Auto merge of #126597 - estebank:unicode-output, r=fmease

Add Unicode block-drawing compiler output support

Add nightly-only theming support to rustc output using Unicode box
drawing characters instead of ASCII-art to draw the terminal UI.

In order to enable, the flags `-Zunstable-options=yes --error-format=human-unicode` must be passed in.

After:

```
error: foo
  ╭▸ test.rs:3:3
  │
3 │       X0 Y0 Z0
  │   ┌───╿──│──┘
  │  ┌│───│──┘
  │ ┏││━━━┙
  │ ┃││
4 │ ┃││   X1 Y1 Z1
5 │ ┃││   X2 Y2 Z2
  │ ┃│└────╿──│──┘ `Z` label
  │ ┃└─────│──┤
  │ ┗━━━━━━┥  `Y` is a good letter too
  │        `X` is a good letter
  ╰╴
note: bar
  ╭▸ test.rs:4:3
  │
4 │ ┏   X1 Y1 Z1
5 │ ┃   X2 Y2 Z2
6 │ ┃   X3 Y3 Z3
  │ ┗━━━━━━━━━━┛
  ├ note: bar
  ╰ note: baz
note: qux
  ╭▸ test.rs:4:3
  │
4 │   X1 Y1 Z1
  ╰╴  ━━━━━━━━
```

Before:

```
error: foo
 --> test.rs:3:3
  |
3 |       X0 Y0 Z0
  |    ___^__-__-
  |   |___|__|
  |  ||___|
  | |||
4 | |||   X1 Y1 Z1
5 | |||   X2 Y2 Z2
  | |||____^__-__- `Z` label
  | ||_____|__|
  | |______|  `Y` is a good letter too
  |        `X` is a good letter
  |
note: bar
 --> test.rs:4:3
  |
4 | /   X1 Y1 Z1
5 | |   X2 Y2 Z2
6 | |   X3 Y3 Z3
  | |__________^
  = note: bar
  = note: baz
note: qux
 --> test.rs:4:3
  |
4 |   X1 Y1 Z1
  |   ^^^^^^^^
```

After:

![rustc output with unicode box drawing characters](https://github.com/rust-lang/rust/assets/1606434/d210b79a-6579-4407-9706-ba8edc6e9f25)

Before:
![current rustc output with ASCII art](https://github.com/rust-lang/rust/assets/1606434/5aecccf8-a6ee-4469-8b39-72fb0d979a9f)

2 files changed, 35 insertions, 14 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index fa2403db925..41cbca89af5 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1721,6 +1721,9 @@ pub fn parse_json(early_dcx: &EarlyDiagCtxt, matches: &getopts::Matches) -> Json
         for sub_option in option.split(',') {
             match sub_option {
                 "diagnostic-short" => json_rendered = HumanReadableErrorType::Short,
+                "diagnostic-unicode" => {
+                    json_rendered = HumanReadableErrorType::Unicode;
+                }
                 "diagnostic-rendered-ansi" => json_color = ColorConfig::Always,
                 "artifacts" => json_artifact_notifications = true,
                 "unused-externs" => json_unused_externs = JsonUnusedExterns::Loud,
@@ -1767,14 +1770,17 @@ pub fn parse_error_format(
                 ErrorOutputType::Json { pretty: true, json_rendered, color_config: json_color }
             }
             Some("short") => ErrorOutputType::HumanReadable(HumanReadableErrorType::Short, color),
+            Some("human-unicode") => {
+                ErrorOutputType::HumanReadable(HumanReadableErrorType::Unicode, color)
+            }
             Some(arg) => {
                 early_dcx.abort_if_error_and_set_error_format(ErrorOutputType::HumanReadable(
                     HumanReadableErrorType::Default,
                     color,
                 ));
                 early_dcx.early_fatal(format!(
-                    "argument for `--error-format` must be `human`, `json` or \
-                     `short` (instead was `{arg}`)"
+                    "argument for `--error-format` must be `human`, `human-annotate-rs`, \
+                    `human-unicode`, `json`, `pretty-json` or `short` (instead was `{arg}`)"
                 ))
             }
         }
@@ -1827,18 +1833,21 @@ pub fn parse_crate_edition(early_dcx: &EarlyDiagCtxt, matches: &getopts::Matches
 fn check_error_format_stability(
     early_dcx: &EarlyDiagCtxt,
     unstable_opts: &UnstableOptions,
-    error_format: ErrorOutputType,
+    format: ErrorOutputType,
 ) {
-    if !unstable_opts.unstable_options {
-        if let ErrorOutputType::Json { pretty: true, .. } = error_format {
-            early_dcx.early_fatal("`--error-format=pretty-json` is unstable");
-        }
-        if let ErrorOutputType::HumanReadable(HumanReadableErrorType::AnnotateSnippet, _) =
-            error_format
-        {
-            early_dcx.early_fatal("`--error-format=human-annotate-rs` is unstable");
-        }
-    }
+    if unstable_opts.unstable_options {
+        return;
+    }
+    let format = match format {
+        ErrorOutputType::Json { pretty: true, .. } => "pretty-json",
+        ErrorOutputType::HumanReadable(format, _) => match format {
+            HumanReadableErrorType::AnnotateSnippet => "human-annotate-rs",
+            HumanReadableErrorType::Unicode => "human-unicode",
+            _ => return,
+        },
+        _ => return,
+    };
+    early_dcx.early_fatal(format!("`--error-format={format}` is unstable"))
 }
 
 fn parse_output_types(
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 9d9434a7776..d8d6b79974f 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -16,7 +16,9 @@ use rustc_data_structures::sync::{
 };
 use rustc_errors::annotate_snippet_emitter_writer::AnnotateSnippetEmitter;
 use rustc_errors::codes::*;
-use rustc_errors::emitter::{DynEmitter, HumanEmitter, HumanReadableErrorType, stderr_destination};
+use rustc_errors::emitter::{
+    DynEmitter, HumanEmitter, HumanReadableErrorType, OutputTheme, stderr_destination,
+};
 use rustc_errors::json::JsonEmitter;
 use rustc_errors::registry::Registry;
 use rustc_errors::{
@@ -966,6 +968,11 @@ fn default_emitter(
                     .macro_backtrace(macro_backtrace)
                     .track_diagnostics(track_diagnostics)
                     .terminal_url(terminal_url)
+                    .theme(if let HumanReadableErrorType::Unicode = kind {
+                        OutputTheme::Unicode
+                    } else {
+                        OutputTheme::Ascii
+                    })
                     .ignored_directories_in_source_blocks(
                         sopts.unstable_opts.ignore_directory_in_diagnostics_source_blocks.clone(),
                     );
@@ -1470,6 +1477,11 @@ fn mk_emitter(output: ErrorOutputType) -> Box<DynEmitter> {
             let short = kind.short();
             Box::new(
                 HumanEmitter::new(stderr_destination(color_config), fallback_bundle)
+                    .theme(if let HumanReadableErrorType::Unicode = kind {
+                        OutputTheme::Unicode
+                    } else {
+                        OutputTheme::Ascii
+                    })
                     .short_message(short),
             )
         }