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feat(compiler-cli): lower some exported expressions (#30038)
The compiler uses metadata to represent what it statically knows about various expressions in a program. Occasionally, expressions in the program for which metadata is extracted may contain sub-expressions which are not representable in metadata. One such construct is an arrow function. The compiler does not always need to understand such expressions completely. For example, for a provider defined with `useValue`, the compiler does not need to understand the value at all, only the outer provider definition. In this case, the compiler employs a technique known as "expression lowering", where it rewrites the provider expression into one that can be represented in metadata. Chiefly, this involves extracting out the dynamic part (the `useValue` expression) into an exported constant. Lowering is applied through a heuristic, which considers the containing statement as well as the field name of the expression. Previously, this heuristic was not completely accurate in the case of route definitions and the `loadChildren` field, which is lowered. If the route definition using `loadChildren` existed inside a decorator invocation, lowering was performed correctly. However, if it existed inside a standalone variable declaration with an export keyword, the heuristic would conclude that lowering was unnecessary. For ordinary providers this is true; however the compiler attempts to fully understand the ROUTES token and thus even if an array of routes is declared in an exported variable, any `loadChildren` expressions within still need to be lowered. This commit enables lowering of already exported variables under a limited set of conditions (where the initializer expression is of a specific form). This should enable the use of `loadChildren` in route definitions. PR Close #30038
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