Make librustc_mir pass rustdoc --test

src/librustc_mir/build/matches/mod.rs

@@ -434,7 +434,7 @@ impl<'a,'tcx> Builder<'a,'tcx> {
     /// But there may also be candidates that the test just doesn't
     /// apply to. For example, consider the case of #29740:
     ///
-    /// ```rust
+    /// ```rust,ignore
     /// match x {
     ///     "foo" => ...,
     ///     "bar" => ...,

src/librustc_mir/build/matches/util.rs

@@ -32,14 +32,18 @@ impl<'a,'tcx> Builder<'a,'tcx> {
     /// this function converts the prefix (`x`, `y`) and suffix (`z`) into
     /// distinct match pairs:
     ///
+    /// ```rust,ignore
     ///     lv[0 of 3] @ x  // see ProjectionElem::ConstantIndex (and its Debug impl)
     ///     lv[1 of 3] @ y  // to explain the `[x of y]` notation
     ///     lv[-1 of 3] @ z
+    /// ```
     ///
     /// If a slice like `s` is present, then the function also creates
     /// a temporary like:
     ///
+    /// ```rust,ignore
     ///     tmp0 = lv[2..-1] // using the special Rvalue::Slice
+    /// ```
     ///
     /// and creates a match pair `tmp0 @ s`
     pub fn prefix_suffix_slice<'pat>(&mut self,

src/librustc_mir/build/scope.rs

@@ -47,7 +47,7 @@ set of scheduled drops up front, and so whenever we exit from the
 scope we only drop the values scheduled thus far. For example, consider
 the scope S corresponding to this loop:
 
-```
+```rust,ignore
 loop {
     let x = ...;
     if cond { break; }

src/librustc_mir/pretty.rs

@@ -23,7 +23,7 @@ const INDENT: &'static str = "    ";
 /// If the session is properly configured, dumps a human-readable
 /// representation of the mir into:
 ///
-/// ```
+/// ```text
 /// rustc.node<node_id>.<pass_name>.<disambiguator>
 /// ```
 ///