use interior mutability for SourceFile lines

Using shared ownership for these may or may not have been a mistake in
the first place. We'll find out.

src/lib.rs

@@ -1,4 +1,4 @@
-use std::{io::BufWriter, sync::Arc};
+use std::sync::Arc;
 
 pub(crate) mod prelude {
     #![allow(unused_imports)]
@@ -7,7 +7,7 @@ pub(crate) mod prelude {
         error::Error as StdError,
         ffi::{OsStr, OsString},
         fmt::{Display, Formatter, Result as FmtResult},
-        io::{Error as IoError, Read, Seek, SeekFrom},
+        io::{Error as IoError, Read, Seek, SeekFrom, Write},
         path::{Path, PathBuf},
         process::{Command, ExitCode},
         str::FromStr,
@@ -85,7 +85,6 @@ pub fn get_highest_prio(
         OsStr::new("; }"),
     ]
     .into_iter()
-    .map(ToOwned::to_owned)
     .collect();
 
     // Get the highest priority, and the file its defined in.
@@ -118,11 +117,21 @@ pub fn get_highest_prio(
 pub fn write_next_prio(
     mut source: SourceFile,
     last_line_def: SourceLine,
-    new_prio: u64,
+    last_pri: i64,
+    new_prio: i64,
 ) -> Result<(), BoxDynError> {
-    let lines = source.lines()?;
+    //let lines = source.lines()?;
 
-    for line in lines {}
+    let old_text = last_line_def.text();
+    let new_text = old_text.replace(last_pri.to_string().as_str(), new_prio.to_string().as_str());
+
+    let new_line = SourceLine {
+        line: Line::from_index(last_line_def.line.index() + 1),
+        path: source.path(),
+        text: Arc::from(new_text),
+    };
+
+    source.insert_line(new_line.line, new_line.text())?;
 
     Ok(())
 }