Make tcx.mk_const more permissive wrt kind argument

- Accept `impl Into`
- Implement `From<>` for `ConstKind`

Note: this adds a dependency on `derive_more` (MIT license). It allows
to derive a lot of traits (like `From` here) that would be otherwise
tedious to implement.

Author: WaffleLapkin

Cargo.lock

@@ -870,6 +870,12 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "convert_case"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6245d59a3e82a7fc217c5828a6692dbc6dfb63a0c8c90495621f7b9d79704a0e"
+
 [[package]]
 name = "core"
 version = "0.0.0"
@@ -1060,6 +1066,19 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "derive_more"
+version = "0.99.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4fb810d30a7c1953f91334de7244731fc3f3c10d7fe163338a35b9f640960321"
+dependencies = [
+ "convert_case",
+ "proc-macro2",
+ "quote",
+ "rustc_version",
+ "syn",
+]
+
 [[package]]
 name = "diff"
 version = "0.1.13"
@@ -3979,6 +3998,7 @@ version = "0.0.0"
 dependencies = [
  "bitflags",
  "chalk-ir",
+ "derive_more",
  "either",
  "gsgdt",
  "polonius-engine",

compiler/rustc_middle/Cargo.toml

@@ -8,6 +8,7 @@ edition = "2021"
 [dependencies]
 bitflags = "1.2.1"
 chalk-ir = "0.87.0"
+derive_more = "0.99.17"
 either = "1.5.0"
 gsgdt = "0.1.2"
 polonius-engine = "0.13.0"

compiler/rustc_middle/src/ty/consts/kind.rs

@@ -49,6 +49,7 @@ impl<'tcx> UnevaluatedConst<'tcx> {
 /// Represents a constant in Rust.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable)]
 #[derive(Hash, HashStable, TypeFoldable, TypeVisitable)]
+#[derive(derive_more::From)]
 pub enum ConstKind<'tcx> {
     /// A const generic parameter.
     Param(ty::ParamConst),
@@ -71,12 +72,19 @@ pub enum ConstKind<'tcx> {
 
     /// A placeholder for a const which could not be computed; this is
     /// propagated to avoid useless error messages.
+    #[from(ignore)]
     Error(ErrorGuaranteed),
 
     /// Expr which contains an expression which has partially evaluated items.
     Expr(Expr<'tcx>),
 }
 
+impl<'tcx> From<ty::ConstVid<'tcx>> for ConstKind<'tcx> {
+    fn from(const_vid: ty::ConstVid<'tcx>) -> Self {
+        InferConst::Var(const_vid).into()
+    }
+}
+
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash)]
 #[derive(HashStable, TyEncodable, TyDecodable, TypeVisitable, TypeFoldable)]
 pub enum Expr<'tcx> {

compiler/rustc_middle/src/ty/context.rs

@@ -2598,8 +2598,8 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     #[inline]
-    pub fn mk_const(self, kind: ty::ConstKind<'tcx>, ty: Ty<'tcx>) -> Const<'tcx> {
-        self.mk_const_internal(ty::ConstS { kind, ty })
+    pub fn mk_const(self, kind: impl Into<ty::ConstKind<'tcx>>, ty: Ty<'tcx>) -> Const<'tcx> {
+        self.mk_const_internal(ty::ConstS { kind: kind.into(), ty })
     }
 
     #[inline]