GitLab

Test Plan: validate

Reviewers: simonpj, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2985

GHC Trac Issues: #13132

This fixes #11592.

Test Plan: validate

Reviewers: simonpj, austin, bgamari, goldfire

Reviewed By: goldfire

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2914

GHC Trac Issues: #11592

Summary:
At the moment, data and type declarations using infix formatting produce the
same AST as those using prefix.

So

    type a ++ b = c

and

    type (++) a b = c

cannot be distinguished in the parsed source, without looking at the OccName
details of the constructor being defined.

Having access to the OccName requires an additional constraint which explodes
out over the entire AST because of its recursive definitions.

In keeping with moving the parsed source to more directly reflect the source
code as parsed, add a specific flag to the declaration to indicate the fixity,
as used in a Match now too.

Note: this flag is to capture the fixity used for the lexical definition of the
type, primarily for use by ppr and ghc-exactprint.

Updates haddock submodule.

Test Plan: ./validate

Reviewers: mpickering, goldfire, bgamari, austin

Reviewed By: mpickering

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2828

GHC Trac Issues: #12942

Summary:
Add prettyprinter tests, which take a file, parse it, pretty print it,
re-parse the pretty printed version and then compare the original and
new ASTs (ignoring locations)

Updates haddock submodule to match the AST changes.

There are three issues outstanding

1. Extra parens around a context are not reproduced. This will require an
   AST change and will be done in a separate patch.

2. Currently if an `HsTickPragma` is found, this is not pretty-printed,
   to prevent noise in the output.

   I am not sure what the desired behaviour in this case is, so have left
   it as before. Test Ppr047 is marked as expected fail for this.

3. Apart from in a context, the ParsedSource AST keeps all the parens from
   the original source.  Something is happening in the renamer to remove the
   parens around visible type application, causing T12530 to fail, as the
   dumped splice decl is after the renamer.

   This needs to be fixed by keeping the parens, but I do not know where they
   are being removed.  I have amended the test to pass, by removing the parens
   in the expected output.

Test Plan: ./validate

Reviewers: goldfire, mpickering, simonpj, bgamari, austin

Reviewed By: simonpj, bgamari

Subscribers: simonpj, goldfire, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2752

GHC Trac Issues: #3384

This patch also removes the "catch all" pattern in the function and
explicitly lists constructors to get a warning in the future if a new
`HsType` was added.

Reviewers: bgamari, austin, simonpj

Reviewed By: simonpj

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D2753

GHC Trac Issues: #12711

The flag was:

1. Not documented.
2. Only used as a boolean flag.
3. Has overlapping functionality with -dno-debug-output
4. My poll of #ghc concluded that people didn't know it existed.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2627

GHC Trac Issues: #12691

Reviewers: bgamari, austin

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2586

GHC Trac Issues: #12617

Allows users to explicitly request which approach to `deriving` to use
via keywords, e.g.,

```
newtype Foo = Foo Bar
  deriving Eq
  deriving stock    Ord
  deriving newtype Show
```

Fixes #10598. Updates haddock submodule.

Test Plan: ./validate

Reviewers: hvr, kosmikus, goldfire, alanz, bgamari, simonpj, austin,
erikd, simonmar

Reviewed By: alanz, bgamari, simonpj

Subscribers: thomie, mpickering, oerjan

Differential Revision: https://phabricator.haskell.org/D2280

GHC Trac Issues: #10598

Summary:
This patch implements primitive unboxed sum types, as described in
https://ghc.haskell.org/trac/ghc/wiki/UnpackedSumTypes.

Main changes are:

- Add new syntax for unboxed sums types, terms and patterns. Hidden
  behind `-XUnboxedSums`.

- Add unlifted unboxed sum type constructors and data constructors,
  extend type and pattern checkers and desugarer.

- Add new RuntimeRep for unboxed sums.

- Extend unarise pass to translate unboxed sums to unboxed tuples right
  before code generation.

- Add `StgRubbishArg` to `StgArg`, and a new type `CmmArg` for better
  code generation when sum values are involved.

- Add user manual section for unboxed sums.

