01, [9 - Make more use of `let_chains`](rust-lang/rust#94498) * On 2022-03-08, [Warn users about `||` in let chain expressions](rust-lang/rust#94754) From the first RFC (2017-12-24) to the theoretical future stabilization day (2022-10-22), it can be said that this feature took 4 years, 9 months and 28 days of research, development, discussions, agreements and headaches to be settled. ## Divergent non-terminal matchers More specifically, rust-lang/rust#86730. ```rust macro_rules! mac { ($e:expr) => { if $e { true } else { false } }; } fn main() { // OK! assert_eq!(mac!(true && let 1 = 1), true); // ERROR! Anything starting with `let` is not considered an expression assert_eq!(mac!(let 1 = 1 && true), true); } ``` To the best of my knowledge, such error or divergence is orthogonal, does not prevent stabilization and can be tackled independently in the near future or effectively in the next Rust 2024 edition. If not, then https://github.com/c410-f3r/rust/tree/let-macro-blah contains a set of changes that will consider `let` an expression. It is possible that none of the solutions above satisfies all applicable constraints but I personally don't know of any other plausible answers. ## Alternative syntax Taking into account the usefulness of this feature and the overwhelming desire to use both now and in the past, `let PAT = EXPR` will be utilized for stabilization but it doesn't or shall create any obstacle for a **possible** future addition of `EXPR is PAT`. The introductory snippet would then be written as the following. ```rust if first_flower is Flower::Rose && first_flower_color is Color::Red && second_flower is Flower::Violet && second_flower_color is Color::Blue && pick_up_lines is &[first_pick_up_line, ..] { println!("Roses are red, violets are blue, {}", first_pick_up_line); } ``` Just to reinforce, this PR only unblocks a **possible** future road for `EXPR is PAT` and does emphasize what is better or what is worse. ## Tests * [Verifies the drop order of let chains and ensures it won't change in the future in an unpredictable way](https://github.com/rust-lang/rust/blob/master/src/test/ui/mir/mir_let_chains_drop_order.rs) * [AST lowering does not wrap let chains in an `DropTemps` expression](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/ast-lowering-does-not-wrap-let-chains.rs) * [Checks pretty printing output](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs) * [Verifies uninitialized variables due to MIR modifications](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/chains-without-let.rs) * [A collection of statements where `let` expressions are forbidden](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs) * [All or at least most of the places where let chains are allowed](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs) * [Ensures that irrefutable lets are allowed in let chains](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/irrefutable-lets.rs) * [issue-88498.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-88498.rs), [issue-90722.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-90722.rs), [issue-92145.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-92145.rs) and [issue-93150.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-93150.rs) were bugs found by third parties and fixed overtime. * [Indexing was triggering a ICE due to a wrongly constructed MIR graph](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/no-double-assigments.rs) * [Protects the precedence of `&&` in relation to other things](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs) * [`let_chains`, as well as `if_let_guard`, has a valid MIR graph that evaluates conditional expressions correctly](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/then-else-blocks.rs) Most of the infra-structure used by let chains is also used by `if` expressions in stable compiler versions since rust-lang/rust#80357 and rust-lang/rust#88572. As a result, no bugs were found since the integration of rust-lang/rust#88642. ## Possible future work * Let chains in match guards is implemented and working but stabilization is blocked by `if_let_guard`. * The usage of `let_chains` with `let_else` is possible but not implemented. Regardless, one attempt was introduced and closed in rust-lang/rust#93437. Thanks `@Centril` for creating the RFC and huge thanks (again) to `@matthewjasper` for all the reviews, mentoring and MIR implementations. Fixes #53667

RalfJung · 2022-10-13T07:35:20Z

@rust-lang/lang rust-lang/lang-team#111 has been closed, so what is the plan to make progress here? This seems to be one of the last blocking issues for inline const expr.

TimNN · 2022-11-06T18:58:36Z

Would a lint that triggers whenever an :expr matcher does not match a const { ... } be maybe sufficient to allow stabilizing inline_const?

