Example demonstrating API trait problem.

.gitignore

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+/target
+Cargo.lock

Cargo.toml

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+[package]
+name = "rusttraits"
+version = "0.1.0"
+authors = ["Bill Thiede <git@xinu.tv>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

src/lib.rs

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+//! Illustrates problem with &mut trait implementations and non-&mut generic functions.
+//!
+//! error[E0277]: the trait bound `&mut Concrete: API` is not satisfied
+//!   --> src/lib.rs:32:14
+//!    |
+//! 24 | fn func<A: API>(mut a: A) {
+//!    |            --- required by this bound in `func`
+//! ...
+//! 32 |         func(self);
+//!    |              ^^^^ the trait `API` is not implemented for `&mut Concrete`
+//!    |
+//!    = help: the following implementations were found:
+//!              <Concrete as API>
+trait API {
+    fn change_state(&mut self);
+    fn reset(&mut self);
+}
+
+#[derive(Default)]
+struct Concrete {
+    count: isize,
+}
+
+fn func<A: API>(mut a: A) {
+    a.reset();
+}
+
+impl API for Concrete {
+    // We don't want this function to be `self` because we don't want the method to consume the
+    // object.
+    fn change_state(&mut self) {
+        func(self);
+    }
+    // This mutates the state of `Concrete`, so it needs to be `&mut`, and will be called from
+    // `func`.
+    fn reset(&mut self) {
+        self.count = 0;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        let mut c = Concrete::default();
+        c.count = 1;
+        assert_eq!(c.count, 1);
+        c.change_state();
+        assert_eq!(c.count, 0);
+    }
+}