allow repeat runs of cargo build

Summary:
The cargo config was being unconditionally added, resutling in "duplicate key `crates-io` in table `source`" error

check if snippet is present before writing

Reviewed By: mzr

Differential Revision: D59803494

fbshipit-source-id: 701773d604c91fd6724069b0451b212cf0018251

build/fbcode_builder/getdeps/cargo.py

@@ -126,24 +126,26 @@ def _create_cargo_config(self):
                 if override not in cargo_content:
                     new_content += override
 
+            if self.build_opts.fbsource_dir:
+                # Point to vendored crates.io if possible
+                try:
+                    from .facebook.rust import vendored_crates
+
+                    new_content = vendored_crates(
+                        self.build_opts.fbsource_dir, new_content
+                    )
+                except ImportError:
+                    # This FB internal module isn't shippped to github,
+                    # so just rely on cargo downloading crates on it's own
+                    pass
+
         if new_content != cargo_content:
             with open(cargo_config_file, "w") as f:
                 print(
                     f"Writing cargo config for {self.manifest.name} to {cargo_config_file}"
                 )
                 f.write(new_content)
 
-        if self.build_opts.fbsource_dir:
-            # Point to vendored crates.io if possible
-            try:
-                from .facebook.rust import vendored_crates
-
-                vendored_crates(self.build_opts.fbsource_dir, cargo_config_file)
-            except ImportError:
-                # This FB internal module isn't shippped to github,
-                # so just rely on cargo downloading crates on it's own
-                pass
-
         return dep_to_git
 
     def _prepare(self, reconfigure) -> None: