Add simple markdown formatting to rustc --explain output

Cargo.lock

@@ -4746,9 +4746,9 @@ dependencies = [
 
 [[package]]
 name = "termcolor"
-version = "1.1.3"
+version = "1.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bab24d30b911b2376f3a13cc2cd443142f0c81dda04c118693e35b3835757755"
+checksum = "be55cf8942feac5c765c2c993422806843c9a9a45d4d5c407ad6dd2ea95eb9b6"
 dependencies = [
  "winapi-util",
 ]

compiler/rustc_driver_impl/src/lib.rs

@@ -24,6 +24,7 @@ use rustc_data_structures::profiling::{
 };
 use rustc_data_structures::sync::SeqCst;
 use rustc_errors::registry::{InvalidErrorCode, Registry};
+use rustc_errors::{markdown, ColorConfig};
 use rustc_errors::{
     DiagnosticMessage, ErrorGuaranteed, Handler, PResult, SubdiagnosticMessage, TerminalUrl,
 };
@@ -282,7 +283,7 @@ fn run_compiler(
     interface::set_thread_safe_mode(&sopts.unstable_opts);
 
     if let Some(ref code) = matches.opt_str("explain") {
-        handle_explain(&early_error_handler, diagnostics_registry(), code);
+        handle_explain(&early_error_handler, diagnostics_registry(), code, sopts.color);
         return Ok(());
     }
 
@@ -540,7 +541,7 @@ impl Compilation {
     }
 }
 
-fn handle_explain(handler: &EarlyErrorHandler, registry: Registry, code: &str) {
+fn handle_explain(handler: &EarlyErrorHandler, registry: Registry, code: &str, color: ColorConfig) {
     let upper_cased_code = code.to_ascii_uppercase();
     let normalised =
         if upper_cased_code.starts_with('E') { upper_cased_code } else { format!("E{code:0>4}") };
@@ -564,7 +565,7 @@ fn handle_explain(handler: &EarlyErrorHandler, registry: Registry, code: &str) {
                 text.push('\n');
             }
             if io::stdout().is_terminal() {
-                show_content_with_pager(&text);
+                show_md_content_with_pager(&text, color);
             } else {
                 safe_print!("{text}");
             }
@@ -575,34 +576,72 @@ fn handle_explain(handler: &EarlyErrorHandler, registry: Registry, code: &str) {
     }
 }
 
-fn show_content_with_pager(content: &str) {
+/// If color is always or auto, print formatted & colorized markdown. If color is never or
+/// if formatted printing fails, print the raw text.
+///
+/// Prefers a pager, falls back standard print
+fn show_md_content_with_pager(content: &str, color: ColorConfig) {
+    let mut fallback_to_println = false;
     let pager_name = env::var_os("PAGER").unwrap_or_else(|| {
         if cfg!(windows) { OsString::from("more.com") } else { OsString::from("less") }
     });
 
-    let mut fallback_to_println = false;
+    let mut cmd = Command::new(&pager_name);
+    // FIXME: find if other pagers accept color options
+    let mut print_formatted = if pager_name == "less" {
+        cmd.arg("-r");
+        true
+    } else if ["bat", "catbat", "delta"].iter().any(|v| *v == pager_name) {
+        true
+    } else {
+        false
+    };
 
-    match Command::new(pager_name).stdin(Stdio::piped()).spawn() {
-        Ok(mut pager) => {
-            if let Some(pipe) = pager.stdin.as_mut() {
-                if pipe.write_all(content.as_bytes()).is_err() {
-                    fallback_to_println = true;
-                }
-            }
+    if color == ColorConfig::Never {
+        print_formatted = false;
+    } else if color == ColorConfig::Always {
+        print_formatted = true;
+    }
+
+    let mdstream = markdown::MdStream::parse_str(content);
+    let bufwtr = markdown::create_stdout_bufwtr();
+    let mut mdbuf = bufwtr.buffer();
+    if mdstream.write_termcolor_buf(&mut mdbuf).is_err() {
+        print_formatted = false;
+    }
 
-            if pager.wait().is_err() {
+    if let Ok(mut pager) = cmd.stdin(Stdio::piped()).spawn() {
+        if let Some(pipe) = pager.stdin.as_mut() {
+            let res = if print_formatted {
+                pipe.write_all(mdbuf.as_slice())
+            } else {
+                pipe.write_all(content.as_bytes())
+            };
+
+            if res.is_err() {
                 fallback_to_println = true;
             }
         }
-        Err(_) => {
+
+        if pager.wait().is_err() {
             fallback_to_println = true;
         }
+    } else {
+        fallback_to_println = true;
     }
 
