Fix ICE by rejecting const blocks in patterns during AST lowering (closes #148138)

[Shinonn23]

This PR fixes the ICE reported in #148138

The root cause is that const blocks aren’t allowed in pattern position, but the AST lowering logic still attempted to create PatExprKind::ConstBlock. That allowed invalid HIR to reach type checking, which then hit a span_bug!.

Following the discussion in the issue, this patch removes the ConstBlock lowering path from lower_expr_within_pat.
Any ExprKind::ConstBlock inside a pattern now lowers to PatKind::Err, consistent with how other invalid pattern expressions are handled.

A new UI test is included to ensure the compiler emits a proper error for const blocks in patterns and to prevent regressions.

Closes #148138

[rustbot]

r? @SparrowLii

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[reddevilmidzy]

If you use #![expect(...)], the WARN will not occur.

Suggested change

	#![feature(deref_patterns)]
	//~^ WARN the feature `deref_patterns` is incomplete
	#![feature(deref_patterns)]
	#![expect(incomplete_features)]

[Shinonn23]

Thanks for the suggestion, @reddevilmidzy! I've updated the test file to use #![expect(incomplete_features)] instead of //~^ WARN and force-pushed the changes.

[dianne]

Since they're caught here now, maybe it would make sense to remove the special-casing of const blocks in patterns from the parser?

[Shinonn23]

Thanks! I've removed PatExprKind::ConstBlock and cleaned up the parser, HIR, and tooling (including rust-analyzer) as suggested. Ready for review.

[dianne]

I'd actually not recommend changing rust-analyzer here. Could you remove the rust-analyzer changes from this PR and open a separate cleanup PR to the rust-analyzer repository instead (rust-lang/rust-analyzer)? rust-analyzer uses its own representations of Rust code, so it shouldn't be necessary to change it here, I don't think.

[Shinonn23]

I've reverted the rust-analyzer changes in the latest push

[dianne]

I'm also not seeing the change to the parser you said you did. Are you writing this using an LLM? I'd strongly recommend writing contributions by hand and reviewing your own code and github interactions. If I'm effectively writing this PR by proxy, I can't approve it; it'd at least need another reviewer to sign off on it. I'd like to help you contribute if it's your own contribution, but machine-generated PRs and replies place extra stress on reviewers; we rely on contributors to self-review and understand the changes they're making. See rust-lang/compiler-team#893 for more information. Apologies if I've jumped to conclusions.

[dianne]

Thanks for your patience as well, and sorry for any alarm I might have caused. ^^
It's perfectly fine if you need translation assistance. I would just recommend extra care in that case since LLMs do have significant shortcomings when it comes to communication.

[dianne]

And of course, if anything is unclear, I'd be happy to clarify or explain.

[Shinonn23]

Thank you very much, it my first time to contribute :) So, I have done clean up the parser

[dianne]

r? dianne

[rustbot]

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred in match checking

cc @Nadrieril

[Veykril]

This should not need to touch rust-analyzer source

View changes since this review

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

4 LL |         const { 1 + 7 } => {}
-    |               ^^^^^^^^^
+    |         ^^^^^ expected identifier, found keyword
6    |
-    = help: use a named `const`-item or an `if`-guard (`x if x == const { ... }`) instead
+ help: escape `const` to use it as an identifier
+    |
+ LL |         r#const { 1 + 7 } => {}
+    |         ++
8 
- error: aborting due to 1 previous error
+ error: expected `,`
+   --> $DIR/in-pat-recovery.rs:6:19
+    |
+ LL |         const { 1 + 7 } => {}
+    |         -----     ^
+    |         |
+    |         while parsing the fields for this pattern
10 
+ error[E0422]: cannot find struct, variant or union type `r#const` in this scope
+   --> $DIR/in-pat-recovery.rs:6:9
+    |
+ LL |         const { 1 + 7 } => {}
+    |         ^^^^^ not found in this scope
+ 
+ error: aborting due to 3 previous errors
+ 
+ For more information about this error, try `rustc --explain E0422`.
---
+   --> $DIR/in-pat-recovery.rs:6:9
+    |         ^^^^^ expected identifier, found keyword
+ help: escape `const` to use it as an identifier
+    |
+ LL |         r#const { 1 + 7 } => {}
+    |         ++
+ error: expected `,`
+   --> $DIR/in-pat-recovery.rs:6:19
+    |
+ LL |         const { 1 + 7 } => {}
+    |         -----     ^
+    |         |
+    |         while parsing the fields for this pattern
+ error[E0422]: cannot find struct, variant or union type `r#const` in this scope
+   --> $DIR/in-pat-recovery.rs:6:9
+    |
+ LL |         const { 1 + 7 } => {}
+    |         ^^^^^ not found in this scope
+ 
+ error: aborting due to 3 previous errors
+ 
+ For more information about this error, try `rustc --explain E0422`.
---
To only update this specific test, also pass `--test-args inline-const/in-pat-recovery.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/inline-const/in-pat-recovery.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/inline-const/in-pat-recovery" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error: expected identifier, found keyword `const`
##[error]  --> /checkout/tests/ui/inline-const/in-pat-recovery.rs:6:9
   |
LL |         const { 1 + 7 } => {}
   |         ^^^^^ expected identifier, found keyword
   |
help: escape `const` to use it as an identifier
   |
LL |         r#const { 1 + 7 } => {}
   |         ++

error: expected `,`
##[error]  --> /checkout/tests/ui/inline-const/in-pat-recovery.rs:6:19
   |

[dianne]

We still need to parse inline const blocks for error recovery purposes. Without this, we'll try parsing the const as an identifier. However, the cleanup we can do is removing the check in parse_const_block that the const block isn't from a pattern, since we'll reach an error in that case later.