GitLab

This patch only attempts to fix links that don't automatically re-direct to the correct URL.

- `emitCopySmallArray` now checks size before generating code and
  doesn't generate any code when size is 0. `emitCopyArray` already does
  this so this makes small/large array cases the same in argument
  checking.

- In both `emitCopySmallArray` and `emitCopyArray` read the `dflags`
  after checking the argument.

Any user can now trivially build any number of Haskell packages with
`-split-sections` by using `splitSections`/`splitSectionsIf` on any
existing or new flavour:

    -- build all packages but the ghc library with -split-sections
    splitSections :: Flavour -> Flavour

    -- build all packages that satisfy the given predicate
    -- with --split-sections
    splitSectionsIf :: (Package -> Bool) -> Flavour -> Flavour

See the new section in `doc/user-settings.md`.

We use Cabal to parse, configure, register and copy packages, which are
expensive operations that are currently not visible to Shake's profiling
infrastructure. By using `traced` we tell Shake to add these IO actions
to the profiling report, helping us to identify performance bottlenecks.

We use short tracing keys, as recommended in Shake docs: the name of the
current target is already available in the rest of the profiling
information.

The new release of Shake comes with these relevant features:

* use symlinks for --shared
* add --compact for a Bazel/Buck style output

See #16490.

[ci skip]

This moves all URL references to Trac Wiki to their corresponding
GitLab counterparts.

This substitution is classified as follows:

1. Automated substitution using sed with Ben's mapping rule [1]
    Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...
    New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...

2. Manual substitution for URLs containing `#` index
    Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...#Zzz
    New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...#zzz

3. Manual substitution for strings starting with `Commentary`
    Old: Commentary/XxxYyy...
    New: commentary/xxx-yyy...

See also !539

[1]: https://gitlab.haskell.org/bgamari/gitlab-migration/blob/master/wiki-mapping.json

By separating these two lines, if the first command fails then `ghci` is
not loaded. Before it would still load ghci but display lots of errors
about not being able to find modules.

ghci always exits with exit code 0 so you have to check the output
to see if the modules loaded succesfully.

Adds the mention that extensions "AllowAmbiguousTypes" and "RankNTypes"
are not always compatible with each other.
Specifies the conditions and causes of failing in resolving
of ambiguity.

They are broken and I don't have time to fix them at the moment.

`git fetch`, which we used previously, doesn't update the remote
tracking branches.

Previously we would use the local branch with the name
`$CI_MERGE_REQUEST_TARGET_BRANCH_NAME` to compute the merge base when
linting. However, this branch isn't necessarily up-to-date. We should
rather use `origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME`.

I'm not sure why these steps were done but they seem counterproductive
and unnecessary.

As per https://prime.haskell.org/wiki/Libraries/Proposals/MonadFailCoauthored-by: Ben Gamari <ben@well-typed.com>

This depends on the windows build which is still allowed to fail. If
that job fails then the packaging job will also fail.

Due to https://github.com/haskell/cabal/issues/5953.

Otherwise configure fails in the 32-bit case with
```
This GHC (c:/GitLabRunner/builds/8fc0e283/0/ghc/ghc/toolchain/bin/ghc) does not generate code for the build platform
   GHC target platform    : x86_64-unknown-mingw32
   Desired build platform : i386-unknown-mingw32
```

It seems that it has failed to execute at all since it was implemented.

We now run the linters on merge requests.

!301 removed the `HsArrApp` and `HsArrForm` constructors, which
renders the corresponding extension points `XArrApp` and `XArrForm`
useless. This patch finally rips them out.

As noted in #16466, `System.Environment.getExecutablePath` depends upon
`PathFileExistsW` which is defined by `shlwapi`.

Fixes #16466.

Fixes #16425

When GHC attempts to unify a metavariable with a type containing
foralls, it will be rejected as an occurrence of impredicativity.
GHC was /not/ extending the same treatment to predicate types, such
as in the following (erroneous) example from #11514:

```haskell
foo :: forall a. (Show a => a -> a) -> ()
foo = undefined
```

This will attempt to instantiate `undefined` at
`(Show a => a -> a) -> ()`, which is impredicative. This patch
catches impredicativity arising from predicates in this fashion.

Since GHC is pickier about impredicative instantiations, some test
cases needed to be updated to be updated so as not to fall afoul of
the new validity check. (There were a surprising number of
impredicative uses of `undefined`!) Moreover, the `T14828` test case
now has slightly less informative types shown with `:print`. This is
due to a a much deeper issue with the GHCi debugger (see #14828).

Fixes #11514.

The `build` stage is meant to be a minimal smoke test to weed out broken
commits. The `validate-x86_64-linux-deb9` build will generally catch a
subset of issues caught by `validate-x86_64-linux-deb9-debug` so only
the latter should be in `build`.