Rollup merge of #76500 - richkadel:mir-graphviz-dark, r=tmandry

Add -Zgraphviz_dark_mode and monospace font fix

Many developers use a dark theme with editors and IDEs, but this
typically doesn't extend to graphviz output.

When I bring up a MIR graphviz document, the white background is
strikingly bright. This new option changes the colors used for graphviz
output to work better in dark-themed UIs.

<img width="1305" alt="Screen Shot 2020-09-09 at 3 00 31 PM" src="https://user-images.githubusercontent.com/3827298/92659478-4b9bff00-f2ad-11ea-8894-b40d3a873cb9.png">

Also fixed the monospace font for common graphviz renders (e.g., VS Code extensions), as described in https://github.com/rust-lang/rust/pull/76500#issuecomment-689837948

**Before:**
<img width="943" alt="Screen Shot 2020-09-09 at 2 48 44 PM" src="https://user-images.githubusercontent.com/3827298/92658939-47231680-f2ac-11ea-97ac-96727e4dd622.png">

**Now with fix:**
<img width="943" alt="Screen Shot 2020-09-09 at 2 49 02 PM" src="https://user-images.githubusercontent.com/3827298/92658959-51451500-f2ac-11ea-9aae-de982d466d6a.png">

1 files changed, 5 insertions, 1 deletions

diff --git a/compiler/rustc_mir/src/dataflow/framework/engine.rs b/compiler/rustc_mir/src/dataflow/framework/engine.rs
index d3ad42f6bbc..0b5b437d186 100644
--- a/compiler/rustc_mir/src/dataflow/framework/engine.rs
+++ b/compiler/rustc_mir/src/dataflow/framework/engine.rs
@@ -306,7 +306,11 @@ where
     let mut buf = Vec::new();
 
     let graphviz = graphviz::Formatter::new(body, def_id, results, style);
-    dot::render_opts(&graphviz, &mut buf, &[dot::RenderOption::Monospace])?;
+    let mut render_opts = vec![dot::RenderOption::Monospace];
+    if tcx.sess.opts.debugging_opts.graphviz_dark_mode {
+        render_opts.push(dot::RenderOption::DarkTheme);
+    }
+    dot::render_opts(&graphviz, &mut buf, &render_opts)?;
 
     if let Some(parent) = path.parent() {
         fs::create_dir_all(parent)?;