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, allowing invalid HIR to reach type checking and trigger 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 is now handled consistently with other invalid pattern expressions.

A new UI test is included to ensure the compiler reports a proper error and to prevent regressions.

Closes #148138.

[rustbot]

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[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

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[dianne]

This is looking good now, thanks! Just a few more notes.
@rustbot author

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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[dianne]

Looks good to me; thanks! One last request: could you fix the PR and commit descriptions? Specifically,

Any ExprKind::ConstBlock inside a pattern now lowers to PatKind::Err, consistent with how other invalid pattern expressions are handled.

This isn't the case. Invalid expression patterns lower to PatKind::Expr of PatExprKind::Lit representing a LitKind::Err. This doesn't need to be in the PR description, but I'd like to make sure whatever details are there are accurate. ^^

I think it'd be fine without that sentence, but it also wouldn't hurt to keep the detail about being consistent with how other invalid pattern expressions are handled. I'll leave the particulars up to you.

• Removed Pat::ConstBlockPat from Rust Analyzer.

This is no longer part of the PR.

• Cleaned up Clippy and other tools.

No other tools needed cleanup. And this is just a nit, but I think the clippy change could count as part of removing PatExprKind::ConstBlock from the HIR; there shouldn't be any other cleanup to clippy.

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