Some other changes:

- Generalize `UbxTupleRep` to `MultiRep` and `UbxTupAlt` to
  `MultiValAlt` to be able to use those with both sums and tuples.

- Don't use `tyConPrimRep` in `isVoidTy`: `tyConPrimRep` is really
  wrong, given an `Any` `TyCon`, there's no way to tell what its kind
  is, but `kindPrimRep` and in turn `tyConPrimRep` returns `PtrRep`.

- Fix some bugs on the way: #12375.

Not included in this patch:

- Update Haddock for new the new unboxed sum syntax.

- `TemplateHaskell` support is left as future work.

For reviewers:

- Front-end code is mostly trivial and adapted from unboxed tuple code
  for type checking, pattern checking, renaming, desugaring etc.

- Main translation routines are in `RepType` and `UnariseStg`.
  Documentation in `UnariseStg` should be enough for understanding
  what's going on.

Credits:

- Johan Tibell wrote the initial front-end and interface file
  extensions.

- Simon Peyton Jones reviewed this patch many times, wrote some code,
  and helped with debugging.

Reviewers: bgamari, alanz, goldfire, RyanGlScott, simonpj, austin,
           simonmar, hvr, erikd

Reviewed By: simonpj

Subscribers: Iceland_jack, ggreif, ezyang, RyanGlScott, goldfire,
             thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2259

Summary:
This annotates the splice point with 'HsSpliced ref e' where 'e' is the
result of the splice. 'ref' is a reference that the typechecker will fill with
the local type environment.

The finalizer then reads the ref and uses the local type environment, which
causes 'reify' to find local variables when run in the finalizer.

Test Plan: ./validate

Reviewers: simonpj, simonmar, bgamari, austin, goldfire

Reviewed By: goldfire

Subscribers: simonmar, thomie, mboes

Differential Revision: https://phabricator.haskell.org/D2286

GHC Trac Issues: #11832

This major commit was initially triggered by #11339, but it spiraled
into a major review of the way in which type signatures for bindings
are handled, especially partial type signatures.  On the way I fixed a
number of other bugs, namely
   #12069
   #12033
   #11700
   #11339
   #11670

The main change is that I completely reorganised the way in which type
signatures in bindings are handled. The new story is in TcSigs
Note [Overview of type signatures].  Some specific:

* Changes in the data types for signatures in TcRnTypes:
  TcIdSigInfo and new TcIdSigInst

* New module TcSigs deals with typechecking type signatures
  and pragmas. It contains code mostly moved from TcBinds,
  which is already too big

* HsTypes: I swapped the nesting of HsWildCardBndrs
  and HsImplicitBndsrs, so that the wildcards are on the
  oustide not the insidde in a LHsSigWcType.  This is just
  a matter of convenient, nothing deep.

There are a host of other changes as knock-on effects, and
it all took FAR longer than I anticipated :-).  But it is
a significant improvement, I think.

Lots of error messages changed slightly, some just variants but
some modest improvements.

New tests

* typecheck/should_compile
    * SigTyVars: a scoped-tyvar test
    * ExPat, ExPatFail: existential pattern bindings
    * T12069
    * T11700
    * T11339

* partial-sigs/should_compile
    * T12033
    * T11339a
    * T11670

One thing to check:

* Small change to output from ghc-api/landmines.
  Need to check with Alan Zimmerman

Previously, I had forgotten to omit variables already in scope
from the TypeInType CUSK check. Simple enough to fix.

Test case: typecheck/should_compile/T11811

When the typechecker generates the error message for an out-of-scope
variable, it now uses the GlobalRdrEnv with respect to which the
variable is unbound, not the GlobalRdrEnv which is available at the time
the error is reported.  Doing so ensures we do not provide suggestions
which themselves are out-of-scope (because they are bound in a later
inter-splice group).

Nonetheless, we do note in the error message if an unambiguous, exact
match to the out-of-scope variable is found in a later inter-splice
group, and we specify where that match is not in scope.

Test Plan: ./validate

Reviewers: goldfire, austin, bgamari

Reviewed By: goldfire

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2000

GHC Trac Issues: #11680

This should require -XScopedTypeVariables. It seems this was previously
handled by RnTypes.rnHsBndrSig which called RnTypes.badKindSigErr but
this was broken in Simon's refactor of wildcards,
1e041b73. Here we re-introduce a check
in RnPat. See #11663.

