diff --git a/Cargo.lock b/Cargo.lock
index 3a35985836c..cb245ce0ff8 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3595,6 +3595,7 @@ dependencies = [
  "rustc_data_structures",
  "rustc_errors",
  "rustc_feature",
+ "rustc_macros",
  "rustc_parse",
  "rustc_session",
  "rustc_span",
diff --git a/compiler/rustc_ast_passes/Cargo.toml b/compiler/rustc_ast_passes/Cargo.toml
index 22742b2adbd..37eff9207c1 100644
--- a/compiler/rustc_ast_passes/Cargo.toml
+++ b/compiler/rustc_ast_passes/Cargo.toml
@@ -11,6 +11,7 @@ rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_feature = { path = "../rustc_feature" }
+rustc_macros = { path = "../rustc_macros" }
 rustc_parse = { path = "../rustc_parse" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index e61dfef7bd3..f8eca6a9337 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -27,6 +27,8 @@ use rustc_target::spec::abi;
 use std::mem;
 use std::ops::{Deref, DerefMut};
 
+use crate::errors::ForbiddenLet;
+
 const MORE_EXTERN: &str =
     "for more information, visit https://doc.rust-lang.org/std/keyword.extern.html";
 
@@ -117,23 +119,7 @@ impl<'a> AstValidator<'a> {
 
     /// Emits an error banning the `let` expression provided in the given location.
     fn ban_let_expr(&self, expr: &'a Expr, forbidden_let_reason: ForbiddenLetReason) {
-        let err = "`let` expressions are not supported here";
-        let mut diag = self.session.struct_span_err(expr.span, err);
-        diag.note("only supported directly in conditions of `if` and `while` expressions");
-        match forbidden_let_reason {
-            ForbiddenLetReason::GenericForbidden => {}
-            ForbiddenLetReason::NotSupportedOr(span) => {
-                diag.span_note(span, "`||` operators are not supported in let chain expressions");
-            }
-            ForbiddenLetReason::NotSupportedParentheses(span) => {
-                diag.span_note(
-                    span,
-                    "`let`s wrapped in parentheses are not supported in a context with let \
-                    chains",
-                );
-            }
-        }
-        diag.emit();
+        self.session.emit_err(ForbiddenLet { span: expr.span, reason: forbidden_let_reason });
     }
 
     fn check_gat_where(
@@ -1876,7 +1862,7 @@ pub fn check_crate(session: &Session, krate: &Crate, lints: &mut LintBuffer) ->
 
 /// Used to forbid `let` expressions in certain syntactic locations.
 #[derive(Clone, Copy)]
-enum ForbiddenLetReason {
+pub(crate) enum ForbiddenLetReason {
     /// `let` is not valid and the source environment is not important
     GenericForbidden,
     /// A let chain with the `||` operator
diff --git a/compiler/rustc_ast_passes/src/errors.rs b/compiler/rustc_ast_passes/src/errors.rs
new file mode 100644
index 00000000000..397acd3b609
--- /dev/null
+++ b/compiler/rustc_ast_passes/src/errors.rs
@@ -0,0 +1,32 @@
+//! Errors emitted by ast_passes.
+
+use rustc_errors::fluent;
+use rustc_errors::{AddSubdiagnostic, Diagnostic};
+use rustc_macros::SessionDiagnostic;
+use rustc_span::Span;
+
+use crate::ast_validation::ForbiddenLetReason;
+
+#[derive(SessionDiagnostic)]
+#[error(ast_passes::forbidden_let)]
+#[note]
+pub struct ForbiddenLet {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub(crate) reason: ForbiddenLetReason,
+}
+
+impl AddSubdiagnostic for ForbiddenLetReason {
+    fn add_to_diagnostic(self, diag: &mut Diagnostic) {
+        match self {
+            Self::GenericForbidden => {}
+            Self::NotSupportedOr(span) => {
+                diag.span_note(span, fluent::ast_passes::not_supported_or);
+            }
+            Self::NotSupportedParentheses(span) => {
+                diag.span_note(span, fluent::ast_passes::not_supported_parentheses);
+            },
+        }
+    }
+}
diff --git a/compiler/rustc_ast_passes/src/lib.rs b/compiler/rustc_ast_passes/src/lib.rs
index 2bc3b6f3616..6a826298985 100644
--- a/compiler/rustc_ast_passes/src/lib.rs
+++ b/compiler/rustc_ast_passes/src/lib.rs
@@ -12,6 +12,7 @@
 #![recursion_limit = "256"]
 
 pub mod ast_validation;
+mod errors;
 pub mod feature_gate;
 pub mod node_count;
 pub mod show_span;
diff --git a/compiler/rustc_error_messages/locales/en-US/ast_passes.ftl b/compiler/rustc_error_messages/locales/en-US/ast_passes.ftl
new file mode 100644
index 00000000000..4f4235b6c81
--- /dev/null
+++ b/compiler/rustc_error_messages/locales/en-US/ast_passes.ftl
@@ -0,0 +1,5 @@
+ast_passes_forbidden_let =
+    `let` expressions are not supported here
+    .note = only supported directly in conditions of `if` and `while` expressions
+    .not_supported_or = `||` operators are not supported in let chain expressions
+    .not_supported_parentheses = `let`s wrapped in parentheses are not supported in a context with let chains
diff --git a/compiler/rustc_error_messages/src/lib.rs b/compiler/rustc_error_messages/src/lib.rs
index ee978f04be2..3569c7f0630 100644
--- a/compiler/rustc_error_messages/src/lib.rs
+++ b/compiler/rustc_error_messages/src/lib.rs
@@ -32,6 +32,7 @@ pub use unic_langid::{langid, LanguageIdentifier};
 
 // Generates `DEFAULT_LOCALE_RESOURCES` static and `fluent_generated` module.
 fluent_messages! {
+    ast_passes => "../locales/en-US/ast_passes.ftl",
     borrowck => "../locales/en-US/borrowck.ftl",
     builtin_macros => "../locales/en-US/builtin_macros.ftl",
     const_eval => "../locales/en-US/const_eval.ftl",