     // If pager fails for whatever reason, we should still print the content
     // to standard output
     if fallback_to_println {
-        safe_print!("{content}");
+        let fmt_success = match color {
+            ColorConfig::Auto => io::stdout().is_terminal() && bufwtr.print(&mdbuf).is_ok(),
+            ColorConfig::Always => bufwtr.print(&mdbuf).is_ok(),
+            ColorConfig::Never => false,
+        };
+
+        if !fmt_success {
+            safe_print!("{content}");
+        }
     }
 }
 

compiler/rustc_errors/Cargo.toml

@@ -20,7 +20,7 @@ rustc_hir = { path = "../rustc_hir" }
 rustc_lint_defs = { path = "../rustc_lint_defs" }
 rustc_type_ir = { path = "../rustc_type_ir" }
 unicode-width = "0.1.4"
-termcolor = "1.0"
+termcolor = "1.2.0"
 annotate-snippets = "0.9"
 termize = "0.1.1"
 serde = { version = "1.0.125", features = [ "derive" ] }

compiler/rustc_errors/src/emitter.rs

@@ -616,7 +616,7 @@ pub enum ColorConfig {
 }
 
 impl ColorConfig {
-    fn to_color_choice(self) -> ColorChoice {
+    pub fn to_color_choice(self) -> ColorChoice {
         match self {
             ColorConfig::Always => {
                 if io::stderr().is_terminal() {

compiler/rustc_errors/src/lib.rs

@@ -61,6 +61,7 @@ pub mod emitter;
 pub mod error;
 pub mod json;
 mod lock;
+pub mod markdown;
 pub mod registry;
 mod snippet;
 mod styled_buffer;

compiler/rustc_errors/src/markdown/mod.rs

@@ -0,0 +1,76 @@
+//! A simple markdown parser that can write formatted text to the terminal
+//!
+//! Entrypoint is `MdStream::parse_str(...)`
+use std::io;
+
+use termcolor::{Buffer, BufferWriter, ColorChoice};
+mod parse;
+mod term;
+
+/// An AST representation of a Markdown document
+#[derive(Clone, Debug, Default, PartialEq)]
+pub struct MdStream<'a>(Vec<MdTree<'a>>);
+
+impl<'a> MdStream<'a> {
+    /// Parse a markdown string to a tokenstream
+    #[must_use]
+    pub fn parse_str(s: &str) -> MdStream<'_> {
+        parse::entrypoint(s)
+    }
+
+    /// Write formatted output to a termcolor buffer
+    pub fn write_termcolor_buf(&self, buf: &mut Buffer) -> io::Result<()> {
+        term::entrypoint(self, buf)
+    }
+}
+
+/// Create a termcolor buffer with the `Always` color choice
+pub fn create_stdout_bufwtr() -> BufferWriter {
+    BufferWriter::stdout(ColorChoice::Always)
+}
+
+/// A single tokentree within a Markdown document
+#[derive(Clone, Debug, PartialEq)]
+pub enum MdTree<'a> {
+    /// Leaf types
+    Comment(&'a str),
+    CodeBlock {
+        txt: &'a str,
+        lang: Option<&'a str>,
+    },
+    CodeInline(&'a str),
+    Strong(&'a str),
+    Emphasis(&'a str),
+    Strikethrough(&'a str),
+    PlainText(&'a str),
+    /// [Foo](www.foo.com) or simple anchor <www.foo.com>
+    Link {
+        disp: &'a str,
+        link: &'a str,
+    },
+    /// `[Foo link][ref]`
+    RefLink {
+        disp: &'a str,
+        id: Option<&'a str>,
+    },
+    /// [ref]: www.foo.com
+    LinkDef {
+        id: &'a str,
+        link: &'a str,
+    },
+    /// Break bewtween two paragraphs (double `\n`), not directly parsed but
+    /// added later
+    ParagraphBreak,
+    /// Break bewtween two lines (single `\n`)
+    LineBreak,
+    HorizontalRule,
+    Heading(u8, MdStream<'a>),
+    OrderedListItem(u16, MdStream<'a>),
+    UnorderedListItem(MdStream<'a>),
+}
+
+impl<'a> From<Vec<MdTree<'a>>> for MdStream<'a> {
+    fn from(value: Vec<MdTree<'a>>) -> Self {
+        Self(value)
+    }
+}