Auto merge of #138492 - lcnr:rm-inline_const_pat, r=oli-obk

remove `feature(inline_const_pat)`

Summarizing https://rust-lang.zulipchat.com/#narrow/channel/144729-t-types/topic/remove.20feature.28inline_const_pat.29.20and.20shared.20borrowck.

With https://github.com/rust-lang/types-team/issues/129 we will start to borrowck items together with their typeck parent. This is necessary to correctly support opaque types, blocking the new solver and TAIT/ATPIT stabilization with the old one. This means that we cannot really support `inline_const_pat` as they are implemented right now:

- we want to typeck inline consts together with their parent body to allow inference to flow both ways and to allow the const to refer to local regions of its parent.This means we also need to borrowck the inline const together with its parent as that's necessary to properly support opaque types
- we want the inline const pattern to participate in exhaustiveness checking
- to participate in exhaustiveness checking we need to evaluate it, which requires borrowck, which now relies on borrowck of the typeck root, which ends up checking exhaustiveness again. **This is a query cycle**.

There are 4 possible ways to handle this:
- stop typechecking inline const patterns together with their parent
  - causes inline const patterns to be different than inline const exprs
  - prevents bidirectional inference, we need to either fail to compile `if let const { 1 } = 1u32` or `if let const { 1u32 } = 1`
  - region inference for inline consts will be harder, it feels non-trivial to support inline consts referencing local regions from the parent fn
- inline consts no longer participate in exhaustiveness checking. Treat them like `pat if pat == const { .. }`  instead. We then only evaluate them after borrowck
  - difference between `const { 1 }`  and `const FOO: usize = 1; match x { FOO => () }`. This is confusing
  - do they carry their weight if they are now just equivalent to using an if-guard
- delay exhaustiveness checking until after borrowck
  - should be possible in theory, but is a quite involved change and may have some unexpected challenges
- remove this feature for now

I believe we should either delay exhaustiveness checking or remove the feature entirely. As moving exhaustiveness checking to after borrow checking is quite complex I think the right course of action is to fully remove the feature for now and to add it again once/if we've got that implementation figured out.

`const { .. }`-expressions remain stable. These seem to have been the main motivation for https://github.com/rust-lang/rfcs/issues/2920.

r? types

cc `@rust-lang/types` `@rust-lang/lang` #76001

7 files changed, 14 insertions, 29 deletions

diff --git a/compiler/rustc_ast_passes/src/feature_gate.rs b/compiler/rustc_ast_passes/src/feature_gate.rs
index 2fac881f4c4..ea60e083c4c 100644
--- a/compiler/rustc_ast_passes/src/feature_gate.rs
+++ b/compiler/rustc_ast_passes/src/feature_gate.rs
@@ -490,7 +490,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         half_open_range_patterns_in_slices,
         "half-open range patterns in slices are unstable"
     );
-    gate_all!(inline_const_pat, "inline-const in pattern position is experimental");
     gate_all!(associated_const_equality, "associated const equality is incomplete");
     gate_all!(yeet_expr, "`do yeet` expression is experimental");
     gate_all!(dyn_star, "`dyn*` trait objects are experimental");
diff --git a/compiler/rustc_feature/src/removed.rs b/compiler/rustc_feature/src/removed.rs
index 47e4f9a2fe8..6ba7b22a0df 100644
--- a/compiler/rustc_feature/src/removed.rs
+++ b/compiler/rustc_feature/src/removed.rs
@@ -142,6 +142,9 @@ declare_features! (
     /// Allows inferring `'static` outlives requirements (RFC 2093).
     (removed, infer_static_outlives_requirements, "1.63.0", Some(54185),
      Some("removed as it caused some confusion and discussion was inactive for years")),
+    /// Allow anonymous constants from an inline `const` block in pattern position
+    (removed, inline_const_pat, "CURRENT_RUSTC_VERSION", Some(76001),
+     Some("removed due to implementation concerns as it requires significant refactorings")),
     /// Lazily evaluate constants. This allows constants to depend on type parameters.
     (removed, lazy_normalization_consts, "1.46.0", Some(72219), Some("superseded by `generic_const_exprs`")),
     /// Changes `impl Trait` to capture all lifetimes in scope.
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 72468dd4714..7c931e32f63 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/compiler/rustc_feature/src/unstable.rs
@@ -531,8 +531,6 @@ declare_features! (
     (unstable, import_trait_associated_functions, "1.86.0", Some(134691)),
     /// Allows associated types in inherent impls.
     (incomplete, inherent_associated_types, "1.52.0", Some(8995)),
-    /// Allow anonymous constants from an inline `const` block in pattern position
-    (unstable, inline_const_pat, "1.58.0", Some(76001)),
     /// Allows using `pointer` and `reference` in intra-doc links
     (unstable, intra_doc_pointers, "1.51.0", Some(80896)),
     // Allows using the `kl` and `widekl` target features and the associated intrinsics
diff --git a/compiler/rustc_parse/messages.ftl b/compiler/rustc_parse/messages.ftl
index 3253222b8f2..24679692a0b 100644
--- a/compiler/rustc_parse/messages.ftl
+++ b/compiler/rustc_parse/messages.ftl
@@ -838,8 +838,6 @@ parse_unexpected_expr_in_pat_const_sugg = consider extracting the expression int
 