That wouldn't change the behavior of existing macros and the behavior for existing macros is reasonable, IMO:

If a macro currently has rules that explicitly match const $x:expr then:

The macro author can silence the lint by ensuring that any rules matching const $x:expr is placed before any rules matching $x:expr.
A macro user can silence the lint via #[allow(...)].

If a macro does not have rules that explicitly match const $x:expr then:

A macro user can work around this by placing parentheses or braces around the const expression, i.e. (const { x }).
The macro author can, if they believe this would be a common patter for their users, add a rule to explicitly handle const $x:expr.

Whether an :expr2024 matcher is needed or whether the behavior of :expr should change with the next edition could be resolved at some later point.

fix: don't let mbe expr fragments match let exprs and inline consts Fixes #11729 `expr` fragment in mbe should not match let expressions and inline consts for backwards compatibility. See rust-lang/rust#86730 for details. This patch is porting [this logic in rustc](https://github.com/rust-lang/rust/blob/f0c4da49983aa699f715caf681e3154b445fb60b/compiler/rustc_parse/src/parser/nonterminal.rs#L28-L34) (which is called [here in rustc's mbe engine](https://github.com/rust-lang/rust/blob/f0c4da49983aa699f715caf681e3154b445fb60b/compiler/rustc_expand/src/mbe/macro_parser.rs#L576)) to our mbe engine.

traviscross · 2023-11-16T05:21:20Z

Further action here is now gated on acceptance of rust-lang/rfcs#3531, "Macro matcher fragment specifiers edition policy".

RalfJung · 2024-03-18T21:16:30Z

Further action here is now gated on acceptance of rust-lang/rfcs#3531, "Macro matcher fragment specifiers edition policy".

That RFC has been accepted.
Elsewhere, @pnkfelix wrote

re todo item "resolve expr fragment specifier issue", see also rust-lang/rfcs#3531, "Macro matcher fragment specifiers edition policy".

So -- what does that RFC being accepted mean for this issue, and for inline-const? Can the issue be closed? Can it be removed from the "blocking" list of inline consts?

traviscross · 2024-03-25T22:24:58Z

The acceptance of rust-lang/rfcs#3531 means that there's now a policy for how to handle this, but someone (help wanted, cc @rust-lang/wg-macros) needs to apply this policy to macro fragment specifiers that have diverged from the grammar, e.g. expr, as discussed above.

iamahuman · 2024-04-10T14:44:53Z

The acceptance of rust-lang/rfcs#3531 means that there's now a policy for how to handle this, but someone (help wanted, cc @rust-lang/wg-macros) needs to apply this policy to macro fragment specifiers that have diverged from the grammar, e.g. expr, as discussed above.

How about reviving PR #84364? Minor changes (e.g., s/expr202x/expr_2021/g; s/expr2015/expr_2015/g) should be enough to make it compliant, I think.

RalfJung · 2024-04-17T12:53:45Z

The acceptance of rust-lang/rfcs#3531 means that there's now a policy for how to handle this, but someone (help wanted, cc @rust-lang/wg-macros) needs to apply this policy to macro fragment specifiers that have diverged from the grammar, e.g. expr, as discussed above.

Okay, thanks.

Is this a blocker for #104087 (stabilizing inline const) or something that can also be done after stabilization?

eholk · 2024-04-17T15:31:39Z

Is this a blocker for #104087 (stabilizing inline const) or something that can also be done after stabilization?

I wouldn't think this needs to block stabilizing inline const; if we don't get it ahead of time it just means expr will be a little weird for a little bit.

I've been working with @vincenzopalazzo to implement the RFC. We had a first PR out at #123865.

