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explain why Rvalue::Len still exists #129939

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Sep 6, 2024
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explain why Rvalue::Len still exists #129939

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3 changes: 3 additions & 0 deletions compiler/rustc_middle/src/mir/syntax.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1307,6 +1307,9 @@ pub enum Rvalue<'tcx> {
/// If the type of the place is an array, this is the array length. For slices (`[T]`, not
/// `&[T]`) this accesses the place's metadata to determine the length. This rvalue is
/// ill-formed for places of other types.
///
/// This cannot be a `UnOp(PtrMetadata, _)` because that expects a value, and we only
/// have a place, and `UnOp(PtrMetadata, RawPtr(place))` is not a thing.
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@scottmcm scottmcm Sep 5, 2024

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The other complication here, dunno whether it's worth mentioning, is that PtrMetadata needs a wide pointer, so if the place is an array you also need to unsize it to slice.

BTW, are there [ST]B issues with adding a short-lived raw-ref to use PtrMetadata if the place is something like (*_2).1? Or is it just that it's a bit annoying to make the temporary?

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@RalfJung RalfJung Sep 5, 2024

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The other complication here, dunno whether it's worth mentioning, is that PtrMetadata needs a wide pointer, so if the place is an array you also need to unsize it to slice.

Not really, as you said above -- if the place is an array we can just emit a constant instead.

BTW, are there [ST]B issues with adding a short-lived raw-ref to use PtrMetadata if the place is something like (*_2).1? Or is it just that it's a bit annoying to make the temporary?

There also shouldn't be any -- creating a raw pointer you never use should be effectively a NOP.

Len(Place<'tcx>),

/// Performs essentially all of the casts that can be performed via `as`.
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