Remove MultiGet from the codebase (pantsbuild#23057)

Removes the migration goal. Anyone migrating needs to
be on 2.31 or earlier. 

Removes the migration guide, and switches references to
it to point to the version in the 2.30 docs in perpetuity.

Also gets rid of the "await in loop" custom flake8 check, 
as it doesn't work with call-by-name, and getting it to would
be complex and not worth the effort. This may have been
some worthwhile nannying in the early days of the engine
but does not seem important now.

A followup will remove remaining traces of `Get`. A further
followup after that may remove engine code that is no 
longer needed.

Author: benjyw

build-support/flake8/.flake8

@@ -24,4 +24,3 @@ extend-ignore:
 [flake8:local-plugins]
 extension =
     PNT20 = dedent_use_checker:check_for_dedent_imports
-    PNT30 = await_in_loop:check_for_await_in_loop

build-support/flake8/await_in_loop.py

@@ -9,7 +9,7 @@ How to adjust for changes made to the Plugin API.
 
 ## 2.30
 
-`Get` and `MultiGet` have been deprecated in favor of calling rules by name. See the [migration guide](../the-rules-api/migrating-gets.mdx) for how to update your plugins to use this idiom.
+`Get` and `MultiGet` have been deprecated in favor of calling rules by name. See the [migration guide](https://www.pantsbuild.org/2.30/docs/writing-plugins/the-rules-api/migrating-gets) for how to update your plugins to use this idiom.
 
 ## 2.23
 

docs/docs/writing-plugins/common-plugin-tasks/plugin-upgrade-guide.mdx

@@ -17,5 +17,4 @@ Adding logic to your plugin.
 - [Union rules (advanced)](./union-rules-advanced.mdx)
 - [Logging and dynamic output](./logging-and-dynamic-output.mdx)
 - [Testing rules](./testing-plugins.mdx)
-- [Tips and debugging](./tips-and-debugging.mdx)
-- [Migrating from call-by-type](./migrating-gets.mdx)
+- [Tips and debugging](./tips-and-debugging.mdx)

docs/docs/writing-plugins/the-rules-api/index.mdx

@@ -12,8 +12,12 @@ Thank you to [Klaviyo](https://www.klaviyo.com/) for their Platinum tier support
 
 ### Highlights
 
+Support for the deprecated `Get()`/`MultiGet()` in the plugin API has been removed.
+
 ### Deprecations
 
+The plugin API's `Get()` and `MultiGet()` constructs, deprecated in 2.30, are now unsupported. If your plugin still relies on them you must [migrate](https://www.pantsbuild.org/2.30/docs/writing-plugins/the-rules-api/migrating-gets) before upgrading to this version.
+
 ### General
 
 ### Goals

docs/docs/writing-plugins/the-rules-api/migrating-gets.mdx

@@ -804,7 +804,7 @@ class _DependencyMapping:
 async def transitive_dependency_mapping(request: _DependencyMappingRequest) -> _DependencyMapping:
     """This uses iteration, rather than recursion, so that we can tolerate dependency cycles.
 
-    Unlike a traditional BFS algorithm, we batch each round of traversals via `MultiGet` for
+    Unlike a traditional BFS algorithm, we batch each round of traversals via `concurrently` for
     improved performance / concurrency.
     """
     roots_as_targets = await resolve_unexpanded_targets(Addresses(request.tt_request.roots))

docs/notes/2.32.x.md

@@ -24,7 +24,7 @@
     AwaitableConstraints,
     Effect,
     GetParseError,
-    MultiGet,
+    concurrently,
 )
 from pants.util.docutil import doc_url
 from pants.util.memo import memoized
@@ -427,7 +427,7 @@ def visit_Assign(self, assign_node: ast.Assign) -> None:
                 continue
             if isinstance(node, ast.Call):
                 f = self._lookup(node.func)
-                if f is MultiGet:
+                if f is concurrently:
                     value = tuple(get.output_type for get in collected_awaitables)
                 elif f is not None:
                     value = _lookup_return_type(f)

src/python/pants/engine/internals/graph.py

@@ -15,7 +15,7 @@
 
 from pants.base.exceptions import RuleTypeError
 from pants.engine.internals.rule_visitor import collect_awaitables
-from pants.engine.internals.selectors import Get, GetParseError, MultiGet
+from pants.engine.internals.selectors import Get, GetParseError, concurrently
 from pants.engine.rules import implicitly, rule
 from pants.util.strutil import softwrap
 
@@ -129,15 +129,15 @@ async def rule():
 @pytest.mark.call_by_type
 def test_multiget_homogeneous() -> None:
     async def rule():
-        await MultiGet(Get(STR, INT(x)) for x in range(5))
+        await concurrently(Get(STR, INT(x)) for x in range(5))
 
     assert_awaitables(rule, [(str, int)])
 
 
 @pytest.mark.call_by_type
 def test_multiget_heterogeneous() -> None:
     async def rule():
-        await MultiGet(Get(STR, INT, 42), Get(INT, STR("bob")))
+        await concurrently(Get(STR, INT, 42), Get(INT, STR("bob")))
 
     assert_awaitables(rule, [(str, int), (int, str)])
 
@@ -392,7 +392,7 @@ async def rule(request: Request):
         s = request.arg2
         Get(bool, s)
         # 3, 4
-        a, b = await MultiGet(
+        a, b = await concurrently(
             Get(list, str),
             Get(tuple, str),
         )
@@ -403,7 +403,7 @@ async def rule(request: Request):
         # 7 -- this is huge!
         c = Request.create_get()
         # 8 -- the `c` is already accounted for, make sure it's not duplicated.
-        await MultiGet([c, Get(str, dict)])
+        await concurrently([c, Get(str, dict)])
         # 9
         Get(float, request._helped())