diff --git a/Cargo.lock b/Cargo.lock
index 9464c87fc72..1c1869a470c 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4253,8 +4253,10 @@ name = "rustc_monomorphize"
 version = "0.0.0"
 dependencies = [
  "rustc_data_structures",
+ "rustc_errors",
  "rustc_hir",
  "rustc_index",
+ "rustc_macros",
  "rustc_middle",
  "rustc_session",
  "rustc_span",
@@ -4995,9 +4997,9 @@ checksum = "da73c8f77aebc0e40c300b93f0a5f1bece7a248a36eee287d4e095f35c7b7d6e"
 
 [[package]]
 name = "snapbox"
-version = "0.3.3"
+version = "0.2.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "44d199ccf8f606592df2d145db26f2aa45344e23c64b074cc5a4047f1d99b0f7"
+checksum = "767a1d5da232b6959cd1bd5c9e8db8a7cce09c3038e89deedb49a549a2aefd93"
 dependencies = [
  "concolor",
  "content_inspector",
@@ -5013,9 +5015,9 @@ dependencies = [
 
 [[package]]
 name = "snapbox-macros"
-version = "0.3.0"
+version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8a253e6f894cfa440cba00600a249fa90869d8e0ec45ab274a456e043a0ce8f2"
+checksum = "c01dea7e04cbb27ef4c86e9922184608185f7cd95c1763bc30d727cda4a5e930"
 
 [[package]]
 name = "socket2"
diff --git a/compiler/rustc_error_messages/locales/en-US/monomorphize.ftl b/compiler/rustc_error_messages/locales/en-US/monomorphize.ftl
new file mode 100644
index 00000000000..4c67c6f5cb4
--- /dev/null
+++ b/compiler/rustc_error_messages/locales/en-US/monomorphize.ftl
@@ -0,0 +1,19 @@
+monomorphize_recursion_limit =
+    reached the recursion limit while instantiating `{$shrunk}`
+    .note = `{$def_path_str}` defined here
+
+monomorphize_written_to_path = the full type name has been written to '{$path}'
+
+monomorphize_type_length_limit = reached the type-length limit while instantiating `{$shrunk}`
+
+monomorphize_consider_type_length_limit =
+    consider adding a `#![type_length_limit="{$type_length}"]` attribute to your crate
+
+monomorphize_fatal_error = {$error_message}
+
+monomorphize_unused_generic_params = item has unused generic parameters
+
+monomorphize_large_assignments =
+    moving {$size} bytes
+    .label = value moved from here
+    .note = The current maximum size is {$limit}, but it can be customized with the move_size_limit attribute: `#![move_size_limit = "..."]`
diff --git a/compiler/rustc_error_messages/src/lib.rs b/compiler/rustc_error_messages/src/lib.rs
index 2d001d445be..3f3f99fa9c0 100644
--- a/compiler/rustc_error_messages/src/lib.rs
+++ b/compiler/rustc_error_messages/src/lib.rs
@@ -39,6 +39,7 @@ fluent_messages! {
     expand => "../locales/en-US/expand.ftl",
     interface => "../locales/en-US/interface.ftl",
     lint => "../locales/en-US/lint.ftl",
+    monomorphize => "../locales/en-US/monomorphize.ftl",
     parser => "../locales/en-US/parser.ftl",
     passes => "../locales/en-US/passes.ftl",
     plugin_impl => "../locales/en-US/plugin_impl.ftl",
diff --git a/compiler/rustc_monomorphize/Cargo.toml b/compiler/rustc_monomorphize/Cargo.toml
index 41ba4d4b64a..59ca04ec868 100644
--- a/compiler/rustc_monomorphize/Cargo.toml
+++ b/compiler/rustc_monomorphize/Cargo.toml
@@ -7,11 +7,13 @@ edition = "2021"
 doctest = false
 
