PDB import scopes refactoring.
Changes: To enable producing both Portable and native PDB formats we need to move the format specific imports encoding down to the PdbWriter. Instead of constructing NamespaceScope objects containing strings that encode usings/imports the C# and VB constructs that hold on to bound imports (ImportChain in C#, SourceFile in VB) now implement a new CCI-level interface IImportScope that provides an array of UsedTypeOrNamespace for each import scope. UsedTypeOrNamespace struct now doesn’t contain strings but symbols that represent the entities that re referenced in usings/Imports. IImportScope only represents imports on file level and below. Project level imports are exposed via new API on Cci.IModule (GetImports method). To represent a namespace a new CCI-level interface INamespace is introduced and implemented by NamespaceSymbol. In future we could replace usage of qualified namespace strings with this interface in other places in the compiler-CCI interface, but to minimize the impact I have not done so in this change. Background: Currently C# and VB compilers build a list of namespace scopes in NamespaceScopeBuilder that are then passed via MethodBody to the MetadataWriter, which then passes them to the PdbWriter. The NamespaceScopeBuilder encodes various forms of usings, extern aliases and imports in specially formatted strings that the EE understands how to decode. It constructs a list of all usings/imports/extern-aliases/etc. for each method. The PdbWriter then writes these string lists to PDB namespace scope records and implements a forwarding optimization – within a set of methods that have the same usings we designate one that we associate the strings with and the others forward to it. The forwarding is implemented differently by VB and C#. VB also defines some additional namespace scope records for default namespace, NoPia assembly names and method namespace. The new portable PDB format has a different, simpler and more efficient representation of import scopes with the same encoding for C# and VB. Import scopes are first class entities that have parent pointer and thus form a tree. The structure of the tree is language specific. The root of the tree represents a project/compilation-wide import scope. For C# this scope contains assembly reference aliases (/r:Alias=Reference.dll). For VB this scope declares project level imports. The import scope tree for VB is 2 level deep – file level and project level. C# may define deeper trees as namespaces may define their own usings. Also some information that Dev12 encodes in PDB namespace scopes is not used anymore by our new EEs (such as VB NoPia names, VB method namespace). These are not emitted to Portable PDB since the old EEs won’t be able to consume the new format anyways. (changeset 1410225)
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