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While we previously checked the safety of safe imported modules we
didn't do this check transitively. This can be a problem when we depend
on a trustworthy module in a package that is no longer trusted, so we
should fail compilation. We already stored in an interface file the
transitive list of packages a module depends on. Now we extend that list
to include a flag saying if we depend on that package being trusted as
well.

This patch disables the use of some GHC extensions in
Safe mode and also the use of certain flags. Some
are disabled completely while others are only allowed
on the command line and not in source PRAGMAS.

We also check that Safe imports are indeed importing
a Safe or Trustworthy module.

See the long Note [Binders in Template Haskell] in Convert.lhs
which explains it all.  This patch fixes Trac #5037.

The key change is that NameU binders (ones made up by newName in
Template Haskell, and by TH quotations) now make Exact RdrNames again,
rather than making RdrNames with heavily encoded OccNames like x[03cv].
(This encoding is what was making #5037 fail.)

The issue here was: what import declaration brings into
scope the 'op here

   import qualified Foo( op )
   import Bar( C(op) )
   instance C Int where
     op = ...

Well, the import of Bar, obviously.  But what if the
import Bar had been
   import Bar( C )
Then the instance is still supposed to work, getting
op from the Foo.op imported from Foo.  (I'm assuming its
the same op, of course.)

This is a merge of a patch contributed by Michal Terepeta and the
recent generics changes.

The general rule is now that we don't complain about
a duplicate export from a "...".  See RnNames.dupExport_ok

The "Unhelpful" cases are now in a separate type. This allows us to
improve various things, e.g.:
* Most of the panic's in SrcLoc are now gone
* The Lexer now works with RealSrcSpans rather than SrcSpans, i.e. it
  knows that it has real locations and thus can assume that the line
  number etc really exists
* Some of the more suspicious cases are no longer necessary, e.g.
  we no longer need this case in advanceSrcLoc:
      advanceSrcLoc loc _ = loc -- Better than nothing

More improvements can probably be made, e.g. tick locations can
probably use RealSrcSpans too.

This was making mc17 fail.

Fixes Trac #4851

They belonged to the old generic deriving mechanism, so they can go. Adapted a lot of code as a consequence.

* Do-notation in arrows is marked with HsStmtContext = ArrowExpr
* tcMDoStmt (which was only used for arrows) is moved
  to TcArrows, and renamed tcArrDoStmt
* Improved documentation in the user manual
* Lots of other minor changes

Add a new flag XDefaultSignatures to enable just the signatures on the default methods. Redefine the behavior of XGenerics to mean enable XDefaultSignatures and XDeriveRepresentable.

Lots of refactoring. In particular I have now combined
TansformStmt and GroupStmt into a single constructor TransStmt.
This gives lots of useful code sharing.

This is the work of Nils Schweinsberg <mail@n-sch.de>

It adds the language extension -XMonadComprehensions, which
generalises list comprehension syntax [ e | x <- xs] to work over
arbitrary monads.

Also get rid of the old {| |} brackets in the lexer.

Fewer keywords!

In RnBinds.rnValBindsRHS I had
      (sig_dus `plusDU` bind_dus)
when it should be
      (bind_dus `plusDU` sig_dus)

So the fix is easy.

See the paper "Practical aspects of evidence based compilation in System FC"

* Coercion becomes a data type, distinct from Type

* Coercions become value-level things, rather than type-level things,
  (although the value is zero bits wide, like the State token)
  A consequence is that a coerion abstraction increases the arity by 1
  (just like a dictionary abstraction)

* There is a new constructor in CoreExpr, namely Coercion, to inject
  coercions into terms

(See his Haskell Symposium 2010 paper
    "A generic deriving mechaism for Haskell")

We collect variables introduced by the {...} part of a let-like record wildcard
pattern and do not warn if the user then doesn't actually use them.

Also change the behaviour slightly, to warn only for
*unqualified* imports.  See Trac #4977.

- Added a pragma {-# VECTORISE var = exp #-} that prevents
  the vectoriser from vectorising the definition of 'var'.
  Instead it uses the binding '$v_var = exp' to vectorise
  'var'.  The vectoriser checks that the Core type of 'exp'
  matches the vectorised Core type of 'var'.  (It would be
  quite complicated to perform that check in the type checker
  as the vectorisation of a type needs the state of the VM
  monad.)
- Added parts of a related VECTORISE SCALAR pragma
- Documented -ddump-vect
- Added -ddump-vt-trace
- Some clean up

Fixes Trac #4877.

This patch fixes Trac #4875.  The main point is to do dependency
analysis on type and class declarations, and kind-check them in
dependency order, so as to improve error messages.

This patch means that a few programs that would typecheck before won't
typecheck any more; but before we were (naughtily) going beyond
Haskell 98 without any language-extension flags, and Trac #4875
convinces me that doing so is a Bad Idea.

Here's an example that won't typecheck any more
       data T a b = MkT (a b)
       type F k = T k Maybe

If you look at T on its own you'd default 'a' to kind *->*;
and then kind-checking would fail on F.

But GHC currently accepts this program beause it looks at
the *occurrences* of T.

...so that you get helpful suggestions when you mis-spell a name
Based on Max's patch in Trac #2442, but heavily refactored.

For a long time an 'mdo' expression has had a SyntaxTable
attached to it.  However, we're busy deprecating SyntaxTables
in favour of rebindable syntax attached to individual Stmts,
and MDoExpr was totally inconsistent with DoExpr in this
regard.

This patch tidies it all up.  Now there's no SyntaxTable on
MDoExpr, and 'modo' is generally handled much more like 'do'.

There is resulting small change in behaviour: now MonadFix is
required only if you actually *use* recursion in mdo. This
seems consistent with the implicit dependency analysis that
is done for mdo.

Still to do:
  * Deal with #4148 (this patch is on the way)
  * Get rid of the last remaining SyntaxTable on HsCmdTop

  
The renamer wasn't attaching the right used-variables to a
TransformStmt constructor.

The real modification is in RnExpr; the rest is just
pretty-printing and white space.

But only when the module exports refer to different modules.
See ticket #4478.