fix: Nail destructuring assignment once and for all #18254
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Because our lint infra *can* handle allows from within macro expansions! (Also, what did this reason have to do with something that is a hard error and not a lint? I'm puzzled). I wonder how many such diagnostics we have... Maybe that also mean we can change `unused_mut` to no-longer-experimental? But this is a change I'm afraid to do without checking.
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Instead of lowering them to `<expr> = <expr>`, then hacking on-demand to resolve them, we lower them to `<pat> = <expr>`, and use the pattern infrastructure to handle them. It turns out, destructuring assignments are surprisingly similar to pattern bindings, and so only minor modifications are needed. This fixes few bugs that arose because of the non-uniform handling (for example, MIR lowering not handling slice and record patterns, and closure capture calculation not handling destructuring assignments at all), and furthermore, guarantees we won't have such bugs in the future, since the programmer will always have to explicitly handle `Expr::Assignment`. Tests don't pass yet; that's because the generated patterns do not exist in the source map. The next commit will fix that.
And few more fixups. I was worried this will lead to more memory usage since `ExprOrPatId` is double the size of `ExprId`, but this does not regress `analysis-stats .`. If this turns out to be a problem, we can easily use the high bit to encode this information.
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By lowering it into patterns instead of expressions.
This is a large PR (phew!) but it also removes a lot of code, almost as much as it inserts. All present and probably even future code dedicated to handling destructuring assignments is thrown out of the window. Besides fixing a bunch of bugs, this also means that any future expansion is going to work much less to work with destructuring assignments, and is much more likely to not have bugs/rough edges/forgotten cases.
rustc goes even more than this and desugars destructuring assignment completely; coming to the HIR it vanishes. But we cannot do that, because this will severely affect our ability to map destructuring assignments up and down the lowering chain (for instance, consider what happens when a macro expands to the "pattern". With rustc's approach, the macro expansion will be split, and we have no way to represent that).
Fixes #18209.