Only invoke decorate if the diag can eventually be emitted

compiler/rustc_middle/src/lint.rs

@@ -398,8 +398,16 @@ pub fn lint_level(
             }
         }
 
-        // Finally, run `decorate`.
+        // Finally, run `decorate`. This is guarded by a `can_emit_warnings()` check so that any
-        decorate(&mut err);
+        // `def_path_str` called within `decorate` won't trigger a `must_produce_diag` ICE if the
+        // `err` isn't eventually emitted (e.g. due to `-A warnings`). If an `err` is force-warn,
+        // it's going to be emitted anyway.
+        if matches!(err_level, rustc_errors::Level::ForceWarning(_))
+            || sess.dcx().can_emit_warnings()
+        {
+            decorate(&mut err);
+        }
+
         explain_lint_level_source(lint, level, src, &mut err);
         err.emit()
     }

tests/ui/lint/decorate-def-path-str-ice.rs

@@ -0,0 +1,14 @@
+// Checks that compiling this file with
+// `-Dunused_must_use -Awarnings --cap-lints=warn --crate-type=lib` does not ICE when emitting
+// diagnostics.
+// Issue: <https://github.com/rust-lang/rust/issues/121774>.
+
+//@ compile-flags: -Dunused_must_use -Awarnings --cap-lints=warn --crate-type=lib
+//@ check-pass
+
+#[must_use]
+fn f() {}
+
+pub fn g() {
+    f();
+}