interpreter: Add a flatten() method to arrays

This allows users to do two things, flatten potentially nested arrays
themselves, and, to safely convert types that may be an array to not an
array.

```meson
x = [meson.get_external_property('may_be_array)].flatten()
```
```meson
x = ['a', ['b', 'c']]
assert(x.flatten() == ['a', 'b', 'c'])
```

1 files changed, 8 insertions, 1 deletions

diff --git a/test cases/common/56 array methods/meson.build b/test cases/common/56 array methods/meson.build
index e9e4969c7..3707775ec 100644
--- a/test cases/common/56 array methods/meson.build
+++ b/test cases/common/56 array methods/meson.build
@@ -1,4 +1,4 @@
-project('array methods')
+project('array methods', meson_version : '>= 1.9')
 
 empty = []
 one = ['abc']
@@ -68,3 +68,10 @@ endif
 if not combined.contains('ghi')
   error('Combined claims not to contain ghi.')
 endif
+
+# test array flattening
+x = ['a', ['b'], [[[[[[['c'], 'd']]], 'e']]]]
+assert(x.length() == 3)
+assert(x.flatten().length() == 5)
+assert(x.flatten() == ['a', 'b', 'c', 'd', 'e'])
+assert(['a', ['b', 'c']].flatten() == ['a', 'b', 'c'])