Rollup merge of #143088 - firefighterduck:improve-doc-discr-tag, r=RalfJung

Improve documentation of `TagEncoding`

This PR is follow-up from the [discussion here](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/.E2.9C.94.20VariantId.3DDiscriminant.20when.20tag.20is.20niche.20encoded.3F/with/524384295).

It aims at making the `TagEncoding` documentation less ambiguous and more detailed with references to relevant implementation sides. It especially clears up the ambiguous use of discriminant/variant index, which sparked the discussion referenced above.

PS: While working with layout data, I somehow ended up looking at the docs for `FakeBorrowKind` and noticed that the one example was not in a doc comment. I hope that this is minor enough of a fix for it to be okay in this otherwise unrelated PR.

1 files changed, 9 insertions, 9 deletions

diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index c50a30cf7f3..92eefd89848 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -284,15 +284,15 @@ pub enum FakeBorrowKind {
     ///
     /// This is used when lowering deref patterns, where shallow borrows wouldn't prevent something
     /// like:
-    // ```compile_fail
-    // let mut b = Box::new(false);
-    // match b {
-    //     deref!(true) => {} // not reached because `*b == false`
-    //     _ if { *b = true; false } => {} // not reached because the guard is `false`
-    //     deref!(false) => {} // not reached because the guard changed it
-    //     // UB because we reached the unreachable.
-    // }
-    // ```
+    /// ```compile_fail
+    /// let mut b = Box::new(false);
+    /// match b {
+    ///     deref!(true) => {} // not reached because `*b == false`
+    ///     _ if { *b = true; false } => {} // not reached because the guard is `false`
+    ///     deref!(false) => {} // not reached because the guard changed it
+    ///     // UB because we reached the unreachable.
+    /// }
+    /// ```
     Deep,
 }