Linear types (#15981)

This is the first step towards implementation of the linear types proposal
(https://github.com/ghc-proposals/ghc-proposals/pull/111).

It features

* A language extension -XLinearTypes
* Syntax for linear functions in the surface language
* Linearity checking in Core Lint, enabled with -dlinear-core-lint
* Core-to-core passes are mostly compatible with linearity
* Fields in a data type can be linear or unrestricted; linear fields
  have multiplicity-polymorphic constructors.
  If -XLinearTypes is disabled, the GADT syntax defaults to linear fields

The following items are not yet supported:

* a # m -> b syntax (only prefix FUN is supported for now)
* Full multiplicity inference (multiplicities are really only checked)
* Decent linearity error messages
* Linear let, where, and case expressions in the surface language
  (each of these currently introduce the unrestricted variant)
* Multiplicity-parametric fields
* Syntax for annotating lambda-bound or let-bound with a multiplicity
* Syntax for non-linear/multiple-field-multiplicity records
* Linear projections for records with a single linear field
* Linear pattern synonyms
* Multiplicity coercions (test LinearPolyType)

A high-level description can be found at
https://ghc.haskell.org/trac/ghc/wiki/LinearTypes/Implementation
Following the link above you will find a description of the changes made to Core.
This commit has been authored by

* Richard Eisenberg
* Krzysztof Gogolewski
* Matthew Pickering
* Arnaud Spiwack

With contributions from:

* Mark Barbone
* Alexander Vershilov

Updates haddock submodule.

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/GHC/Stg/Lift/Monad.hs b/compiler/GHC/Stg/Lift/Monad.hs
index b693730eca..21097cc59f 100644
--- a/compiler/GHC/Stg/Lift/Monad.hs
+++ b/compiler/GHC/Stg/Lift/Monad.hs
@@ -39,6 +39,7 @@ import GHC.Types.Unique.Supply
 import GHC.Utils.Misc
 import GHC.Types.Var.Env
 import GHC.Types.Var.Set
+import GHC.Core.Multiplicity
 
 import Control.Arrow ( second )
 import Control.Monad.Trans.Class
@@ -274,7 +275,7 @@ withLiftedBndr abs_ids bndr inner = do
         -- See Note [transferPolyIdInfo] in GHC.Types.Id. We need to do this at least
         -- for arity information.
         = transferPolyIdInfo bndr (dVarSetElems abs_ids)
-        . mkSysLocal (mkFastString str) uniq
+        . mkSysLocal (mkFastString str) uniq Many
         $ ty
   LiftM $ RWS.local
     (\e -> e