 [dependencies]
-smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+smallvec = { version = "1.8.1", features = [ "union", "may_dangle" ] }
 tracing = "0.1"
 rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_errors = { path = "../rustc_errors" }
 rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
+rustc_macros = { path = "../rustc_macros" }
 rustc_middle = { path = "../rustc_middle" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 82ef16a7f72..8f0d98563af 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -207,6 +207,8 @@ use std::iter;
 use std::ops::Range;
 use std::path::PathBuf;
 
+use crate::errors::{FatalError, LargeAssignmentsLint, RecursionLimit, TypeLengthLimit};
+
 #[derive(PartialEq)]
 pub enum MonoItemCollectionMode {
     Eager,
@@ -604,17 +606,24 @@ fn check_recursion_limit<'tcx>(
     // more than the recursion limit is assumed to be causing an
     // infinite expansion.
     if !recursion_limit.value_within_limit(adjusted_recursion_depth) {
+        let def_span = tcx.def_span(def_id);
+        let def_path_str = tcx.def_path_str(def_id);
         let (shrunk, written_to_path) = shrunk_instance_name(tcx, &instance, 32, 32);
-        let error = format!("reached the recursion limit while instantiating `{}`", shrunk);
-        let mut err = tcx.sess.struct_span_fatal(span, &error);
-        err.span_note(
-            tcx.def_span(def_id),
-            &format!("`{}` defined here", tcx.def_path_str(def_id)),
-        );
-        if let Some(path) = written_to_path {
-            err.note(&format!("the full type name has been written to '{}'", path.display()));
-        }
-        err.emit()
+        let mut path = PathBuf::new();
+        let was_written = if written_to_path.is_some() {
+            path = written_to_path.unwrap();
+            Some(())
+        } else {
+            None
+        };
+        tcx.sess.emit_fatal(RecursionLimit {
+            span,
+            shrunk,
+            def_span,
+            def_path_str,
+            was_written,
+            path,
+        });
     }
 
     recursion_depths.insert(def_id, recursion_depth + 1);
@@ -642,16 +651,15 @@ fn check_type_length_limit<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) {
     // Bail out in these cases to avoid that bad user experience.
     if !tcx.type_length_limit().value_within_limit(type_length) {
         let (shrunk, written_to_path) = shrunk_instance_name(tcx, &instance, 32, 32);
-        let msg = format!("reached the type-length limit while instantiating `{}`", shrunk);
-        let mut diag = tcx.sess.struct_span_fatal(tcx.def_span(instance.def_id()), &msg);
-        if let Some(path) = written_to_path {
-            diag.note(&format!("the full type name has been written to '{}'", path.display()));
-        }
-        diag.help(&format!(
-            "consider adding a `#![type_length_limit=\"{}\"]` attribute to your crate",
-            type_length
-        ));
-        diag.emit()
+        let span = tcx.def_span(instance.def_id());
+        let mut path = PathBuf::new();
+        let was_written = if written_to_path.is_some() {
+            path = written_to_path.unwrap();
+            Some(())
+        } else {
+            None
+        };
+        tcx.sess.emit_fatal(TypeLengthLimit { span, shrunk, was_written, path, type_length });
     }
 }
 
@@ -914,17 +922,16 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirNeighborCollector<'a, 'tcx> {
                     // but correct span? This would make the lint at least accept crate-level lint attributes.
                     return;
                 };
-                self.tcx.struct_span_lint_hir(
+                self.tcx.emit_spanned_lint(
                     LARGE_ASSIGNMENTS,
                     lint_root,
                     source_info.span,
-                    |lint| {
-                        let mut err = lint.build(&format!("moving {} bytes", layout.size.bytes()));
-                        err.span_label(source_info.span, "value moved from here");
-                        err.note(&format!(r#"The current maximum size is {}, but it can be customized with the move_size_limit attribute: `#![move_size_limit = "..."]`"#, limit.bytes()));
-                        err.emit();
+                    LargeAssignmentsLint {
+                        span: source_info.span,
+                        size: layout.size.bytes(),
+                        limit: limit.bytes(),
                     },
-                );
+                )
             }
         }
     }
@@ -1321,7 +1328,9 @@ impl<'v> RootCollector<'_, 'v> {
 
