Implement const expressions and patterns (RFC 2920) #77124

spastorino · 2020-09-23T21:43:21Z

cc @ecstatic-morse @lcnr @oli-obk @petrochenkov

rust-highfive · 2020-09-23T21:43:25Z

r? @oli-obk

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compiler/rustc_parse/src/parser/pat.rs

compiler/rustc_ast/src/ast.rs

lcnr

I would personally somewhat strongly prefer a different expression name here, for example InlineConst. AnonConst is already in use to describe all unnamed constants (array lengths etc) of which
these expressions are only a subset.

[u8; 3 + 4]
    ^^^^^^
StructFoo<{3 + 7}>
           ^^^^^^

Apart from this I think these changes look good

src/test/ui/parser/keyword-const-as-identifier.rs

compiler/rustc_ast/src/ast.rs

src/test/ui/parser/keyword-const-as-identifier.rs

compiler/rustc_parse/src/parser/mod.rs

compiler/rustc_ast_pretty/src/pprust.rs

compiler/rustc_parse/src/parser/stmt.rs

compiler/rustc_typeck/src/check/expr.rs

compiler/rustc_mir_build/src/build/expr/category.rs

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

compiler/rustc_parse/src/parser/mod.rs

src/test/ui/parser/keyword-const-as-identifier.rs

compiler/rustc_parse/src/parser/pat.rs

bors · 2020-10-17T14:44:57Z

⌛ Testing commit 03321b8 with merge 6af9846...

bors · 2020-10-17T16:57:12Z

☀️ Test successful - checks-actions, checks-azure
Approved by: oli-obk
Pushing 6af9846 to master...

leonardo-m · 2020-10-18T08:31:26Z

I've tried:

#![allow(incomplete_features)]
#![feature(inline_const)]
fn main() {
    const N: u32 = 10;
    let x: u32 = 3;

    match x {
        1 .. const { N - 1 } => {},
        _ => {},
    }
}

It gave me errors:

error: expected one of `=>`, `if`, or `|`, found keyword `const`
 --> ...\temp.rs:9:14
  |
9 |         1 .. const { N - 1 } => {},
  |              ^^^^^ expected one of `=>`, `if`, or `|`

error[E0658]: half-open range patterns are unstable
 --> ...\temp.rs:9:9
  |
9 |         1 .. const { N - 1 } => {},
  |         ^^^^
  |
  = note: see issue #67264 <https://github.com/rust-lang/rust/issues/67264> for more information
  = help: add `#![feature(half_open_range_patterns)]` to the crate attributes to enable

jplatte · 2020-10-19T16:55:51Z

@leonardo-m You should probably open a new issue about that.

leonardo-m · 2020-10-19T17:01:18Z

@leonardo-m You should probably open a new issue about that.

OK, filed as #78108

petrochenkov · 2020-10-19T17:57:28Z

compiler/rustc_hir_pretty/src/lib.rs

+        self.s.word_space("const");
+        self.s.word("{");
+        self.print_anon_const(anon_const);
+        self.s.word("}");


Similarly to AST printing, the expr is already a block, printing the braces again shouldn't be necessary.

petrochenkov · 2020-10-19T18:04:05Z

compiler/rustc_parse/src/parser/mod.rs

@@ -545,6 +546,11 @@ impl<'a> Parser<'a> {
        self.check_or_expected(self.token.can_begin_const_arg(), TokenType::Const)
    }

+    fn check_inline_const(&mut self) -> bool {
+        self.check_keyword(kw::Const)
+            && self.look_ahead(1, |t| t == &token::OpenDelim(DelimToken::Brace))


token::NtBlock should probably also work as a next token.
(At least it will be successfully parsed by parse_const_expr.)

leonardo-m · 2020-10-19T19:29:01Z

One use case I've found in my code:

let _m = [const { Vec::<u32>::new() }; 40];

Rustc could suggest to write that when a programmer tries to write:

let _m = [vec![]; 40];

@petrochenkov

…r=petrochenkov Inline const followups r? @petrochenkov Follow ups of rust-lang#77124

@petrochenkov

…r=petrochenkov Inline const followups r? @petrochenkov Follow ups of rust-lang#77124

@petrochenkov

…t, r=petrochenkov Make inline const work in range patterns Fixes rust-lang#78108 which is a follow up of rust-lang#77124 r? @petrochenkov

@petrochenkov

…t, r=petrochenkov Make inline const work in range patterns Fixes rust-lang#78108 which is a follow up of rust-lang#77124 r? @petrochenkov

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

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jyn514 added A-const-eval Area: Constant evaluation (MIR interpretation) F-inline_const Inline constants (aka: const blocks, const expressions, anonymous constants) T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 25, 2020

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bors added the merged-by-bors This PR was explicitly merged by bors. label Oct 17, 2020

bors mentioned this pull request Oct 17, 2020

Implement destructuring assignment #71156

Closed

bors merged commit 6af9846 into rust-lang:master Oct 17, 2020

rustbot added this to the 1.49.0 milestone Oct 17, 2020

This was referenced Oct 17, 2020

Add #[cfg(panic = '...')] #74754

Merged

Mark repr128 as incomplete_features #77488

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spastorino mentioned this pull request Oct 19, 2020

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

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petrochenkov reviewed Oct 19, 2020

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spastorino mentioned this pull request Oct 19, 2020

Make inline const work in range patterns #78116

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spastorino mentioned this pull request Oct 19, 2020

Inline const followups #78118

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camelid mentioned this pull request Oct 21, 2020

ICE: "cannot convert ReErased to a region vid" for let const { "foo" } = "foo" #78174

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JohnTitor added a commit to JohnTitor/rust that referenced this pull request Oct 21, 2020

Rollup merge of rust-lang#78118 - spastorino:inline-const-followups, …

34c2223

…r=petrochenkov Inline const followups r? @petrochenkov Follow ups of rust-lang#77124

JohnTitor added a commit to JohnTitor/rust that referenced this pull request Oct 21, 2020

Rollup merge of rust-lang#78118 - spastorino:inline-const-followups, …

de24210

…r=petrochenkov Inline const followups r? @petrochenkov Follow ups of rust-lang#77124

camelid mentioned this pull request Dec 16, 2020

regression 1.49: macro_rules unexpected tokens #79908

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nbdd0121 mentioned this pull request Nov 7, 2022

Stabilise inline_const #104087

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Implement const expressions and patterns (RFC 2920) #77124

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Implement const expressions and patterns (RFC 2920) #77124

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