Show original version_req for locked dependency

When encounter resolver error with locked dependency, we now show
original version req along with the exact locked version.

src/cargo/core/dependency.rs

@@ -339,6 +339,16 @@ impl Dependency {
         self
     }
 
+    /// Locks this dependency to a specified version.
+    ///
+    /// Mainly used in dependency patching like `[patch]` or `[replace]`, which
+    /// doesn't need to lock the entire dependency to a specific [`PackageId`].
+    pub fn lock_version(&mut self, version: &semver::Version) -> &mut Dependency {
+        let me = Rc::make_mut(&mut self.inner);
+        me.req.lock_to(version);
+        self
+    }
+
     /// Returns `true` if this is a "locked" dependency. Basically a locked
     /// dependency has an exact version req, but not vice versa.
     pub fn is_locked(&self) -> bool {

src/cargo/core/resolver/errors.rs

@@ -387,10 +387,16 @@ pub(crate) fn describe_path<'a>(
             } else {
                 dep.name_in_toml().to_string()
             };
+            let locked_version = dep
+                .version_req()
+                .locked_version()
+                .map(|v| format!("(locked to {}) ", v))
+                .unwrap_or_default();
+
             write!(
                 dep_path_desc,
-                "\n    ... which satisfies {}dependency `{}` of package `{}`",
-                source_kind, requirement, pkg
+                "\n    ... which satisfies {}dependency `{}` {}of package `{}`",
+                source_kind, requirement, locked_version, pkg
             )
             .unwrap();
         }

tests/testsuite/build_script.rs

@@ -1000,7 +1000,7 @@ fn links_duplicates_old_registry() {
     but a native library can be linked only once
 
 package `bar v0.1.0`
-    ... which satisfies dependency `bar = \"=0.1.0\"` of package `foo v0.1.0 ([..]foo)`
+    ... which satisfies dependency `bar = \"^0.1\"` (locked to 0.1.0) of package `foo v0.1.0 ([..]foo)`
 links to native library `a`
 
 package `foo v0.1.0 ([..]foo)`