         let start_def_id = match self.tcx.lang_items().require(LangItem::Start) {
             Ok(s) => s,
-            Err(err) => self.tcx.sess.fatal(&err),
+            Err(error_message) => {
+                self.tcx.sess.emit_fatal(FatalError { error_message: error_message.clone() });
+            }
         };
         let main_ret_ty = self.tcx.fn_sig(main_def_id).output();
 
diff --git a/compiler/rustc_monomorphize/src/errors.rs b/compiler/rustc_monomorphize/src/errors.rs
new file mode 100644
index 00000000000..62ebac97136
--- /dev/null
+++ b/compiler/rustc_monomorphize/src/errors.rs
@@ -0,0 +1,81 @@
+use std::path::PathBuf;
+
+use rustc_errors::ErrorGuaranteed;
+use rustc_macros::{LintDiagnostic, SessionDiagnostic};
+use rustc_session::SessionDiagnostic;
+// use rustc_session::SessionDiagnostic;
+use rustc_span::Span;
+
+#[derive(SessionDiagnostic)]
+#[diag(monomorphize::recursion_limit)]
+pub struct RecursionLimit {
+    #[primary_span]
+    pub span: Span,
+    pub shrunk: String,
+    #[note]
+    pub def_span: Span,
+    pub def_path_str: String,
+    #[note(monomorphize::written_to_path)]
+    pub was_written: Option<()>,
+    pub path: PathBuf,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(monomorphize::type_length_limit)]
+#[help(monomorphize::consider_type_length_limit)]
+pub struct TypeLengthLimit {
+    #[primary_span]
+    pub span: Span,
+    pub shrunk: String,
+    #[note(monomorphize::written_to_path)]
+    pub was_written: Option<()>,
+    pub path: PathBuf,
+    pub type_length: usize,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(monomorphize::fatal_error)]
+pub struct FatalError {
+    pub error_message: String,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(monomorphize::fatal_error)]
+pub struct SpanFatalError {
+    #[primary_span]
+    pub span: Span,
+    pub error_message: String,
+}
+
+pub struct UnusedGenericParams {
+    pub span: Span,
+    pub param_spans: Vec<Span>,
+    pub param_names: Vec<String>,
+}
+
+impl SessionDiagnostic<'_> for UnusedGenericParams {
+    fn into_diagnostic(
+        self,
+        sess: &'_ rustc_session::parse::ParseSess,
+    ) -> rustc_errors::DiagnosticBuilder<'_, ErrorGuaranteed> {
+        let mut diag = sess.struct_err(rustc_errors::fluent::monomorphize::unused_generic_params);
+        diag.set_span(self.span);
+        for (span, name) in self.param_spans.into_iter().zip(self.param_names) {
+            // FIXME: I can figure out how to do a label with a fluent string with a fixed message,
+            // or a label with a dynamic value in a hard-coded string, but I haven't figured out
+            // how to combine the two. 😢
+            diag.span_label(span, format!("generic parameter `{}` is unused", name));
+        }
+        diag
+    }
+}
+
+#[derive(LintDiagnostic)]
+#[diag(monomorphize::large_assignments)]
+#[note]
+pub struct LargeAssignmentsLint {
+    #[label]
+    pub span: Span,
+    pub size: u64,
+    pub limit: u64,
+}
diff --git a/compiler/rustc_monomorphize/src/lib.rs b/compiler/rustc_monomorphize/src/lib.rs
index ef4560b5ec4..d64de44705b 100644
--- a/compiler/rustc_monomorphize/src/lib.rs
+++ b/compiler/rustc_monomorphize/src/lib.rs
@@ -3,6 +3,8 @@
 #![feature(let_else)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 
 #[macro_use]
 extern crate tracing;
@@ -16,6 +18,7 @@ use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
 mod collector;
+mod errors;
 mod partitioning;
 mod polymorphize;
 mod util;
diff --git a/compiler/rustc_monomorphize/src/partitioning/mod.rs b/compiler/rustc_monomorphize/src/partitioning/mod.rs
index ff2d3869328..d88b7e0a813 100644
--- a/compiler/rustc_monomorphize/src/partitioning/mod.rs
+++ b/compiler/rustc_monomorphize/src/partitioning/mod.rs
@@ -108,6 +108,7 @@ use rustc_span::symbol::Symbol;
 