Test Plan: Validate with `T11663`

Reviewers: austin, simonpj

Reviewed By: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2054

GHC Trac Issues: #11663

We now check that a CUSK is really a CUSK and issue an error if
it isn't. This also involves more solving and zonking in
kcHsTyVarBndrs, which was the outright bug reported in #11648.

Test cases: polykinds/T11648{,b}

This updates the haddock submodule.

[skip ci]

Both gcc and clang tell which warning flag a reported warning can be
controlled with, this patch makes ghc do the same. More generally, this
allows for annotated compiler output, where an optional annotation is
displayed in brackets after the severity.

This also adds a new flag `-f(no-)show-warning-groups` to control
whether to show which warning-group (such as `-Wall` or `-Wcompat`)
a warning belongs to. This flag is on by default.

This implements #10752

Reviewed By: quchen, bgamari, hvr

Differential Revision: https://phabricator.haskell.org/D1943

When renaming a type, now looks for wildcards in bound variables'
kinds.

testcase: dependent/should_compile/T11241

Summary:
Previously, `-Wunused-matches` would fire whenever it detected unused type
variables in a type family or data family instance. This can be annoying for
users who wish to use type variable names as documentation, as being
`-Wall`-compliant would mean that they'd have to prefix many of their type
variable names with underscores, making the documentation harder to read.

To avoid this, a new warning `-Wunused-type-variables` was created that only
encompasses unused variables in family instances. `-Wunused-matches` reverts
back to its role of only warning on unused term-level pattern names. Unlike
`-Wunused-matches`, `-Wunused-type-variables` is not implied by `-Wall`.

Fixes #11451.

Test Plan: ./validate

Reviewers: goldfire, ekmett, austin, hvr, simonpj, bgamari

Reviewed By: simonpj, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1825

GHC Trac Issues: #11451

Summary:
In the past the canonical way for constructing an SDoc string literal was the
composition `ptext . sLit`.  But for some time now we have function `text` that
does the same.  Plus it has some rules that optimize its runtime behaviour.
This patch takes all uses of `ptext . sLit` in the compiler and replaces them
with calls to `text`.  The main benefits of this patch are clener (shorter) code
and less dependencies between module, because many modules now do not need to
import `FastString`.  I don't expect any performance benefits - we mostly use
SDocs to report errors and it seems there is little to be gained here.

Test Plan: ./validate

Reviewers: bgamari, austin, goldfire, hvr, alanz

Subscribers: goldfire, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1784

Summary:
Certain syntactic elements have integers in them, such as fixity
specifications, SPECIALISE pragmas and so on.

The lexer will accept mult-radix literals, with arbitrary leading zeros
in these.

Bring in a SourceText field to each affected AST element to capture the
original literal text for use with API Annotations.

Affected hsSyn elements are

```
     -- See note [Pragma source text]
     data Activation = NeverActive
                     | AlwaysActive
                     | ActiveBefore SourceText PhaseNum
                          -- Active only *strictly before* this phase
                     | ActiveAfter SourceText PhaseNum
                           -- Active in this phase and later
                     deriving( Eq, Data, Typeable )
                               -- Eq used in comparing rules in HsDecls

     data Fixity = Fixity SourceText Int FixityDirection
       -- Note [Pragma source text]
       deriving (Data, Typeable)
 ```

and

```
      | HsTickPragma         -- A pragma introduced tick
         SourceText           -- Note [Pragma source text] in BasicTypes
         (StringLiteral,(Int,Int),(Int,Int))
                                          -- external span for this tick
         ((SourceText,SourceText),(SourceText,SourceText))
            -- Source text for the four integers used in the span.
            -- See note [Pragma source text] in BasicTypes
         (LHsExpr id)
```

Updates haddock submodule

Test Plan: ./validate

Reviewers: goldfire, bgamari, austin

Reviewed By: bgamari

Subscribers: thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1781

GHC Trac Issues: #11430

Differential Revision: https://phabricator.haskell.org/D1785

We now check for unused variables one at a time, instead of
all at the top.

