Migrate stable let_chains error to session diagnostics

Author: Noratrieb

Diff for: compiler/rustc_ast_passes/src/ast_validation.rs

@@ -121,30 +121,9 @@ impl<'a> AstValidator<'a> {
     fn ban_let_expr(&self, expr: &'a Expr, forbidden_let_reason: ForbiddenLetReason) {
         let sess = &self.session;
         if sess.opts.unstable_features.is_nightly_build() {
-            let err = "`let` expressions are not supported here";
-            let mut diag = sess.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();
+            sess.emit_err(ForbiddenLet { span: expr.span, reason: forbidden_let_reason });
         } else {
-            sess.struct_span_err(expr.span, "expected expression, found statement (`let`)")
-                .note("variable declaration using `let` is a statement")
-                .emit();
+            sess.emit_err(ForbiddenLetStable { span: expr.span });
         }
     }
 

Diff for: compiler/rustc_ast_passes/src/errors.rs

@@ -30,6 +30,14 @@ impl AddSubdiagnostic for ForbiddenLetReason {
     }
 }
 
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::forbidden_let_stable)]
+#[note]
+pub struct ForbiddenLetStable {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(SessionDiagnostic)]
 #[diag(ast_passes::forbidden_assoc_constraint)]
 pub struct ForbiddenAssocConstraint {

Diff for: compiler/rustc_error_messages/locales/en-US/ast_passes.ftl

@@ -4,6 +4,10 @@ ast_passes_forbidden_let =
     .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
 
+ast_passes_forbidden_let_stable =
+    expected expression, found statement (`let`)
+    .note = variable declaration using `let` is a statement
+
 ast_passes_deprecated_where_clause_location =
     where clause not allowed here