Allow users to format rust code in markdown code blocks

Now users can envoke rustfmt directly with a markdown file and the rust
code within that file will be reformatted e.g. `rustfmt README.md`

Author: ytmimi

src/formatting.rs

@@ -2,6 +2,7 @@
 
 use std::collections::HashMap;
 use std::io::{self, Write};
+use std::path::PathBuf;
 use std::time::{Duration, Instant};
 
 use rustc_ast::ast;
@@ -11,6 +12,7 @@ use self::newline_style::apply_newline_style;
 use crate::comment::{CharClasses, FullCodeCharKind};
 use crate::config::{Config, FileName, Verbosity};
 use crate::formatting::generated::is_generated_file;
+use crate::markdown::rewrite_markdown;
 use crate::modules::Module;
 use crate::parse::parser::{DirectoryOwnership, Parser, ParserError};
 use crate::parse::session::ParseSess;
@@ -35,6 +37,14 @@ impl<'b, T: Write + 'b> Session<'b, T> {
             return Err(ErrorKind::VersionMismatch);
         }
 
+        match input {
+            Input::File(ref path) if path.extension().map(|ext| ext == "md").unwrap_or(false) => {
+                let config = self.config.clone();
+                return format_markdown_code_snippets(path.clone(), &config, self);
+            }
+            _ => {}
+        }
+
         rustc_span::create_session_if_not_set_then(self.config.edition().into(), |_| {
             if self.config.disable_all_formatting() {
                 // When the input is from stdin, echo back the input.
@@ -174,6 +184,28 @@ fn format_project<T: FormatHandler>(
     Ok(context.report)
 }
 
+fn format_markdown_code_snippets<T: FormatHandler>(
+    path: PathBuf,
+    config: &Config,
+    handler: &mut T,
+) -> Result<FormatReport, ErrorKind> {
+    let input = std::fs::read_to_string(&path)?;
+    let mut result = rewrite_markdown(&input, config).into_owned();
+
+    if !result.ends_with('\n') {
+        // Add a trailing newline if needed
+        result.push('\n');
+    }
+
+    apply_newline_style(config.newline_style(), &mut result, &input);
+
+    let file_name = FileName::Real(path);
+    let mut report = FormatReport::new();
+    handler.handle_formatted_file(None, file_name, result, &mut report)?;
+
+    Ok(report)
+}
+
 // Used for formatting files.
 struct FormatContext<'a, T: FormatHandler> {
     krate: &'a ast::Crate,