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, 6 insertions, 8 deletions

diff --git a/compiler/GHC/Builtin/primops.txt.pp b/compiler/GHC/Builtin/primops.txt.pp
index ee7bdac29a..a9ebb5645f 100644
--- a/compiler/GHC/Builtin/primops.txt.pp
+++ b/compiler/GHC/Builtin/primops.txt.pp
@@ -194,17 +194,15 @@ section "The word size story."
 -- This type won't be exported directly (since there is no concrete
 -- syntax for this sort of export) so we'll have to manually patch
 -- export lists in both GHC and Haddock.
-primtype (->) a b
-  {The builtin function type, written in infix form as {\tt a -> b} and
-   in prefix form as {\tt (->) a b}. Values of this type are functions
-   taking inputs of type {\tt a} and producing outputs of type {\tt b}.
+primtype FUN m a b
+  {The builtin function type, written in infix form as {\tt a # m -> b}.
+   Values of this type are functions taking inputs of type {\tt a} and
+   producing outputs of type {\tt b}. The multiplicity of the input is
+   {\tt m}.
 
-   Note that {\tt a -> b} permits levity-polymorphism in both {\tt a} and
+   Note that {\tt FUN m a b} permits levity-polymorphism in both {\tt a} and
    {\tt b}, so that types like {\tt Int\# -> Int\#} can still be well-kinded.
   }
-  with fixity = infixr -1
-         -- This fixity is only the one picked up by Haddock. If you
-         -- change this, do update 'ghcPrimIface' in 'GHC.Iface.Load'.
 
 ------------------------------------------------------------------------
 section "Char#"