finally came up with some repro code

tests/ui/auxiliary/proc_macro_attr.rs

@@ -11,8 +11,8 @@ use quote::{quote, quote_spanned};
 use syn::spanned::Spanned;
 use syn::token::Star;
 use syn::{
-    parse_macro_input, parse_quote, FnArg, ImplItem, ItemImpl, ItemTrait, Lifetime, Pat, PatIdent, PatType, Signature,
-    TraitItem, Type,
+    parse_macro_input, parse_quote, FnArg, ImplItem, ItemFn, ItemImpl, ItemTrait, Lifetime, Pat, PatIdent, PatType,
+    Signature, TraitItem, Type,
 };
 
 #[proc_macro_attribute]
@@ -95,3 +95,33 @@ pub fn rename_my_lifetimes(_args: TokenStream, input: TokenStream) -> TokenStrea
 
     TokenStream::from(quote!(#item))
 }
+
+#[proc_macro_attribute]
+pub fn fake_main(_attr: TokenStream, item: TokenStream) -> TokenStream {
+    let mut item = parse_macro_input!(item as ItemFn);
+    let span = item.block.brace_token.span;
+
+    if item.sig.asyncness.is_some() {
+        item.sig.asyncness = None;
+    }
+
+    let crate_name = quote! { fake_crate };
+    let block = item.block;
+    item.block = syn::parse_quote_spanned! {
+        span =>
+        {
+            #crate_name::block_on(async {
+                #block
+            })
+        }
+    };
+
+    quote! {
+        mod #crate_name {
+            pub fn block_on<F: ::std::future::Future>(_fut: F) {}
+        }
+
+        #item
+    }
+    .into()
+}

tests/ui/semicolon_if_nothing_returned.fixed

@@ -1,6 +1,11 @@
+//@aux-build:proc_macro_attr.rs
+
 #![warn(clippy::semicolon_if_nothing_returned)]
 #![allow(clippy::redundant_closure, clippy::uninlined_format_args, clippy::needless_late_init)]
 
+#[macro_use]
+extern crate proc_macro_attr;
+
 fn get_unit() {}
 
 // the functions below trigger the lint
@@ -120,3 +125,25 @@ fn let_else_stmts() {
         return;
     };
 }
+
+mod issue12123 {
+    #[rustfmt::skip]
+    mod this_triggers {
+        #[fake_main];
+        async fn main() {
+            
+        }
+    }
+
+    mod and_this {
+        #[fake_main];
+        async fn main() {
+            println!("hello");
+        }
+    }
+
+    mod but_this_does_not {
+        #[fake_main]
+        async fn main() {}
+    }
+}

tests/ui/semicolon_if_nothing_returned.rs

@@ -1,6 +1,11 @@
+//@aux-build:proc_macro_attr.rs
+
 #![warn(clippy::semicolon_if_nothing_returned)]
 #![allow(clippy::redundant_closure, clippy::uninlined_format_args, clippy::needless_late_init)]
 
+#[macro_use]
+extern crate proc_macro_attr;
+
 fn get_unit() {}
 
 // the functions below trigger the lint
@@ -120,3 +125,25 @@ fn let_else_stmts() {
         return;
     };
 }
+
+mod issue12123 {
+    #[rustfmt::skip]
+    mod this_triggers {
+        #[fake_main]
+        async fn main() {
+            
+        }
+    }
+
+    mod and_this {
+        #[fake_main]
+        async fn main() {
+            println!("hello");
+        }
+    }
+
+    mod but_this_does_not {
+        #[fake_main]
+        async fn main() {}
+    }
+}

tests/ui/semicolon_if_nothing_returned.stderr

@@ -1,5 +1,5 @@
 error: consider adding a `;` to the last statement for consistent formatting
-  --> $DIR/semicolon_if_nothing_returned.rs:8:5
+  --> $DIR/semicolon_if_nothing_returned.rs:13:5
    |
 LL |     println!("Hello")
    |     ^^^^^^^^^^^^^^^^^ help: add a `;` here: `println!("Hello");`
@@ -8,28 +8,40 @@ LL |     println!("Hello")
    = help: to override `-D warnings` add `#[allow(clippy::semicolon_if_nothing_returned)]`
 
 error: consider adding a `;` to the last statement for consistent formatting
-  --> $DIR/semicolon_if_nothing_returned.rs:12:5
+  --> $DIR/semicolon_if_nothing_returned.rs:17:5
    |
 LL |     get_unit()
    |     ^^^^^^^^^^ help: add a `;` here: `get_unit();`
 
 error: consider adding a `;` to the last statement for consistent formatting
-  --> $DIR/semicolon_if_nothing_returned.rs:17:5
+  --> $DIR/semicolon_if_nothing_returned.rs:22:5
    |
 LL |     y = x + 1
    |     ^^^^^^^^^ help: add a `;` here: `y = x + 1;`
 
 error: consider adding a `;` to the last statement for consistent formatting
-  --> $DIR/semicolon_if_nothing_returned.rs:23:9
+  --> $DIR/semicolon_if_nothing_returned.rs:28:9
    |
 LL |         hello()
    |         ^^^^^^^ help: add a `;` here: `hello();`
 
 error: consider adding a `;` to the last statement for consistent formatting
-  --> $DIR/semicolon_if_nothing_returned.rs:34:9
+  --> $DIR/semicolon_if_nothing_returned.rs:39:9
    |
 LL |         ptr::drop_in_place(s.as_mut_ptr())
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: add a `;` here: `ptr::drop_in_place(s.as_mut_ptr());`
 
-error: aborting due to 5 previous errors
+error: consider adding a `;` to the last statement for consistent formatting
+  --> $DIR/semicolon_if_nothing_returned.rs:132:9
+   |
+LL |         #[fake_main]
+   |         ^^^^^^^^^^^^ help: add a `;` here: `#[fake_main];`
+
+error: consider adding a `;` to the last statement for consistent formatting
+  --> $DIR/semicolon_if_nothing_returned.rs:139:9
+   |
+LL |         #[fake_main]
+   |         ^^^^^^^^^^^^ help: add a `;` here: `#[fake_main];`
+
+error: aborting due to 7 previous errors