Serialize GitReference::Branch("master") as GitReference::DefaultBranch

src/cargo/core/resolver/encode.rs

@@ -98,7 +98,7 @@ use serde::de;
 use serde::ser;
 use serde::{Deserialize, Serialize};
 
-use crate::core::{Dependency, Package, PackageId, SourceId, Workspace};
+use crate::core::{Dependency, GitReference, Package, PackageId, SourceId, Workspace};
 use crate::util::errors::{CargoResult, CargoResultExt};
 use crate::util::interning::InternedString;
 use crate::util::{internal, Graph};
@@ -659,6 +659,13 @@ pub fn encodable_package_id(id: PackageId, state: &EncodeState<'_>) -> Encodable
 fn encode_source(id: SourceId) -> Option<SourceId> {
     if id.is_path() {
         None
+    } else if let Some(git_reference) = id.git_reference() {
+        match git_reference {
+            GitReference::Branch(branch) if branch == "master" => {
+                Some(id.with_default_git_reference())
+            }
+            _ => Some(id),
+        }
     } else {
         Some(id)
     }

src/cargo/core/source/source_id.rs

@@ -322,6 +322,15 @@ impl SourceId {
         }
     }
 
+    /// Creates a new `SourceId` from this source with Git reference turned into default
+    pub fn with_default_git_reference(self) -> SourceId {
+        assert!(self.is_git());
+        SourceId::wrap(SourceIdInner {
+            kind: SourceKind::Git(GitReference::DefaultBranch),
+            ..(*self.inner).clone()
+        })
+    }
+
     /// Creates a new `SourceId` from this source with the given `precise`.
     pub fn with_precise(self, v: Option<String>) -> SourceId {
         SourceId::wrap(SourceIdInner {

tests/testsuite/lockfile_compat.rs

@@ -566,6 +566,39 @@ dependencies = [
     assert_lockfiles_eq(&lockfile, &lock);
 }
 
+// If explicitly specifying the master branch makes no difference
+#[cargo_test]
+fn master_branch_not_mentioned() {
+    let git = git::new("bar", |p| {
+        p.file("Cargo.toml", &basic_manifest("bar", "0.1.0"))
+            .file("src/lib.rs", "")
+    });
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+            [project]
+            name = "foo"
+            version = "0.0.1"
+            authors = []
+
+            [dependencies]
+            bar = {{ git = '{}', branch = "master" }}
+        "#,
+                git.url()
+            ),
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("generate-lockfile").run();
+
+    let lockfile = p.read_lockfile();
+    assert!(!lockfile.contains("master"), "master in {}", lockfile);
+}
+
 #[cargo_test]
 fn v2_path_and_crates_io() {
     let cksum010 = Package::new("a", "0.1.0").publish();