 use crate::collector::InliningMap;
 use crate::collector::{self, MonoItemCollectionMode};
+use crate::errors::{FatalError, SpanFatalError};
 
 pub struct PartitioningCx<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
@@ -149,7 +150,10 @@ fn get_partitioner<'tcx>(tcx: TyCtxt<'tcx>) -> Box<dyn Partitioner<'tcx>> {
 
     match strategy {
         "default" => Box::new(default::DefaultPartitioning),
-        _ => tcx.sess.fatal("unknown partitioning strategy"),
+        _ => {
+            let error_message = "unknown partitioning strategy".to_string();
+            tcx.sess.emit_fatal(FatalError { error_message: error_message.clone() });
+        }
     }
 }
 
@@ -334,9 +338,9 @@ where
             let error_message = format!("symbol `{}` is already defined", sym1);
 
             if let Some(span) = span {
-                tcx.sess.span_fatal(span, &error_message)
+                tcx.sess.emit_fatal(SpanFatalError { span, error_message: error_message.clone() });
             } else {
-                tcx.sess.fatal(&error_message)
+                tcx.sess.emit_fatal(FatalError { error_message: error_message.clone() });
             }
         }
     }
diff --git a/compiler/rustc_monomorphize/src/polymorphize.rs b/compiler/rustc_monomorphize/src/polymorphize.rs
index 394843e510d..88f8e24fbd5 100644
--- a/compiler/rustc_monomorphize/src/polymorphize.rs
+++ b/compiler/rustc_monomorphize/src/polymorphize.rs
@@ -22,6 +22,8 @@ use rustc_span::symbol::sym;
 use std::convert::TryInto;
 use std::ops::ControlFlow;
 
+use crate::errors::UnusedGenericParams;
+
 /// Provide implementations of queries relating to polymorphization analysis.
 pub fn provide(providers: &mut Providers) {
     providers.unused_generic_params = unused_generic_params;
@@ -206,22 +208,28 @@ fn emit_unused_generic_params_error<'tcx>(
         _ => tcx.def_span(def_id),
     };
 
-    let mut err = tcx.sess.struct_span_err(fn_span, "item has unused generic parameters");
-
+    let mut param_spans = Vec::new();
+    let mut param_names = Vec::new();
     let mut next_generics = Some(generics);
     while let Some(generics) = next_generics {
         for param in &generics.params {
             if unused_parameters.contains(param.index).unwrap_or(false) {
                 debug!(?param);
                 let def_span = tcx.def_span(param.def_id);
-                err.span_label(def_span, &format!("generic parameter `{}` is unused", param.name));
+                // 🤔 The docs say
+                //
+                //     Any attribute applied to a Vec<T> will be repeated for each element of the vector.
+                //
+                // But they don't say what template variable to use to substitute each value into the message!?
+                param_spans.push(def_span);
+                param_names.push(param.name.to_string());
             }
         }
 
         next_generics = generics.parent.map(|did| tcx.generics_of(did));
     }
 
-    err.emit();
+    tcx.sess.emit_err(UnusedGenericParams { span: fn_span, param_spans, param_names });
 }
 
 /// Visitor used to aggregate generic parameter uses.
diff --git a/src/tools/cargo b/src/tools/cargo
index 6da726708a4..9809f8ff33c 160000
--- a/src/tools/cargo
+++ b/src/tools/cargo
@@ -1 +1 @@
-Subproject commit 6da726708a4406f31f996d813790818dce837161
+Subproject commit 9809f8ff33c2b998919fd0432c626f0f7323697a