Test: dependent/should_compile/T11405

Since GHC 8.1/8.2 only needs to be bootstrap-able by GHC 7.10 and
GHC 8.0 (and GHC 8.2), we can now finally drop all that pre-AMP
compatibility CPP-mess for good!

Reviewers: austin, goldfire, bgamari

Subscribers: goldfire, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D1724

This re-working of the typechecker algorithm is based on
the paper "Visible type application", by Richard Eisenberg,
Stephanie Weirich, and Hamidhasan Ahmed, to be published at
ESOP'16.

This patch introduces -XTypeApplications, which allows users
to say, for example `id @Int`, which has type `Int -> Int`. See
the changes to the user manual for details.

This patch addresses tickets #10619, #5296, #10589.

Michal's work on #10982, #11098, refactored the handling of named
wildcards by making them more like ordinary type variables.

This patch takes the same idea to its logical conclusion, resulting
in a much tidier, tighter implementation.

Read Note [The wildcard story for types] in HsTypes.

Changes:

 * Named wildcards are ordinary type variables, throughout

 * HsType no longer has a data constructor for named wildcards
   (was NamedWildCard in HsWildCardInfo).  Named wildcards are
   simply HsTyVars

 * Similarly named wildcards disappear from Template Haskell

 * I refactored RnTypes to avoid polluting LocalRdrEnv with something
   as narrow as named wildcards.  Instead the named wildcard set is
   carried in RnTyKiEnv.

There is a submodule update for Haddock.

An HsAppInfix can carry a qconop/varop preceded by a SIMPLEQUOTE as a
Located RdrName.

In this case AnnSimpleQuote is attached to the Located HsAppType.

    | SIMPLEQUOTE qconop            {% ams (sLL $1 $> $ HsAppInfix $2)
                                           [mj AnnSimpleQuote $1] }
    | SIMPLEQUOTE varop             {% ams (sLL $1 $> $ HsAppInfix $2)
                                           [mj AnnSimpleQuote $1] }

This patch changes

    data HsType name
      ...
      | HsAppsTy [HsAppType name]

to

    data HsType name
      ...
      | HsAppsTy [LHsAppType name]

so that the annotation is not discarded when it reaches the ParsedSource

Until now GADTs were supported in Template Haskell by encoding them using
normal data types.  This patch adds proper support for representing GADTs
in TH.

Test Plan: T10828

Reviewers: goldfire, austin, bgamari

Subscribers: thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1465

GHC Trac Issues: #10828

The warnings are enabled with the flag -fwarn-unused-matches, the same
one that enables warnings on the term level.

Identifiers starting with an underscore are now always parsed as type
variables.  When the NamedWildCards extension is enabled, the renamer
replaces those variables with named wildcards.

An additional NameSet nwcs is added to LocalRdrEnv. It's used to keep
names of the type variables that should be replaced with wildcards.

While renaming HsForAllTy, when a name is explicitly bound it is removed
from the nwcs NameSet. As a result, the renamer doesn't replace them in
the quantifier body. (Trac #11098)

Fixes #10982, #11098

Reviewers: alanz, bgamari, hvr, austin, jstolarek

Reviewed By: jstolarek

Subscribers: goldfire, mpickering, RyanGlScott, thomie

Differential Revision: https://phabricator.haskell.org/D1576

GHC Trac Issues: #10982

Summary:
Post #11019, there have been some new instances of RdrName that are not
located, in particular

```#!hs
data FieldOcc name = FieldOcc { rdrNameFieldOcc  :: RdrName
                              , selectorFieldOcc :: PostRn name name
                              }

data AmbiguousFieldOcc name
  = Unambiguous RdrName (PostRn name name)
  | Ambiguous   RdrName (PostTc name name)
  deriving (Typeable)
```
Add locations to them

Updates haddock submodule to match

Test Plan: ./validate

Reviewers: goldfire, hvr, bgamari, austin

Reviewed By: hvr

Subscribers: hvr, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1670

GHC Trac Issues: #11258

This exposes `template-haskell` functions for querying the language
extensions which are enabled when compiling a module,

- an `isExtEnabled` function to check whether an extension is enabled
- an `extsEnabled` function to obtain a full list of enabled extensions

To avoid code duplication this adds a `GHC.LanguageExtensions` module to
`ghc-boot` and moves `DynFlags.ExtensionFlag` into it. A happy
consequence of this is that the ungainly `DynFlags` lost around 500
lines. Moreover, flags corresponding to language extensions are now
clearly distinguished from other flags due to the `LangExt.*` prefix.

