Championed issue: dotnet/csharplang#275
Speclet: https://github.com/dotnet/csharplang/blob/master/proposals/static-lambdas.md
Main PR: #39121

Compiler scenarios

• LangVer (DisallowInNonPreview)
• static lambda cannot reference locals, parameters or this (StaticLambdaCannotCaptureInstanceField, StaticLambdaCannotCaptureParameter, StaticLambdaCannotReferenceBase and following)
• static lambda can reference static fields/methods/properties (StaticLambdaCanReferenceStaticField, StaticLambdaCanCallStaticMethod and following)
• static lambda can reference const field/local (StaticLambdaCanReferenceConstField and following)
• static lambda inside regular lambda (InstanceLambdaCannotHaveLocalsCapturedByInnerStaticLambda,
• regular lambda inside static lambda (still cannot capture outer locals, parameters or this) (StaticLambdaCanHaveLocalsCapturedByInnerInstanceLambda
• similar mixing of static and regular with lambdas and local functions (StaticLambdaCannotReferenceInstanceLocalFunction, StaticLambdaCanReferenceStaticLocalFunction, StaticLambdaCannotHaveLocalsCapturedByInnerStaticLambda, StaticLambdaCanHaveLocalsCapturedByInnerInstanceLocalFunction): outer and inner, either may be lambda or local function, either may be static or instance, check referencing variables from outer and middle scope.
• verify nameof (can still reference)
• can reference type parameters from enclosing method (default(T)), should be same as local functions
• verify with various lambda syntaxes (with/without types)
• test in top-level statements, with nint,
• success cases should be executed (produce an output), verify diagnostics and emit diagnostics
• static delegate (including parse tree test)
• parse tree test for all supported lambdas
• verify accessibility of private members (instance and static)
• capture what metadata we emit (although no guarantee, same IL as non-static lambda to avoid inefficiency)
• static should not interfere with caching (this falls under the "should not effect emit/validity" umbrella)
• verify that static lambda can be used in expression tree
• verify semantic model, verify symbol API, (verify .IsStatic), verify data flow
• verify IOperation (IAnonymousFunction, symbol covers need for API)
• Static anonymous funcs as call arguments:
  • M(static ...)
  • M(ref static ...)
  • M(param: static ...)
  • Also indexers: obj[static ...] and variants
• test with lambda discard parameters
• test in ternary (with cast on one side, or with target-typing with new feature), ref ternary (error), and switch expression
• review language parser for "is possible lambda" function
• check diagnostic and parse tree on trying to convert a static lambda to a function pointer (review message)

Productivity

• converting static anonymous function to static local function This refactoring does not seem to exist for regular anonymous functions.
• make anonymous function 'static' Add "make anonymous function static" code fix #45832
• recommend static keyword
• verify what captures are displayed (empty set)
  • in practice we display whatever is being captured, including things which are an error to capture.

LDM

• confirm static delegate (current implementation allows)