Do rustc's byte indices work?

cf. rust-lang/rust#35164

Author: killercup

examples/do-byte-indices-work.rs

@@ -0,0 +1,62 @@
+extern crate serde_json;
+extern crate rustfix;
+
+use std::process::Command;
+
+use rustfix::diagnostics::Diagnostic;
+
+fn main() {
+    let sorry_for_the_churn = Command::new("touch").arg("src/diagnostics.rs").output();
+    let output = Command::new("cargo")
+        .args(&["rustc",
+                "--lib",
+                "--quiet",
+                "--color", "never",
+                "--",
+                "-Z", "unstable-options",
+                "--error-format", "json"])
+        .output()
+        .unwrap();
+    let content = String::from_utf8(output.stderr).unwrap();
+
+    let suggestions = content.lines()
+        .filter(|s| !s.trim().is_empty())
+        .flat_map(|line| serde_json::from_str::<Diagnostic>(line))
+        .flat_map(|diag| diag.spans.into_iter())
+        .take(5);
+
+    for s in suggestions {
+        let file = read_file_to_string(&s.file_name).unwrap();
+        let content_by_byte_index = file[s.byte_start..s.byte_end].to_string();
+        let content_by_line_col_index: String = file.lines()
+            .skip(s.line_start - 1)
+            .take(s.line_end - s.line_start + 1)
+            .enumerate()
+            .map(|(i, line)| {
+                let start = 0;
+                let end = s.line_end - s.line_start;
+                if i == start && i == end {
+                    line.chars().skip(s.column_start - 1).take(s.column_end - s.column_start).collect()
+                } else if i == start {
+                    line.chars().skip(s.column_start - 1).collect()
+                } else if i == end {
+                    line.chars().take(s.column_end).collect()
+                } else {
+                    line.to_string()
+                }
+            })
+            .collect();
+        assert_eq!(content_by_byte_index, content_by_line_col_index);
+        println!("Worked.");
+    }
+}
+
+fn read_file_to_string(file_name: &str) -> Result<String, std::io::Error> {
+    use std::fs::File;
+    use std::io::Read;
+
+    let mut file = File::open(file_name)?;
+    let mut buffer = String::new();
+    file.read_to_string(&mut buffer)?;
+    Ok(buffer)
+}

src/diagnostics.rs

@@ -1,7 +1,9 @@
 //! Rustc Diagnostic JSON Output
-//! 
+//!
 //! The following data types are copied from [rust-lang/rust](https://github.com/rust-lang/rust/blob/de78655bca47cac8e783dbb563e7e5c25c1fae40/src/libsyntax/json.rs)
 
+use std::fmt::Write; // <- unused on purpose!
+
 #[derive(Serialize, Deserialize, Debug)]
 pub struct Diagnostic {
     /// The primary error message.

src/lib.rs

@@ -5,6 +5,8 @@ extern crate serde_json;
 pub mod diagnostics;
 use diagnostics::{Diagnostic, DiagnosticSpan};
 
+use std::fmt::Write; // <- unused on purpose!
+
 #[derive(Debug, Copy, Clone, Hash, PartialEq)]
 pub struct LinePosition {
     pub line: usize,