Stabilise inline_const # Stabilisation Report ## Summary This PR will stabilise `inline_const` feature in expression position. `inline_const_pat` is still unstable and will *not* be stabilised. The feature will allow code like this: ```rust foo(const { 1 + 1 }) ``` which is roughly desugared into ```rust struct Foo; impl Foo { const FOO: i32 = 1 + 1; } foo(Foo::FOO) ``` This feature is from rust-lang/rfcs#2920 and is tracked in rust-lang#76001 (the tracking issue should *not* be closed as it needs to track inline const in pattern position). The initial implementation is done in rust-lang#77124. ## Difference from RFC There are two major differences (enhancements) as implemented from the RFC. First thing is that the RFC says that the type of an inline const block inferred from the content *within* it, but we currently can infer the type using the information from outside the const block as well. This is a frequently requested feature to the initial implementation (e.g. rust-lang#89964). The inference is implemented in rust-lang#89561 and is done by treating inline const similar to a closure and therefore share inference context with its parent body. This allows code like: ```rust let v: Vec<i32> = const { Vec::new() }; ``` Another enhancement that differs from the RFC is that we currently allow inline consts to reference generic parameters. This is implemented in rust-lang#96557. This allows code like: ```rust fn create_none_array<T, const N: usize>() -> [Option<T>; N] { [const { None::<T> }; N] } ``` This enhancement also makes inline const usable as static asserts: ```rust fn require_zst<T>() { const { assert!(std::mem::size_of::<T>() == 0) } } ``` ## Documentation Reference: rust-lang/reference#1295 ## Unresolved issues We still have a few issues that are not resolved, but I don't think it necessarily has to block stabilisation: * expr fragment specifier issue: rust-lang#86730 * ~~`const {}` behaves similar to `async {}` but not to `{}` and `unsafe {}` (they are treated as `ExpressionWithoutBlock` rather than `ExpressionWithBlock`): https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/const.20blocks.20differ.20from.20normal.20and.20from.20unsafe.20blocks/near/290229453~~ ## Tests There are a few tests in https://github.com/rust-lang/rust/tree/master/src/test/ui/inline-const

pnkfelix mentioned this issue Jun 29, 2021

Add expr202x macro pattern #84364

Closed

pnkfelix added the T-lang Relevant to the language team, which will review and decide on the PR/issue. label Jun 29, 2021

This was referenced Jun 29, 2021

Tracking Issue for inline const patterns (RFC 2920) #76001

Open

Tracking issue for eRFC 2497, "if- and while-let-chains, take 2" #53667

Open

pnkfelix changed the title ~~Expr non-terminal has diverged between parser and macro matcher~~ Non-terminals, e.g. expr, have diverged between parser and macro matcher Jun 30, 2021

pnkfelix mentioned this issue Jul 20, 2021

Non-terminal divergence between parser and macro matcher rust-lang/lang-team#111

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c410-f3r mentioned this issue Mar 14, 2022

Stabilize let_chains in Rust 1.64 #94927

Merged

scottmcm mentioned this issue Oct 26, 2022

Parse error recovery is obversable by macros in several cases #103534

Open

nbdd0121 mentioned this issue Nov 7, 2022

Stabilise inline_const #104087

Merged

lowr mentioned this issue Dec 20, 2022

fix: don't let mbe expr fragments match let exprs and inline consts rust-lang/rust-analyzer#13800

Merged

dtolnay mentioned this issue Feb 20, 2023

inline-const is not an expr in macros, but it should be #84155

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dtolnay added the A-maybe-future-edition Something we may consider for a future edition. label Feb 20, 2023

dtolnay mentioned this issue Feb 20, 2023

Support const block syntax in ensure! macro dtolnay/anyhow#298

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eholk mentioned this issue Apr 17, 2024

Tracking Issue for updating expr macro fragment specifier for Rust 2024 #123742

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Non-terminals, e.g. expr, have diverged between parser and macro matcher #86730

Non-terminals, e.g. expr, have diverged between parser and macro matcher #86730

pnkfelix commented Jun 29, 2021 •

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eholk commented Apr 17, 2024

Non-terminals, e.g. expr, have diverged between parser and macro matcher #86730

Non-terminals, e.g. expr, have diverged between parser and macro matcher #86730

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pnkfelix commented Jun 29, 2021 • edited Loading

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bstrie commented Jan 27, 2022

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memoryruins commented Jul 15, 2022

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