rust: suggest correct dependency when found in Cargo.lock

As we refer to crates using API version instead of plain version, it can
be confusing to user which is left with a simple "Dependency X not
found".

Thus, when we detect that a rust dependency is missing, and is not
available using a wrap file, if we find something compatible using
Cargo.lock import, we just list the correct dependencies names.

mesonbuild/interpreter/dependencyfallbacks.py

@@ -340,6 +340,23 @@ def lookup(self, kwargs: TYPE_nkwargs, force_fallback: bool = False) -> Dependen
                         self._subproject_impl(subp_name, varname)
                     break
 
+                # Check if we could have a matching Rust crate imported from
+                # Cargo.lock, with expected api version. We know there is no
+                # wrap-file defined to match this crate.
+                if name.endswith('-rs') and required:
+                    s = name.split('-')
+                    if len(s) < 3:
+                        continue
+                    s.pop() # -rs
+                    s.pop() # version
+                    crate = '-'.join(s)
+                    cargo_wraps = self.wrap_resolver.get_cargo_wraps(crate)
+                    if not cargo_wraps:
+                        continue
+                    imports = ', '.join([f'"{n}"' for n in cargo_wraps])
+                    raise DependencyException(f'Rust dependency "{name}" not found, '
+                                              f'but following versions were found using Cargo.lock import: {imports}')
+
         candidates = self._get_candidates()
 
         # writing just "dependency('')" is an error, because it can only fail

mesonbuild/wrap/wrap.py

@@ -427,6 +427,24 @@ def find_program_provider(self, names: T.List[str]) -> T.Optional[str]:
                 return wrap_name
         return None
 
+    def get_cargo_wraps(self, cratename: str) -> T.List[str]:
+        found = []
+        for wrap in self.wraps.values():
+            if wrap.get('method') != 'cargo':
+                continue
+            if not wrap.name.endswith('-rs'):
+                continue
+            s = wrap.name.split('-')
+            if len(s) < 3:
+                continue
+            s.pop() # -rs
+            s.pop() # version
+            wrap_cratename = '-'.join(s)
+
+            if wrap_cratename == cratename:
+                found.append(wrap.name)
+        return found
+
     def resolve(self, packagename: str, force_method: T.Optional[Method] = None) -> T.Tuple[str, Method]:
         wrap = self.wraps.get(packagename)
         if wrap is None:

test cases/rust/26 cargo lock suggest dependency/meson.build

@@ -0,0 +1,3 @@
+project('cargo lock')
+
+dependency('bar-1.0-rs')

test cases/rust/26 cargo lock suggest dependency/test.json

@@ -0,0 +1,7 @@
+{
+  "stdout": [
+    {
+      "line": "test cases/rust/26 cargo lock suggest dependency/meson.build:3:0: ERROR: Rust dependency \"bar-1.0-rs\" not found, but following versions were found using Cargo.lock import: \"bar-1-rs\", \"bar-2-rs\", \"bar-0-rs\", \"bar-0.8-rs\""
+    }
+  ]
+}