Auto merge of #96094 - Elliot-Roberts:fix_doctests, r=compiler-errors

Begin fixing all the broken doctests in `compiler/`

Begins to fix #95994.
All of them pass now but 24 of them I've marked with `ignore HELP (<explanation>)` (asking for help) as I'm unsure how to get them to work / if we should leave them as they are.
There are also a few that I marked `ignore` that could maybe be made to work but seem less important.
Each `ignore` has a rough "reason" for ignoring after it parentheses, with

- `(pseudo-rust)` meaning "mostly rust-like but contains foreign syntax"
- `(illustrative)` a somewhat catchall for either a fragment of rust that doesn't stand on its own (like a lone type), or abbreviated rust with ellipses and undeclared types that would get too cluttered if made compile-worthy.
- `(not-rust)` stuff that isn't rust but benefits from the syntax highlighting, like MIR.
- `(internal)` uses `rustc_*` code which would be difficult to make work with the testing setup.

Those reason notes are a bit inconsistently applied and messy though. If that's important I can go through them again and try a more principled approach. When I run `rg '```ignore \(' .` on the repo, there look to be lots of different conventions other people have used for this sort of thing. I could try unifying them all if that would be helpful.

I'm not sure if there was a better existing way to do this but I wrote my own script to help me run all the doctests and wade through the output. If that would be useful to anyone else, I put it here: https://github.com/Elliot-Roberts/rust_doctest_fixing_tool

compiler/rustc_parse/src/parser/item.rs

@@ -485,7 +485,7 @@ impl<'a> Parser<'a> {
 
     /// Parses an implementation item.
     ///
-    /// ```
+    /// ```ignore (illustrative)
     /// impl<'a, T> TYPE { /* impl items */ }
     /// impl<'a, T> TRAIT for TYPE { /* impl items */ }
     /// impl<'a, T> !TRAIT for TYPE { /* impl items */ }
@@ -493,7 +493,7 @@ impl<'a> Parser<'a> {
     /// ```
     ///
     /// We actually parse slightly more relaxed grammar for better error reporting and recovery.
-    /// ```
+    /// ```ebnf
     /// "impl" GENERICS "const"? "!"? TYPE "for"? (TYPE | "..") ("where" PREDICATES)? "{" BODY "}"
     /// "impl" GENERICS "const"? "!"? TYPE ("where" PREDICATES)? "{" BODY "}"
     /// ```
@@ -806,7 +806,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a `type` alias with the following grammar:
-    /// ```
+    /// ```ebnf
     /// TypeAlias = "type" Ident Generics {":" GenericBounds}? {"=" Ty}? ";" ;
     /// ```
     /// The `"type"` has already been eaten.
@@ -930,7 +930,7 @@ impl<'a> Parser<'a> {
     ///
     /// # Examples
     ///
-    /// ```
+    /// ```ignore (illustrative)
     /// extern crate foo;
     /// extern crate bar as foo;
     /// ```
@@ -1630,7 +1630,7 @@ impl<'a> Parser<'a> {
 
     /// Parses a declarative macro 2.0 definition.
     /// The `macro` keyword has already been parsed.
-    /// ```
+    /// ```ebnf
     /// MacBody = "{" TOKEN_STREAM "}" ;
     /// MacParams = "(" TOKEN_STREAM ")" ;
     /// DeclMac = "macro" Ident MacParams? MacBody ;