Unconditionally lower match arm even if it's unneeded for never pattern in match

Author: mu001999

compiler/rustc_ast_lowering/src/expr.rs

@@ -672,10 +672,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let span = self.lower_span(arm.span);
         self.lower_attrs(hir_id, &arm.attrs);
         let is_never_pattern = pat.is_never_pattern();
-        let body = if let Some(body) = &arm.body
+        // We need to lower the body even if it's unneeded for never pattern in match,
+        // ensure that we can get HirId for DefId if need (issue #137708).
+        let body = arm.body.as_ref().map(|x| self.lower_expr(x));
+        let body = if let Some(body) = body
             && !is_never_pattern
         {
-            self.lower_expr(body)
+            body
         } else {
             // Either `body.is_none()` or `is_never_pattern` here.
             if !is_never_pattern {

tests/ui/never_type/unused_trait_in_never_pattern_body.rs

@@ -0,0 +1,12 @@
+fn a() {
+    match 0 {
+        ! => || { //~ ERROR `!` patterns are experimental
+        //~^ ERROR a never pattern is always unreachable
+        //~^^ ERROR mismatched types
+            use std::ops::Add;
+            0.add(1)
+        },
+    }
+}
+
+fn main() {}

tests/ui/never_type/unused_trait_in_never_pattern_body.stderr

@@ -0,0 +1,36 @@
+error[E0658]: `!` patterns are experimental
+  --> $DIR/unused_trait_in_never_pattern_body.rs:3:9
+   |
+LL |         ! => || {
+   |         ^
+   |
+   = note: see issue #118155 <https://github.com/rust-lang/rust/issues/118155> for more information
+   = help: add `#![feature(never_patterns)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: a never pattern is always unreachable
+  --> $DIR/unused_trait_in_never_pattern_body.rs:3:14
+   |
+LL |           ! => || {
+   |  ______________^
+LL | |
+LL | |
+LL | |             use std::ops::Add;
+LL | |             0.add(1)
+LL | |         },
+   | |         ^
+   | |         |
+   | |_________this will never be executed
+   |           help: remove this expression
+
+error: mismatched types
+  --> $DIR/unused_trait_in_never_pattern_body.rs:3:9
+   |
+LL |         ! => || {
+   |         ^ a never pattern must be used on an uninhabited type
+   |
+   = note: the matched value is of type `i32`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.