Merge #7147

7147: ssr: Allow replacing expressions with statements r=davidlattimore a=MarijnS95

Depends on #6587

Until that is merged, the diff is https://github.com/MarijnS95/rust-analyzer/compare/stmt..replace-expr-with-stmt

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Now that statements can be matched and replaced (#6587) some usecases require expressions to be replaced with statements as well. This happens when something that can ambiguously be an expression or statement like `if` and loop blocks appear in the last position of a block, as trailing expression. In this case a replacement pattern of the form `if foo(){$a();}==>>$a();` will only substitute `if` blocks in the list of statements but not if they (implicitly) end up in the trailing expression, where they are not wrapped by an EXPR_STMT (but the pattern and template are, as parsing only succeeds for the `stmt ==>> stmt` case).

Instead of adding two rules that match an expression - and emit duplicate matching errors - allow the template for expressions to be a statement if it fails to parse as an expression.

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Another gross change that does not seem to break any tests currently, but perhaps a safeguard should be added to only allow this kind of replacement in blocks by "pushing" the replacement template to the statement list and clearing the trailing expression?

CC @davidlattimore

Co-authored-by: Marijn Suijten <marijn@traverseresearch.nl>