From 6aea014fbfc1b9bdba22bc3dcd03ab64d9785b63 Mon Sep 17 00:00:00 2001
From: Camelid <camelidcamel@gmail.com>
Date: Sat, 26 Dec 2020 17:55:39 -0800
Subject: [PATCH 1/2] Document `InferTy` & co.

---
 compiler/rustc_middle/src/ty/sty.rs | 33 ++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 4ce76409c6f..0631f2c5378 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1424,28 +1424,33 @@ pub struct EarlyBoundRegion {
     pub name: Symbol,
 }
 
+/// A **ty**pe **v**ariable **ID**.
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
 pub struct TyVid {
     pub index: u32,
 }
 
+/// A **`const`** **v**ariable **ID**.
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
 pub struct ConstVid<'tcx> {
     pub index: u32,
     pub phantom: PhantomData<&'tcx ()>,
 }
 
+/// An **int**egral (`u32`, `i32`, `usize`, etc.) type **v**ariable **ID**.
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
 pub struct IntVid {
     pub index: u32,
 }
 
+/// An **float**ing-point (`f32` or `f64`) type **v**ariable **ID**.
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
 pub struct FloatVid {
     pub index: u32,
 }
 
 rustc_index::newtype_index! {
+    /// A **region** (lifetime) **v**ariable **ID**.
     pub struct RegionVid {
         DEBUG_FORMAT = custom,
     }
@@ -1457,18 +1462,40 @@ impl Atom for RegionVid {
     }
 }
 
+/// A placeholder for a type that hasn't been inferred yet.
+///
+/// E.g., if we have an empty array (`[]`), then we create a fresh
+/// type variable for the element type since we won't know until it's
+/// used what the element type is supposed to be.
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
 #[derive(HashStable)]
 pub enum InferTy {
+    /// A type variable.
     TyVar(TyVid),
+    /// An integral type variable.
+    ///
+    /// These are created when the compiler sees an integer literal like
+    /// `1` that could be several different types (`u8`, `i32`, `u32`, etc.).
+    /// We don't know until it's used what type it's supposed to be, so
+    /// we create a fresh type variable.
     IntVar(IntVid),
+    /// A floating-point type variable.
+    ///
+    /// These are created when the compiler sees an float literal like
+    /// `1.0` that could be either an `f32` or an `f64`.
+    /// We don't know until it's used what type it's supposed to be, so
+    /// we create a fresh type variable.
     FloatVar(FloatVid),
 
-    /// A `FreshTy` is one that is generated as a replacement for an
-    /// unbound type variable. This is convenient for caching etc. See
-    /// `infer::freshen` for more details.
+    /// A [`FreshTy`][Self::FreshTy] is one that is generated as a replacement
+    /// for an unbound type variable. This is convenient for caching etc. See
+    /// `rustc_infer::infer::freshen` for more details.
+    ///
+    /// Compare with [`TyVar`][Self::TyVar].
     FreshTy(u32),
+    /// Like [`FreshTy`][Self::FreshTy], but as a replacement for [`IntVar`][Self::IntVar].
     FreshIntTy(u32),
+    /// Like [`FreshTy`][Self::FreshTy], but as a replacement for [`FloatVar`][Self::FloatVar].
     FreshFloatTy(u32),
 }
 

From cdad0c80ef93dc64dd9ab8ec2410ec08f60b4120 Mon Sep 17 00:00:00 2001
From: Camelid <camelidcamel@gmail.com>
Date: Sun, 27 Dec 2020 11:05:40 -0800
Subject: [PATCH 2/2] Also show the displayed version of IntVar and FloatVar

Co-authored-by: Joshua Nelson <joshua@yottadb.com>
---
 compiler/rustc_middle/src/ty/sty.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 0631f2c5378..744c7a541a5 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1472,14 +1472,14 @@ impl Atom for RegionVid {
 pub enum InferTy {
     /// A type variable.
     TyVar(TyVid),
-    /// An integral type variable.
+    /// An integral type variable (`{integer}`).
     ///
     /// These are created when the compiler sees an integer literal like
     /// `1` that could be several different types (`u8`, `i32`, `u32`, etc.).
     /// We don't know until it's used what type it's supposed to be, so
     /// we create a fresh type variable.
     IntVar(IntVid),
-    /// A floating-point type variable.
+    /// A floating-point type variable (`{float}`).
     ///
     /// These are created when the compiler sees an float literal like
     /// `1.0` that could be either an `f32` or an `f64`.