diff --git a/compiler/rustc_attr/src/builtin.rs b/compiler/rustc_attr/src/builtin.rs
index 49c6b84a4a5..8ffd322b93f 100644
--- a/compiler/rustc_attr/src/builtin.rs
+++ b/compiler/rustc_attr/src/builtin.rs
@@ -13,7 +13,6 @@ use rustc_session::parse::{feature_err, ParseSess};
 use rustc_session::{RustcVersion, Session};
 use rustc_span::hygiene::Transparency;
 use rustc_span::{symbol::sym, symbol::Symbol, Span};
-use std::fmt::{self, Display};
 use std::num::NonZeroU32;
 
 use crate::session_diagnostics::{self, IncorrectReprFormatGenericCause};
@@ -736,12 +735,12 @@ pub enum DeprecatedSince {
     RustcVersion(RustcVersion),
     /// Deprecated in the future ("to be determined").
     Future,
-    /// `feature(staged_api)` is off, or it's on but the deprecation version
-    /// cannot be parsed as a RustcVersion. In the latter case, an error has
-    /// already been emitted. In the former case, deprecation versions outside
-    /// the standard library are allowed to be arbitrary strings, for better or
-    /// worse.
+    /// `feature(staged_api)` is off. Deprecation versions outside the standard
+    /// library are allowed to be arbitrary strings, for better or worse.
     Symbol(Symbol),
+    /// Failed to parse a deprecation version. An error has already been
+    /// emitted.
+    Err,
 }
 
 impl Deprecation {
@@ -754,18 +753,8 @@ impl Deprecation {
             Some(DeprecatedSince::Future) => false,
             // The `since` field doesn't have semantic purpose without `#![staged_api]`.
             Some(DeprecatedSince::Symbol(_)) => true,
-            // Assume deprecation is in effect if "since" field is missing.
-            None => true,
-        }
-    }
-}
-
-impl Display for DeprecatedSince {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            DeprecatedSince::RustcVersion(since) => Display::fmt(since, formatter),
-            DeprecatedSince::Future => formatter.write_str("TBD"),
-            DeprecatedSince::Symbol(since) => Display::fmt(since, formatter),
+            // Assume deprecation is in effect if "since" field is absent or invalid.
+            None | Some(DeprecatedSince::Err) => true,
         }
     }
 }
@@ -885,7 +874,7 @@ pub fn find_deprecation(
                 Some(DeprecatedSince::RustcVersion(version))
             } else {
                 sess.emit_err(session_diagnostics::InvalidSince { span: attr.span });
-                Some(DeprecatedSince::Symbol(since))
+                Some(DeprecatedSince::Err)
             }
         } else if is_rustc {
             sess.emit_err(session_diagnostics::MissingSince { span: attr.span });
diff --git a/compiler/rustc_middle/src/middle/stability.rs b/compiler/rustc_middle/src/middle/stability.rs
index 27c2010fe29..4d27bcad911 100644
--- a/compiler/rustc_middle/src/middle/stability.rs
+++ b/compiler/rustc_middle/src/middle/stability.rs
@@ -155,15 +155,16 @@ fn deprecation_message(
     let message = if is_in_effect {
         format!("use of deprecated {kind} `{path}`")
     } else {
-        if let Some(DeprecatedSince::Future) = since {
-            format!("use of {kind} `{path}` that will be deprecated in a future Rust version")
-        } else {
-            format!(
-                "use of {} `{}` that will be deprecated in future version {}",
-                kind,
-                path,
-                since.unwrap()
-            )
+        match since {
+            Some(DeprecatedSince::RustcVersion(version)) => format!(
+                "use of {kind} `{path}` that will be deprecated in future version {version}"
+            ),
+            Some(DeprecatedSince::Future) => {
+                format!("use of {kind} `{path}` that will be deprecated in a future Rust version")
+            }
+            Some(DeprecatedSince::Symbol(_)) | Some(DeprecatedSince::Err) | None => {
+                unreachable!("this deprecation is always in effect; {since:?}")
+            }
         }
     };
 
diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index cf54dc3d6b7..b64bfa2ae8a 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/src/librustdoc/html/render/mod.rs
@@ -619,18 +619,19 @@ fn short_item_info(
     if let Some(depr @ Deprecation { note, since, suggestion: _ }) = item.deprecation(cx.tcx()) {
         // We display deprecation messages for #[deprecated], but only display
         // the future-deprecation messages for rustc versions.
-        let mut message = if let Some(since) = since {
-            if !depr.is_in_effect() {
-                if let DeprecatedSince::Future = since {
-                    String::from("Deprecating in a future Rust version")
+        let mut message = match since {
+            Some(DeprecatedSince::RustcVersion(version)) => {
+                if depr.is_in_effect() {
+                    format!("Deprecated since {version}")
                 } else {
-                    format!("Deprecating in {}", Escape(&since.to_string()))
+                    format!("Deprecating in {version}")
                 }
-            } else {
-                format!("Deprecated since {}", Escape(&since.to_string()))
             }
-        } else {
-            String::from("Deprecated")
+            Some(DeprecatedSince::Future) => String::from("Deprecating in a future Rust version"),
+            Some(DeprecatedSince::Symbol(since)) => {
+                format!("Deprecated since {}", Escape(since.as_str()))
+            }
+            Some(DeprecatedSince::Err) | None => String::from("Deprecated"),
         };
 
         if let Some(note) = note {
diff --git a/src/librustdoc/json/conversions.rs b/src/librustdoc/json/conversions.rs
index ff2e6b1ebc9..acc928360d2 100644
--- a/src/librustdoc/json/conversions.rs
+++ b/src/librustdoc/json/conversions.rs
@@ -7,6 +7,7 @@
 use std::fmt;
 
 use rustc_ast::ast;
+use rustc_attr::DeprecatedSince;
 use rustc_hir::{def::CtorKind, def::DefKind, def_id::DefId};
 use rustc_metadata::rendered_const;
 use rustc_middle::ty::{self, TyCtxt};
@@ -139,7 +140,13 @@ where
 
 pub(crate) fn from_deprecation(deprecation: rustc_attr::Deprecation) -> Deprecation {
     let rustc_attr::Deprecation { since, note, suggestion: _ } = deprecation;
-    Deprecation { since: since.map(|s| s.to_string()), note: note.map(|s| s.to_string()) }
+    let since = match since {
+        Some(DeprecatedSince::RustcVersion(version)) => Some(version.to_string()),
+        Some(DeprecatedSince::Future) => Some("TBD".to_owned()),
+        Some(DeprecatedSince::Symbol(since)) => Some(since.to_string()),
+        Some(DeprecatedSince::Err) | None => None,
+    };
+    Deprecation { since, note: note.map(|s| s.to_string()) }
 }
 
 impl FromWithTcx<clean::GenericArgs> for GenericArgs {