 parse_unexpected_expr_in_pat_create_guard_sugg = consider moving the expression to a match arm guard
 
-parse_unexpected_expr_in_pat_inline_const_sugg = consider wrapping the expression in an inline `const` (requires `{"#"}![feature(inline_const_pat)]`)
-
 parse_unexpected_expr_in_pat_update_guard_sugg = consider moving the expression to the match arm guard
 
 parse_unexpected_if_with_if = unexpected `if` in the condition expression
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 1b4341ec28d..55cb7687823 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -2769,17 +2769,6 @@ pub(crate) enum UnexpectedExpressionInPatternSugg {
         /// The statement's block's indentation.
         indentation: String,
     },
-
-    #[multipart_suggestion(
-        parse_unexpected_expr_in_pat_inline_const_sugg,
-        applicability = "maybe-incorrect"
-    )]
-    InlineConst {
-        #[suggestion_part(code = "const {{ ")]
-        start_span: Span,
-        #[suggestion_part(code = " }}")]
-        end_span: Span,
-    },
 }
 
 #[derive(Diagnostic)]
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index a79b4048288..d865fd42764 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -1370,9 +1370,6 @@ impl<'a> Parser<'a> {
 
     /// Parses inline const expressions.
     fn parse_const_block(&mut self, span: Span, pat: bool) -> PResult<'a, P<Expr>> {
-        if pat {
-            self.psess.gated_spans.gate(sym::inline_const_pat, span);
-        }
         self.expect_keyword(exp!(Const))?;
         let (attrs, blk) = self.parse_inner_attrs_and_block(None)?;
         let anon_const = AnonConst {
@@ -1380,7 +1377,17 @@ impl<'a> Parser<'a> {
             value: self.mk_expr(blk.span, ExprKind::Block(blk, None)),
         };
         let blk_span = anon_const.value.span;
-        Ok(self.mk_expr_with_attrs(span.to(blk_span), ExprKind::ConstBlock(anon_const), attrs))
+        let kind = if pat {
+            let guar = self
+                .dcx()
+                .struct_span_err(blk_span, "`inline_const_pat` has been removed")
+                .with_help("use a named `const`-item or an `if`-guard instead")
+                .emit();
+            ExprKind::Err(guar)
+        } else {
+            ExprKind::ConstBlock(anon_const)
+        };
+        Ok(self.mk_expr_with_attrs(span.to(blk_span), kind, attrs))
     }
 
     /// Parses mutability (`mut` or nothing).
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index ec14c5718da..174cc929fa7 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -631,15 +631,6 @@ impl<'a> Parser<'a> {
                             ident,
                             indentation,
                         });
-
-                        // help: wrap the expr in a `const { expr }`
-                        // FIXME(inline_const_pat): once stabilized, remove this check and remove the `(requires #[feature(inline_const_pat)])` note from the message
-                        if self.parser.psess.unstable_features.is_nightly_build() {
-                            err.subdiagnostic(UnexpectedExpressionInPatternSugg::InlineConst {
-                                start_span: expr_span.shrink_to_lo(),
-                                end_span: expr_span.shrink_to_hi(),
-                            });
-                        }
                     },
                 );
             }