Rollup merge of #87035 - GuillaumeGomez:fix-implementors-display, r=notriddle

Fix implementors display

Part of https://github.com/rust-lang/rust/issues/86632.

This PR does a few things:
 * It fixes of the JS rendered implementors.
 * It generates anchors for JS rendered implementors to make it coherent with the others.
 * It adds a test to ensure that we won't have the same issue again.
 * It changes the way we render the rustdoc-gui crates to simplify it a bit and also to allow to have dependencies without going through compiletest.

Before:

![Screenshot from 2021-07-10 13-30-13](https://user-images.githubusercontent.com/3050060/125174172-b4048700-e1c3-11eb-8f0e-c46081371d4f.png)

After:

![Screenshot from 2021-07-10 21-11-15](https://user-images.githubusercontent.com/3050060/125174173-b49d1d80-e1c3-11eb-8740-1dbbff70c2eb.png)

I plan to add the `[src]` links in another PR because this one is already big enough.

cc `@Mark-Simulacrum` (for the bootstrap changes)

r? `@Nemo157`

src/test/rustdoc-gui/implementors.goml

@@ -0,0 +1,16 @@
+// The goal of this test is to check that the external trait implementors, generated with JS,
+// have the same display than the "local" ones.
+goto: file://|DOC_PATH|/implementors/trait.Whatever.html
+assert: "#implementors-list"
+// There are supposed to be two implementors listed.
+assert-count: ("#implementors-list > .impl", 2)
+// Now we check that both implementors have an anchor, an ID and a similar DOM.
+assert: ("#implementors-list > .impl:nth-child(1) > a.anchor")
+assert-attribute: ("#implementors-list > .impl:nth-child(1)", {"id": "impl-Whatever"})
+assert-attribute: ("#implementors-list > .impl:nth-child(1) > a.anchor", {"href": "#impl-Whatever"})
+assert: "#implementors-list > .impl:nth-child(1) > code.in-band"
+
+assert: ("#implementors-list > .impl:nth-child(2) > a.anchor")
+assert-attribute: ("#implementors-list > .impl:nth-child(2)", {"id": "impl-Whatever-1"})
+assert-attribute: ("#implementors-list > .impl:nth-child(2) > a.anchor", {"href": "#impl-Whatever-1"})
+assert: "#implementors-list > .impl:nth-child(2) > code.in-band"

src/test/rustdoc-gui/src/Cargo.lock

@@ -0,0 +1,18 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "implementors"
+version = "0.1.0"
+
+[[package]]
+name = "lib2"
+version = "0.1.0"
+dependencies = [
+ "implementors",
+]
+
+[[package]]
+name = "test_docs"
+version = "0.1.0"

src/test/rustdoc-gui/src/Cargo.toml

@@ -0,0 +1,6 @@
+[workspace]
+members = [
+    "test_docs",
+    "lib2",
+    "implementors",
+]

src/test/rustdoc-gui/src/implementors/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "implementors"
+version = "0.1.0"
+edition = "2018"
+
+[lib]
+path = "lib.rs"

src/test/rustdoc-gui/src/implementors/lib.rs

@@ -0,0 +1,7 @@
+pub trait Whatever {
+    fn method() {}
+}
+
+pub struct Struct;
+
+impl Whatever for Struct {}

src/test/rustdoc-gui/src/lib2/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "lib2"
+version = "0.1.0"
+edition = "2018"
+
+[lib]
+path = "lib.rs"
+
+[dependencies]
+implementors = { path = "../implementors" }

src/test/rustdoc-gui/src/lib2/lib.rs

@@ -31,3 +31,5 @@ impl Trait for Foo {
     type X = u32;
     const Y: u32 = 0;
 }
+
+impl implementors::Whatever for Foo {}

src/test/rustdoc-gui/src/lib2/src/lib.rs

@@ -0,0 +1,7 @@
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn it_works() {
+        assert_eq!(2 + 2, 4);
+    }
+}

src/test/rustdoc-gui/src/test_docs/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "test_docs"
+version = "0.1.0"
+edition = "2018"
+
+[lib]
+path = "lib.rs"