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:

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
  |   ^^^^^^^^
```

2 files changed, 22 insertions, 1 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index d733e32f209..352152a1bd4 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1714,6 +1714,9 @@ 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,
@@ -1760,6 +1763,9 @@ 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,
@@ -1831,6 +1837,9 @@ fn check_error_format_stability(
         {
             early_dcx.early_fatal("`--error-format=human-annotate-rs` is unstable");
         }
+        if let ErrorOutputType::HumanReadable(HumanReadableErrorType::Unicode, _) = error_format {
+            early_dcx.early_fatal("`--error-format=human-unicode` is unstable");
+        }
     }
 }
 
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 1963cf4eb7c..cc16073db76 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::{
@@ -965,6 +967,11 @@ 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(),
                     );
@@ -1468,6 +1475,11 @@ 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),
             )
         }