interpreter: Warn when environment() ops are overriden

Warn when someone tries to use append() or prepend() on an env var
which already has an operation set on it. People seem to think that
multiple append/prepend operations stack, but they don't.

Closes https://github.com/mesonbuild/meson/issues/5087

2 files changed, 21 insertions, 0 deletions

diff --git a/test cases/unit/59 test env doesn't stack/meson.build b/test cases/unit/59 test env doesn't stack/meson.build
new file mode 100644
index 000000000..01f2637de
--- /dev/null
+++ b/test cases/unit/59 test env doesn't stack/meson.build
@@ -0,0 +1,12 @@
+project('test env var stacking')
+
+testenv = environment()
+testenv.set('TEST_VAR_SET', 'some-value')
+testenv.set('TEST_VAR_APPEND', 'some-value')
+testenv.set('TEST_VAR_PREPEND', 'some-value')
+
+testenv.append('TEST_VAR_APPEND', 'another-value-append', separator: ':')
+testenv.prepend('TEST_VAR_PREPEND', 'another-value-prepend', separator: ':')
+testenv.set('TEST_VAR_SET', 'another-value-set')
+
+test('check env', find_program('script.py'), env: testenv)
diff --git a/test cases/unit/59 test env doesn't stack/script.py b/test cases/unit/59 test env doesn't stack/script.py
new file mode 100755
index 000000000..2a766732e
--- /dev/null
+++ b/test cases/unit/59 test env doesn't stack/script.py
@@ -0,0 +1,9 @@
+#!/usr/bin/env python3
+
+import os
+
+for name in ('append', 'prepend', 'set'):
+    envname = 'TEST_VAR_' + name.upper()
+    value = 'another-value-' + name
+    envvalue = os.environ[envname]
+    assert (envvalue == value), (name, envvalue)