intro-install_plan: fix destinations for build_targets with custom install_dir

There are a couple issues that combine to make the current handling a
bit confusing.

- we call it "install_dir_name" but it is only ever the class default

- CustomTarget always has it set to None, and then we check if it is
  None then create a different variable with a safe fallback. The if is
  useless -- it cannot fail, but if it did we'd get an undefined
  variable error when we tried to use `dir_name`

Remove the special handling for CustomTarget. Instead, just always
accept None as a possible value of outdir_name when constructing install
data, and, if it is None, fall back to {prefix}/outdir regardless of
what type it used to be.

1 files changed, 3 insertions, 2 deletions

diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index 245f612..3de5ef4 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -624,13 +624,14 @@ class Target(HoldableObject):
 
     def get_install_dir(self) -> T.Tuple[T.List[T.Union[str, Literal[False]]], str, Literal[False]]:
         # Find the installation directory.
-        default_install_dir, install_dir_name = self.get_default_install_dir()
+        default_install_dir, default_install_dir_name = self.get_default_install_dir()
         outdirs = self.get_custom_install_dir()
         if outdirs and outdirs[0] != default_install_dir and outdirs[0] is not True:
             # Either the value is set to a non-default value, or is set to
             # False (which means we want this specific output out of many
             # outputs to not be installed).
             custom_install_dir = True
+            default_install_dir_name = None
         else:
             custom_install_dir = False
             # if outdirs is empty we need to set to something, otherwise we set
@@ -640,7 +641,7 @@ class Target(HoldableObject):
             else:
                 outdirs = [default_install_dir]
 
-        return outdirs, install_dir_name, custom_install_dir
+        return outdirs, default_install_dir_name, custom_install_dir
 
     def get_basename(self) -> str:
         return self.name