Updates haddock submodule.

This fixes #10820.

Test Plan: validate

Reviewers: austin, spinda, hvr, goldfire, alanz

Reviewed By: goldfire

Subscribers: mpickering, RyanGlScott, hvr, simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D1200

GHC Trac Issues: #10820

As mentioned in #4426 these warnings are now errors since the Great
Wildcards Refactor of 2015 (1e041b73).
I've opened #11221 to ensure we remove the last traces of the option in
8.2.

Test Plan: validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1615

GHC Trac Issues: #4426

This implements the ideas originally put forward in
"System FC with Explicit Kind Equality" (ICFP'13).

There are several noteworthy changes with this patch:
 * We now have casts in types. These change the kind
   of a type. See new constructor `CastTy`.

 * All types and all constructors can be promoted.
   This includes GADT constructors. GADT pattern matches
   take place in type family equations. In Core,
   types can now be applied to coercions via the
   `CoercionTy` constructor.

 * Coercions can now be heterogeneous, relating types
   of different kinds. A coercion proving `t1 :: k1 ~ t2 :: k2`
   proves both that `t1` and `t2` are the same and also that
   `k1` and `k2` are the same.

 * The `Coercion` type has been significantly enhanced.
   The documentation in `docs/core-spec/core-spec.pdf` reflects
   the new reality.

 * The type of `*` is now `*`. No more `BOX`.

 * Users can write explicit kind variables in their code,
   anywhere they can write type variables. For backward compatibility,
   automatic inference of kind-variable binding is still permitted.

 * The new extension `TypeInType` turns on the new user-facing
   features.

 * Type families and synonyms are now promoted to kinds. This causes
   trouble with parsing `*`, leading to the somewhat awkward new
   `HsAppsTy` constructor for `HsType`. This is dispatched with in
   the renamer, where the kind `*` can be told apart from a
   type-level multiplication operator. Without `-XTypeInType` the
   old behavior persists. With `-XTypeInType`, you need to import
   `Data.Kind` to get `*`, also known as `Type`.

 * The kind-checking algorithms in TcHsType have been significantly
   rewritten to allow for enhanced kinds.

 * The new features are still quite experimental and may be in flux.

 * TODO: Several open tickets: #11195, #11196, #11197, #11198, #11203.

 * TODO: Update user manual.

Tickets addressed: #9017, #9173, #7961, #10524, #8566, #11142.
Updates Haddock submodule.

The ConDecl type in HsDecls is an uneasy compromise. For the most part,
HsSyn directly reflects the syntax written by the programmer; and that
gives just the right "pegs" on which to hang Alan's API annotations. But
ConDecl doesn't properly reflect the syntax of Haskell-98 and GADT-style
data type declarations.

To be concrete, here's a draft new data type

```lang=hs
data ConDecl name
  | ConDeclGADT
      { con_names   :: [Located name]
      , con_type    :: LHsSigType name  -- The type after the ‘::’
      , con_doc     :: Maybe LHsDocString }

  | ConDeclH98
      { con_name    :: Located name

      , con_qvars     :: Maybe (LHsQTyVars name)
        -- User-written forall (if any), and its implicit
        -- kind variables
        -- Non-Nothing needs -XExistentialQuantification

      , con_cxt       :: Maybe (LHsContext name)
        -- ^ User-written context (if any)

      , con_details   :: HsConDeclDetails name
          -- ^ Arguments

      , con_doc       :: Maybe LHsDocString
          -- ^ A possible Haddock comment.
      } deriving (Typeable)
```

Note that

    For GADTs, just keep a type. That's what the user writes.
    NB:HsType can represent records on the LHS of an arrow:

      { x:Int,y:Bool} -> T

    con_qvars and con_cxt are both Maybe because they are both
    optional (the forall and the context of an existential data type

    For ConDeclGADT the type variables of the data type do not scope
    over the con_type; whereas for ConDeclH98 they do scope over con_cxt
    and con_details.

