Implement better defaulting for build-type

This implements the following defaulting rules:

 * For `cabal-version:2.0` and below, default to the `Custom`
   build-type unconditionally (legacy defaulting)

 * Otherwise, if a `custom-setup` stanza is defined, default to
   the `Custom` build-type; else default to `Simple` build-type.

This gets us better defaults for the two most popular use-cases, and
which can be statically inferred by only looking at the `.cabal` file.

This allows us to bring down the minimal (modern) trivial cabal
package definition down to a single file with 4 lines:

    cabal-version: 2.1
    name: mu
    version: 0
    library

NB: We don't need any `Setup.hs` file, as `cabal sdist` will magically
generate one on the fly.

Author: hvr

Cabal/Distribution/PackageDescription/Check.hs

@@ -448,7 +448,7 @@ checkFields pkg =
            "Package names with the prefix 'z-' are reserved by Cabal and "
         ++ "cannot be used."
 
-  , check (isNothing (buildTypeRaw pkg)) $
+  , check (isNothing (buildTypeRaw pkg) && specVersion pkg < mkVersion [2,1]) $
       PackageBuildWarning $
            "No 'build-type' specified. If you do not need a custom Setup.hs or "
         ++ "./configure script then use 'build-type: Simple'."

Cabal/Distribution/Types/PackageDescription.hs

@@ -190,11 +190,22 @@ descCabalVersion pkg = case specVersionRaw pkg of
 -- /effective/ @build-type@. This function implements the following
 -- defaulting rules:
 --
---  * For @cabal-version:2.0@ and below, default to @Custom@ build-type.
+--  * For @cabal-version:2.0@ and below, default to the @Custom@
+--    build-type unconditionally.
+--
+--  * Otherwise, if a @custom-setup@ stanza is defined, default to
+--    the @Custom@ build-type; else default to @Simple@ build-type.
 --
 -- @since 2.2
 buildType :: PackageDescription -> BuildType
-buildType = fromMaybe Custom . buildTypeRaw
+buildType pkg
+  | specVersion pkg >= mkVersion [2,1]
+    = fromMaybe newDefault (buildTypeRaw pkg)
+  | otherwise -- cabal-version < 2.1
+    = fromMaybe Custom (buildTypeRaw pkg)
+  where
+    newDefault | isNothing (setupBuildInfo pkg) = Simple
+               | otherwise                      = Custom
 
 emptyPackageDescription :: PackageDescription
 emptyPackageDescription

Cabal/doc/developing-packages.rst

@@ -798,12 +798,20 @@ describe the package as a whole:
 
 .. pkg-field:: build-type: identifier
 
-    :default: ``Custom``
+    :default: ``Custom`` or ``Simple``
 
     The type of build used by this package. Build types are the
     constructors of the
     `BuildType <../release/cabal-latest/doc/API/Cabal/Distribution-PackageDescription.html#t:BuildType>`__
-    type, defaulting to ``Custom``.
+    type. This field is optional and when missing, its default value
+    is inferred according to the following rules:
+
+     - When :pkg-field:`cabal-version` is set to ``2.1`` or higher,
+       the default is ``Simple`` unless a :pkg-section:`custom-setup`
+       exists, in which case the inferred default is ``Custom``.
+
+     - For lower :pkg-field:`cabal-version` values, the default is
+       ``Custom`` unconditionally.
 
     If the build type is anything other than ``Custom``, then the
     ``Setup.hs`` file *must* be exactly the standardized content