cargo: improve error message for dependency version conflicts

While developing I often got a confusing 'Neither a subproject directory nor
a None.wrap file was found'.  Replace it with a clear message that explains:

1. Which dependency couldn't be found
2. What versions are available in the root Cargo.lock
3. That this is likely a version conflict
4. How to fix it (update dependencies to match root Cargo.lock)

Example:
ERROR: Dependency 'socket2-0.4-rs' not found. Root Cargo.lock provides:
socket2-0.5-rs, socket2-0.6-rs. This could be a Meson bug, please report it.

I am actually not sure if this is really always a Meson bug.  But to be
safe, suggest reporting it until more experience is gained.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 33 insertions, 1 deletions

diff --git a/mesonbuild/cargo/interpreter.py b/mesonbuild/cargo/interpreter.py
index be3d29337..74d7ae09f 100644
--- a/mesonbuild/cargo/interpreter.py
+++ b/mesonbuild/cargo/interpreter.py
@@ -124,6 +124,17 @@ class Interpreter:
 
     def _fetch_package_from_subproject(self, package_name: str, meson_depname: str) -> T.Tuple[PackageState, bool]:
         subp_name, _ = self.environment.wrap_resolver.find_dep_provider(meson_depname)
+        if subp_name is None:
+            # If Cargo.lock has a different version, this could be a resolution
+            # bug, but maybe also a version mismatch?  I am not sure yet...
+            similar_deps = [pkg.subproject
+                            for pkg in self.cargolock.named(package_name)]
+            if similar_deps:
+                similar_msg = f'Cargo.lock provides: {", ".join(similar_deps)}.'
+            else:
+                similar_msg = 'Cargo.lock does not contain this crate name.'
+            raise MesonException(f'Dependency {meson_depname!r} not found in any wrap files or Cargo.lock; {similar_msg} This could be a Meson bug, please report it.')
+
         subdir, _ = self.environment.wrap_resolver.resolve(subp_name)
         subprojects_dir = os.path.join(subdir, 'subprojects')
         self.environment.wrap_resolver.load_and_merge(subprojects_dir, T.cast('SubProject', meson_depname))
diff --git a/mesonbuild/cargo/manifest.py b/mesonbuild/cargo/manifest.py
index 698bb98a5..01e7c697b 100644
--- a/mesonbuild/cargo/manifest.py
+++ b/mesonbuild/cargo/manifest.py
@@ -5,12 +5,13 @@
 
 from __future__ import annotations
 
+import collections
 import dataclasses
 import os
 import typing as T
 
 from . import version
-from ..mesonlib import MesonException, lazy_property
+from ..mesonlib import MesonException, lazy_property, Version
 from .. import mlog
 
 if T.TYPE_CHECKING:
@@ -489,6 +490,14 @@ class CargoLockPackage:
     checksum: T.Optional[str] = None
     dependencies: T.List[str] = dataclasses.field(default_factory=list)
 
+    @lazy_property
+    def api(self) -> str:
+        return version.api(self.version)
+
+    @lazy_property
+    def subproject(self) -> str:
+        return f'{self.name}-{self.api}-rs'
+
     @classmethod
     def from_raw(cls, raw: raw.CargoLockPackage) -> CargoLockPackage:
         return _raw_to_dataclass(raw, cls, 'Cargo.lock package')
@@ -508,3 +517,15 @@ class CargoLock:
     def from_raw(cls, raw: raw.CargoLock) -> CargoLock:
         return _raw_to_dataclass(raw, cls, 'Cargo.lock',
                                  package=lambda x: [CargoLockPackage.from_raw(p) for p in x])
+
+    def named(self, name: str) -> T.Sequence[CargoLockPackage]:
+        return self._versions[name]
+
+    @lazy_property
+    def _versions(self) -> T.Dict[str, T.List[CargoLockPackage]]:
+        versions = collections.defaultdict(list)
+        for pkg in self.package:
+            versions[pkg.name].append(pkg)
+        for pkg_versions in versions.values():
+            pkg_versions.sort(reverse=True, key=lambda pkg: Version(pkg.version))
+        return versions