Fix rustdoc warnings about invalid Rust syntax

compiler/rustc_mir/src/borrow_check/region_infer/mod.rs

@@ -1225,7 +1225,9 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// it. However, it works pretty well in practice. In particular,
     /// this is needed to deal with projection outlives bounds like
     ///
-    ///     <T as Foo<'0>>::Item: '1
+    /// ```ignore (MIR syntax)
+    /// <T as Foo<'0>>::Item: '1
+    /// ```
     ///
     /// In particular, this routine winds up being important when
     /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the

compiler/rustc_mir/src/util/pretty.rs

@@ -659,7 +659,8 @@ pub fn write_allocations<'tcx>(
 /// Dumps the size and metadata and content of an allocation to the given writer.
 /// The expectation is that the caller first prints other relevant metadata, so the exact
 /// format of this function is (*without* leading or trailing newline):
-/// ```
+///
+/// ```text
 /// size: {}, align: {}) {
 ///     <bytes>
 /// }

compiler/rustc_trait_selection/src/opaque_types.rs

@@ -38,13 +38,16 @@ pub struct OpaqueTypeDecl<'tcx> {
     /// then `substs` would be `['a, T]`.
     pub substs: SubstsRef<'tcx>,
 
-    /// The span of this particular definition of the opaque type.  So
+    /// The span of this particular definition of the opaque type. So
     /// for example:
     ///
     /// ```
     /// type Foo = impl Baz;
     /// fn bar() -> Foo {
-    ///             ^^^ This is the span we are looking for!
+    /// //          ^^^ This is the span we are looking for!
+    /// // ...
+    /// # unimplemented!()
+    /// }
     /// ```
     ///
     /// In cases where the fn returns `(impl Trait, impl Trait)` or

compiler/rustc_typeck/src/check/expr.rs

@@ -1275,7 +1275,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     /// Report an error for a struct field expression when there are fields which aren't provided.
     ///
-    /// ```ignore (diagnostic)
+    /// ```text
     /// error: missing field `you_can_use_this_field` in initializer of `foo::Foo`
     ///  --> src/main.rs:8:5
     ///   |
@@ -1327,7 +1327,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     /// Report an error for a struct field expression when there are no visible fields.
     ///
-    /// ```ignore (diagnostic)
+    /// ```text
     /// error: cannot construct `Foo` with struct literal syntax due to inaccessible fields
     ///  --> src/main.rs:8:5
     ///   |

compiler/rustc_typeck/src/check/pat.rs

@@ -1381,7 +1381,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Returns a diagnostic reporting a struct pattern which is missing an `..` due to
     /// inaccessible fields.
     ///
-    /// ```ignore (diagnostic)
+    /// ```text
     /// error: pattern requires `..` due to inaccessible fields
     ///   --> src/main.rs:10:9
     ///    |
@@ -1431,7 +1431,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     /// Returns a diagnostic reporting a struct pattern which does not mention some fields.
     ///
-    /// ```ignore (diagnostic)
+    /// ```text
     /// error[E0027]: pattern does not mention field `you_cant_use_this_field`
     ///   --> src/main.rs:15:9
     ///    |