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Lifetime Annotations
====================
Problems
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Higher-ranked bounds (lifetimes) can be nested, and typecheck/inferrence can change that nesting after resolve
- Potential solution: each HRB block (`for<...>`) is numbered within one of three scopes, the impl scope, item scope, and body scope.
- Impl scope is for bounds that are visible for the entire `impl` or type block
- Item scope is for bounds visible within the current impl item (`fn`, `type`, ...)
- Body scope is for bounds within the body of a function (e.g. in type annotations)
- Each of these scopes specifies a prefix to the binding IDs, similar to how currently there's three prefixes for generics (impl, item, placeholder)
Region inferrence
- To what extent does type inferrene play into lifetime inferrence? Can lifetimes be inferred as a second pass after
types are concretely known, or do they play into each other?
Notes on Borrowck
=================
- On borrow, calculate lifetime of asignment (using existing lifetime code)
- Ignore reborrows?
- Visit all statements in that lifetime and locate places where the borrow is propagated/stored
- Requires lifetime parameters on functions/&-ptrs to be present
- Assignment of the source value during the lifetime of the borrow is an error
- Dropping of the source value is an error
- Returning the borrow is an error
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