use a more informative error message for invoking meson in a subdir

Explicitly mention that the project definition is invalid, and clarify
that project is `project()` -- a function.

Also try to walk the directory tree upward, and if there are parent
meson.build files, just say this isn't the project root, and "maybe you
meant to run meson there instead?"

This won't catch calls to subdir('foo/bar') but we can't be perfect,
only better than before and catch the *majority* of such cases, and
hopefully it's a lot more clear if meson protests that the project is
"invalid, there is no project() function", where the user should look
for a potential solution.

Fixes #3426

1 files changed, 1 insertions, 1 deletions

diff --git a/test cases/failing/1 project not first/test.json b/test cases/failing/1 project not first/test.json
index 70f3c41..27bae02 100644
--- a/test cases/failing/1 project not first/test.json
+++ b/test cases/failing/1 project not first/test.json
@@ -1,7 +1,7 @@
 {
   "stdout": [
     {
-      "line": "ERROR: First statement must be a call to project"
+      "line": "ERROR: Invalid source tree: first statement must be a call to project()"
     }
   ]
 }