Rollup merge of #75438 - Cldfire:rustdoc/use-adaptive-svg-favicon, r=GuillaumeGomez

Use adaptive SVG favicon for rustdoc like other rust sites

Use the theme-adaptive SVG favicon that was recently introduced [for the Rust site](rust-lang/www.rust-lang.org#1185) (and others).

(This PR is simply copied from the PR linked above, so see that for rationale.)

Closes #72165.

Before, Firefox on Linux:

![image](https://user-images.githubusercontent.com/13814214/89971811-34c0a900-dc2a-11ea-9aa6-a4aa9d66bed4.png)

After, Firefox on Linux (`prefers-color-scheme` set to `dark` by setting `ui.systemUsesDarkTheme` to a number value of `1` in `about:config`):

![image](https://user-images.githubusercontent.com/13814214/89971842-515ce100-dc2a-11ea-92e8-c374aaaf6031.png)

Author: jonas-schievink

src/librustdoc/html/layout.rs

@@ -158,7 +158,9 @@ pub fn render<T: Print, S: Print>(
         keywords = page.keywords,
         favicon = if layout.favicon.is_empty() {
             format!(
-                r#"<link rel="shortcut icon" href="{static_root_path}favicon{suffix}.ico">"#,
+                r##"<link rel="icon" type="image/svg+xml" href="{static_root_path}favicon{suffix}.svg">
+<link rel="alternate icon" type="image/png" href="{static_root_path}favicon-16x16{suffix}.png">
+<link rel="alternate icon" type="image/png" href="{static_root_path}favicon-32x32{suffix}.png">"##,
                 static_root_path = static_root_path,
                 suffix = page.resource_suffix
             )

src/librustdoc/html/render/mod.rs

@@ -754,7 +754,9 @@ fn write_shared(
         write(cx.path("rust-logo.png"), static_files::RUST_LOGO)?;
     }
     if (*cx.shared).layout.favicon.is_empty() {
-        write(cx.path("favicon.ico"), static_files::RUST_FAVICON)?;
+        write(cx.path("favicon.svg"), static_files::RUST_FAVICON_SVG)?;
+        write(cx.path("favicon-16x16.png"), static_files::RUST_FAVICON_PNG_16)?;
+        write(cx.path("favicon-32x32.png"), static_files::RUST_FAVICON_PNG_32)?;
     }
     write(cx.path("brush.svg"), static_files::BRUSH_SVG)?;
     write(cx.path("wheel.svg"), static_files::WHEEL_SVG)?;

src/librustdoc/html/static/favicon-16x16.png

@@ -53,8 +53,10 @@ pub static LICENSE_MIT: &[u8] = include_bytes!("static/LICENSE-MIT.txt");
 
 /// The contents of `rust-logo.png`, the default icon of the documentation.
 pub static RUST_LOGO: &[u8] = include_bytes!("static/rust-logo.png");
-/// The contents of `favicon.ico`, the default favicon of the documentation.
-pub static RUST_FAVICON: &[u8] = include_bytes!("static/favicon.ico");
+/// The default documentation favicons (SVG and PNG fallbacks)
+pub static RUST_FAVICON_SVG: &[u8] = include_bytes!("static/favicon.svg");
+pub static RUST_FAVICON_PNG_16: &[u8] = include_bytes!("static/favicon-16x16.png");
+pub static RUST_FAVICON_PNG_32: &[u8] = include_bytes!("static/favicon-32x32.png");
 
 /// The built-in themes given to every documentation site.
 pub mod themes {