Implement IntoDiagnosticArg for hir::ConstContext

Cargo.lock

@@ -3813,6 +3813,7 @@ dependencies = [
  "atty",
  "rustc_data_structures",
  "rustc_error_messages",
+ "rustc_hir",
  "rustc_lint_defs",
  "rustc_macros",
  "rustc_serialize",

compiler/rustc_errors/Cargo.toml

@@ -13,6 +13,7 @@ rustc_serialize = { path = "../rustc_serialize" }
 rustc_span = { path = "../rustc_span" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_hir = { path = "../rustc_hir" }
 rustc_lint_defs = { path = "../rustc_lint_defs" }
 unicode-width = "0.1.4"
 atty = "0.2"

compiler/rustc_errors/src/diagnostic.rs

@@ -5,6 +5,7 @@ use crate::{
 };
 use rustc_data_structures::stable_map::FxHashMap;
 use rustc_error_messages::FluentValue;
+use rustc_hir as hir;
 use rustc_lint_defs::{Applicability, LintExpectationId};
 use rustc_span::edition::LATEST_STABLE_EDITION;
 use rustc_span::symbol::{Ident, Symbol};
@@ -160,6 +161,16 @@ impl<'source> Into<FluentValue<'source>> for DiagnosticArgValue<'source> {
     }
 }
 
+impl IntoDiagnosticArg for hir::ConstContext {
+    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
+        DiagnosticArgValue::Str(Cow::Borrowed(match self {
+            hir::ConstContext::ConstFn => "constant function",
+            hir::ConstContext::Static(_) => "static",
+            hir::ConstContext::Const => "constant",
+        }))
+    }
+}
+
 /// Trait implemented by error types. This should not be implemented manually. Instead, use
 /// `#[derive(SessionSubdiagnostic)]` -- see [rustc_macros::SessionSubdiagnostic].
 #[rustc_diagnostic_item = "AddSubdiagnostic"]

compiler/rustc_hir/src/hir.rs

@@ -1595,6 +1595,9 @@ impl fmt::Display for ConstContext {
     }
 }
 
+// NOTE: `IntoDiagnosticArg` impl for `ConstContext` lives in `rustc_errors`
+// due to a cyclical dependency between hir that crate.
+
 /// A literal.
 pub type Lit = Spanned<LitKind>;