Updates haddock submodule.

Test Plan: ./validate

Reviewers: simonpj, erikd, hvr, goldfire, austin, bgamari

Subscribers: erikd, goldfire, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1558

GHC Trac Issues: #11028

This patch began as a modest refactoring of HsType and friends, to
clarify and tidy up exactly where quantification takes place in types.
Although initially driven by making the implementation of wildcards more
tidy (and fixing a number of bugs), I gradually got drawn into a pretty
big process, which I've been doing on and off for quite a long time.

There is one compiler performance regression as a result of all
this, in perf/compiler/T3064.  I still need to look into that.

* The principal driving change is described in Note [HsType binders]
  in HsType.  Well worth reading!

* Those data type changes drive almost everything else.  In particular
  we now statically know where

       (a) implicit quantification only (LHsSigType),
           e.g. in instance declaratios and SPECIALISE signatures

       (b) implicit quantification and wildcards (LHsSigWcType)
           can appear, e.g. in function type signatures

* As part of this change, HsForAllTy is (a) simplified (no wildcards)
  and (b) split into HsForAllTy and HsQualTy.  The two contructors
  appear when and only when the correponding user-level construct
  appears.  Again see Note [HsType binders].

  HsExplicitFlag disappears altogether.

* Other simplifications

     - ExprWithTySig no longer needs an ExprWithTySigOut variant

     - TypeSig no longer needs a PostRn name [name] field
       for wildcards

     - PatSynSig records a LHsSigType rather than the decomposed
       pieces

     - The mysterious 'GenericSig' is now 'ClassOpSig'

* Renamed LHsTyVarBndrs to LHsQTyVars

* There are some uninteresting knock-on changes in Haddock,
  because of the HsSyn changes

I also did a bunch of loosely-related changes:

* We already had type synonyms CoercionN/CoercionR for nominal and
  representational coercions.  I've added similar treatment for

      TcCoercionN/TcCoercionR

      mkWpCastN/mkWpCastN

  All just type synonyms but jolly useful.

* I record-ised ForeignImport and ForeignExport

* I improved the (poor) fix to Trac #10896, by making
  TcTyClsDecls.checkValidTyCl recover from errors, but adding a
  harmless, abstract TyCon to the envt if so.

* I did some significant refactoring in RnEnv.lookupSubBndrOcc,
  for reasons that I have (embarrassingly) now totally forgotten.
  It had to do with something to do with import and export

Updates haddock submodule.

At the moment the API Annotations can only be used on the ParsedSource,
as there are changes made to the RenamedSource that prevent it from
being used to round trip source code.

It is possible to build a map from every Located Name in the
RenamedSource from its location to the Name, which can then be used when
resolved names are required when changing the ParsedSource.

However, there are instances where the identifier is not located,
specifically

  (GHC.VarPat name)
  (GHC.HsVar name)
  (GHC.UserTyVar name)
  (GHC.HsTyVar name)

Replace each of the name types above with (Located name)

Updates the haddock submodule.

Test Plan: ./validate

Reviewers: austin, goldfire, bgamari

Reviewed By: bgamari

Subscribers: goldfire, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1512

GHC Trac Issues: #11019

Summary:
At the moment ghc-exactprint, which uses the GHC API Annotations to
provide a framework for roundtripping Haskell source code with optional
AST edits, has to implement a horrible workaround to manage the points
where layout needs to be captured.

These are

    MatchGroup
    HsDo
    HsCmdDo
    HsLet
    LetStmt
    HsCmdLet
    GRHSs

To provide a more natural representation, the contents subject to layout
rules need to be wrapped in a SrcSpan.

This commit does this.

Trac ticket #10250

Test Plan: ./validate

Reviewers: hvr, goldfire, bgamari, austin, mpickering

Reviewed By: mpickering

Subscribers: thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1370

GHC Trac Issues: #10250