meson setup fails on Windows with an absolute path for libdir on E:\ drive and is case sensitive for Windows drive

[azmr]

Describe the bug
I'm learning to use Meson following the tutorial (which for Windows requires quite a lot of additional research to get things to work - something to address another time)

I have python and Meson on the C:\ drive, source and libraries on E:. (E:\ is currently a USB drive if that's relevant.)
When trying to add a dependency to a library, I tried to set the libdir to the library location on E:\ with meson setup build --libdir=E:/Documents/Coding/C/lib (after an rm -rf build) and got a python stack trace in response:

$ meson setup build --libdir=E:/Documents/Coding/C/lib
The Meson build system
Version: 0.52.1
Source dir: E:\Documents\Coding\C\meson_test
Build dir: E:\Documents\Coding\C\meson_test\build
Build type: native build
Traceback (most recent call last):
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\mesonmain.py", line 129, in run
    return options.run_func(options)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\msetup.py", line 245, in run
    app.generate()
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\msetup.py", line 159, in generate
    self._generate(env)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\msetup.py", line 176, in _generate
    intr = interpreter.Interpreter(b)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\interpreter.py", line 2110, in __init__
    self.parse_project()
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\interpreterbase.py", line 397, in parse_project
    self.evaluate_codeblock(self.ast, end=1)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\interpreterbase.py", line 436, in evaluate_codeblock
    raise e
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\interpreterbase.py", line 430, in evaluate_codeblock
    self.evaluate_statement(cur)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\interpreterbase.py", line 441, in evaluate_statement
    return self.function_call(cur)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\interpreterbase.py", line 776, in function_call
    return func(node, posargs, kwargs)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\interpreterbase.py", line 143, in wrapped
    return f(*wrapped_args, **wrapped_kwargs)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\interpreterbase.py", line 174, in wrapped
    return f(*wrapped_args, **wrapped_kwargs)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\interpreter.py", line 2723, in func_project
    self.coredata.set_default_options(default_options, self.subproject, self.environment)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\coredata.py", line 742, in set_default_options
    self.set_options(options, subproject=subproject)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\coredata.py", line 674, in set_options
    if self._try_set_builtin_option(k, v):
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\coredata.py", line 531, in _try_set_builtin_option
    value = self.sanitize_dir_option_value(prefix, optname, value)
  File "C:\Users\<user>\AppData\Roaming\Python\Python38\site-packages\mesonbuild\coredata.py", line 479, in sanitize_dir_option_value
    if os.path.commonpath([value, prefix]) != str(PurePath(prefix)):
  File "c:\users\liz\appdata\local\programs\python\python38\lib\ntpath.py", line 763, in commonpath
    raise ValueError("Paths don't have the same drive")
ValueError: Paths don't have the same drive

This is possibly related to #3239

If I try the same instruction but with the libraries copied to the C:\ drive, I get the following response:

$ meson setup build --libdir=C:/Documents/Coding/C/lib
The Meson build system
Version: 0.52.1
Source dir: E:\Documents\Coding\C\meson_test
Build dir: E:\Documents\Coding\C\meson_test\build
Build type: native build

meson.build:1:0: ERROR: The value of the 'libdir' option is 'C:/Documents/Coding/C/lib' which must be a subdir of the prefix 'c:/'.
Note that if you pass a relative path, it is assumed to be a subdir of prefix.

A full log can be found at E:\Documents\Coding\C\meson_test\build\meson-logs\meson-log.txt

If I do the same again but with $ meson setup build --libdir=c:/Documents/Coding/C/lib (note the lowercase c) I get the same response as for $ meson setup build (i.e. it seems to work).

(I'm aware that I should be setting the prefix argument to the "installation directory" but I'm not yet sure what that is)

Files To Reproduce
I don't think the contents of the files matters. These are as barebones as it gets.
main.c
meson.build

Expected behavior
I would expect Meson to show me an error message and a way to fix things (rather than a stack trace).
I would also expect Windows paths to be case insensitive or to be warned that they are case sensitive.

System Parameters

• Native build
• Windows 10
• Python 3.8.1
• meson --version => 0.52.1
• ninja --version => 1.9.0.git.kitware.dyndep-1.jobserver-1

Notes
Thanks for all your work on Meson, it looks it will be a useful tool once I've figured it out.
If I can provide any more information, do let me know.

[jpakkane]

Can you test if the linked MR fixes this for you?