cargo: include the implicit feature for dependencies in the manifest

Make the implicit `xyz = ["dep:xyz"]` declaration explicit in the Manifest.
This also makes it possible to implement correctly the part of the spec where
"If you specify the optional dependency with the dep: prefix anywhere in
the [features] table, that disables the implicit feature."

Unfortunately, this can only be done after target-specific configurations
are resolved, which makes it hard to write a unit test for this.  Rustix
requires it, though; without this patch, something like

        [package]
        name = "rustix"
        edition = "2021"
        rust-version = "1.63"
        version = "0.38.34"

        [dependencies]
        alloc = { version = "1.0.0", optional = true, package = "rustc-std-workspace-alloc" }
        libc = { version = "0.2.0", optional = true, package = "libc" }
        libc_errno = { version = "0.3.8", optional = true, package = "errno" }

        [features]
        alloc = []
        default = ["std"]
        rustc-dep-of-std = ["dep:alloc"]
        std = ["alloc"]
        use-libc = ["libc_errno", "libc"]

would incorrectly request the rustc-std-workspace-alloc crate via the
chain default->std->alloc.  The patch blocks it because it finds the
"dep:alloc" dependency of feature rustc-dep-of-std.

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

Author: bonzini

mesonbuild/cargo/interpreter.py

@@ -12,6 +12,7 @@
 from __future__ import annotations
 import dataclasses
 import functools
+import itertools
 import os
 import pathlib
 import collections
@@ -428,6 +429,20 @@ def _prepare_package(self, pkg: PackageState) -> None:
         for condition, dependencies in pkg.manifest.target.items():
             if eval_cfg(condition, cfgs):
                 pkg.manifest.dependencies.update(dependencies)
+
+        # If you specify the optional dependency with the dep: prefix anywhere in the [features]
+        # table, that disables the implicit feature.
+        deps = set(feature[4:]
+                   for feature in itertools.chain.from_iterable(pkg.manifest.features.values())
+                   if feature.startswith('dep:'))
+        for name, dep in itertools.chain(pkg.manifest.dependencies.items(),
+                                         pkg.manifest.dev_dependencies.items(),
+                                         pkg.manifest.build_dependencies.items()):
+            if dep.optional and name not in deps:
+                pkg.manifest.features.setdefault(name, [])
+                pkg.manifest.features[name].append(f'dep:{name}')
+                deps.add(name)
+
         # Fetch required dependencies recursively.
         for depname, dep in pkg.manifest.dependencies.items():
             if not dep.optional:
@@ -499,9 +514,6 @@ def _enable_feature(self, pkg: PackageState, feature: str) -> None:
         if feature in cfg.features:
             return
         cfg.features.add(feature)
-        # A feature can also be a dependency.
-        if feature in pkg.manifest.dependencies:
-            self._add_dependency(pkg, feature)
         # Recurse on extra features and dependencies this feature pulls.
         # https://doc.rust-lang.org/cargo/reference/features.html#the-features-section
         for f in pkg.manifest.features.get(feature, []):