[lldb][docs] Resurrect the information on adding a new language (#109427)

This got deleted in e078c95, I presume
accidentally, because it didn't have a corresponding rst file for it.

So I've brought it back and converted it into Markdown. The content
remains accurate, from what I know at least.

It's a bit "now draw the rest of the owl" but if nothing else, it gives
you a bunch of important classes to go and research as a starting point.

You can see the original content here:
https://github.com/llvm/llvm-project/blob/5d71fc5d7b5ffe2323418a09db6eddaf84d6c662/lldb/www/adding-language-support.html

Author: DavidSpickett

lldb/docs/index.rst

@@ -163,6 +163,7 @@ interesting areas to contribute to lldb.
    resources/caveats
    resources/projects
    resources/lldbdap
+   resources/addinglanguagesupport
    Public C++ API <https://lldb.llvm.org/cpp_reference/namespacelldb.html>
    Private C++ API <https://lldb.llvm.org/cpp_reference/index.html>
 

lldb/docs/resources/addinglanguagesupport.md

@@ -0,0 +1,95 @@
+# Adding Programming Language Support
+
+LLDB has been architected to make it straightforward to add support for a
+programming language. Only a small enum in core LLDB needs to be modified to
+make LLDB aware of a new programming language. Everything else can be supplied
+in derived classes that need not even be present in the core LLDB repository.
+This makes it convenient for developers adding language support in downstream
+repositories since it practically eliminates the potential for merge conflicts.
+
+The basic steps are:
+* Add the language to the `LanguageType` enum.
+* Add a `TypeSystem` for the language.
+* Add expression evaluation support.
+
+Additionally, you may want to create a `Language` and `LanguageRuntime` plugin
+for your language, which enables support for advanced features like dynamic
+typing and data formatting.
+
+## Add the Language to the LanguageType enum
+
+The `LanguageType` enum
+(see [lldb-enumerations.h](https://github.com/llvm/llvm-project/blob/main/lldb/include/lldb/lldb-enumerations.h))
+contains a list of every language known to LLDB. It is the one place where
+support for a language must live that will need to merge cleanly with upstream
+LLDB if you are developing your language support in a separate branch. When
+adding support for a language previously unknown to LLDB, start by adding an
+enumeration entry to `LanguageType`.
+
+## Add a TypeSystem for the Language
+
+Both [Module](https://github.com/llvm/llvm-project/blob/main/lldb/include/lldb/Core/Module.h)
+and [Target](https://github.com/llvm/llvm-project/blob/main/lldb/include/lldb/Target/Target.h)
+support the retrieval of a `TypeSystem` instance via `GetTypeSystemForLanguage()`.
+For `Module`, this method is directly on the `Module` instance. For `Target`,
+this is retrieved indirectly via the `TypeSystemMap` for the `Target` instance.
+
+The `TypeSystem` instance returned by the `Target` is expected to be capable of
+evaluating expressions, while the `TypeSystem` instance returned by the `Module`
+is not. If you want to support expression evaluation for your language, you could
+consider one of the following approaches:
+* Implement a single `TypeSystem` class that supports evaluation when given an
+  optional `Target`, implementing all the expression evaluation methods on the
+  `TypeSystem`.
+* Create multiple `TypeSystem` classes, one for evaluation and one for static
+  `Module` usage.
+
+For clang and Swift, the latter approach was chosen. Primarily to make it
+clearer that evaluation with the static `Module`-returned `TypeSystem` instances
+make no sense, and have them error out on those calls. But either approach is
+fine.
+
+# Creating Types
+
+Your `TypeSystem` will need an approach for creating types based on a set of
+`Module`s. If your type info is going to come from DWARF info, you will want to
+subclass [DWARFASTParser](https://github.com/llvm/llvm-project/blob/main/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParser.h).
+
+
+# Add Expression Evaluation Support
+
+Expression Evaluation support is enabled by implementing the relevant methods on
+a `TypeSystem`-derived class. Search for `Expression` in the
+[TypeSystem header](https://github.com/llvm/llvm-project/blob/main/lldb/include/lldb/Symbol/TypeSystem.h)
+to find the methods to implement.
+
+# Type Completion
+
+There are three levels of type completion, each requiring more type information:
+1. Pointer size: When you have a forward decl or a reference, and that's all you
+  need. At this stage, the pointer size is all you need.
+2. Layout info: You need the size of an instance of the type, but you still don't
+  need to know all the guts of the type.
+3. Full type info: Here you need everything, because you're playing with
+  internals of it, such as modifying a member variable.
+
+Ensure you never complete more of a type than is needed for a given situation.
+This will keep your type system from doing more work than necessary.
+
+# Language and LanguageRuntime Plugins
+
+If you followed the steps outlined above, you already have taught LLDB a great
+deal about your language. If your language's runtime model and fundamental data
+types don't differ much from the C model, you are pretty much done.
+
+However it is likely that your language offers its own data types for things
+like strings and arrays, and probably has a notion of dynamic types, where the
+effective type of a variable can only be known at runtime.
+
+These tasks are covered by two plugins:
+* a `LanguageRuntime` plugin, which provides LLDB with a dynamic view of your
+  language; this plugin answers questions that require a live process to acquire
+  information (for example dynamic type resolution).
+* a `Language` plugin, which provides LLDB with a static view of your language;
+  questions that are statically knowable and do not require a process are
+  answered by this plugin (for example data formatters).