rust: change warning_level=0 to "--cap-lints allow"

This is a better and more backwards-compatible way to disable all warnings,
compared to "-A warnings".  The Rust RFC (https://rust-lang.github.io/rfcs/1193-cap-lints.html)
explains the rationale:

> We would very much like to be able to modify lints, however. For example
> rust-lang/rust#26473 updated the missing_docs lint to also look for missing
> documentation on const items. This ended up breaking some crates in the
> ecosystem due to their usage of #![deny(missing_docs)].

While at it, document that Rust deviates from the other languages in its
interpretation of warning_level=0.

Signed-off-by: Paolo Bonzini <[email protected]>

docs/markdown/Builtin-options.md

@@ -98,6 +98,8 @@ machine](#specifying-options-per-machine) section for details.
 | force_fallback_for                     | []            | Force fallback for those dependencies                          | no             | no                |
 | vsenv                                  | false         | Activate Visual Studio environment                             | no             | no                |
 
+(For the Rust language only, `warning_level=0` disables all warnings).
+
 #### Details for `backend`
 
 Several build file formats are supported as command runners to build the

mesonbuild/compilers/rust.py

@@ -69,7 +69,7 @@ class RustCompiler(Compiler):
     id = 'rustc'
 
     _WARNING_LEVELS: T.Dict[str, T.List[str]] = {
-        '0': ['-A', 'warnings'],
+        '0': ['--cap-lints', 'allow'],
         '1': [],
         '2': [],
         '3': ['-W', 'warnings'],