Fix error when using certain config values in Cargo.toml

`pre-build = "file"` had a bad serialization

(cherry picked from commit 6a57d01)

Author: Emilgardis

src/cross_toml.rs

@@ -680,6 +680,28 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    pub fn fully_populated_roundtrip() -> Result<()> {
+        let cfg = r#"
+            [target.a]
+            xargo = false
+            build-std = true
+            image = "local"
+            dockerfile.file = "Dockerfile"
+            dockerfile.context = ".."
+            pre-build = ["sh"]
+            zig = true
+
+            [target.b]
+            pre-build = "sh"
+            zig = "2.17"
+        "#;
+
+        let (cfg, _) = CrossToml::parse_from_cross(cfg, &mut m!())?;
+        serde_json::from_value::<CrossToml>(serde_json::to_value(cfg)?)?;
+        Ok(())
+    }
+
     #[test]
     pub fn merge() -> Result<()> {
         let cfg1_str = r#"

src/docker/custom.rs

@@ -23,14 +23,29 @@ pub enum Dockerfile<'a> {
     },
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+#[derive(Debug, Clone, PartialEq, Eq, serde::Deserialize)]
 pub enum PreBuild {
     /// A path to a file to copy or a single line to `RUN` if line comes from env
     Single { line: String, env: bool },
     /// Lines to execute in a single `RUN`
     Lines(Vec<String>),
 }
 
+impl serde::Serialize for PreBuild {
+    fn serialize<S: serde::Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match self {
+            PreBuild::Single { line, .. } => serializer.serialize_str(line),
+            PreBuild::Lines(lines) => {
+                use serde::ser::SerializeSeq;
+                let mut seq = serializer.serialize_seq(Some(lines.len()))?;
+                for line in lines {
+                    seq.serialize_element(line)?;
+                }
+                seq.end()
+            }
+        }
+    }
+}
 impl FromStr for PreBuild {
     type Err = std::convert::Infallible;
 

src/tests/toml.rs

@@ -60,14 +60,17 @@ fn toml_check() -> Result<(), Box<dyn std::error::Error>> {
                 text_line_no(&contents, fence.range().start),
             );
             let mut msg_info = crate::shell::MessageInfo::default();
-            assert!(if !cargo {
+            let toml = if !cargo {
                 crate::cross_toml::CrossToml::parse_from_cross(&fence_content, &mut msg_info)?
             } else {
                 crate::cross_toml::CrossToml::parse_from_cargo(&fence_content, &mut msg_info)?
                     .unwrap_or_default()
-            }
-            .1
-            .is_empty());
+            };
+            assert!(toml.1.is_empty());
+
+            // TODO: Add serde_path_to_error
+            // Check if roundtrip works, needed for merging Cross.toml and Cargo.toml
+            serde_json::from_value::<crate::cross_toml::CrossToml>(serde_json::to_value(toml.0)?)?;
         }
     }
     Ok(())