From 40ba1c902f0fa4f288242f573408f122c71945ce Mon Sep 17 00:00:00 2001
From: Robin Schroer <git@sulami.xyz>
Date: Thu, 8 Dec 2022 21:42:15 +0100
Subject: [PATCH] Illegal sized bounds: only suggest mutability change if
 needed

In a scenario like

```
struct Type;

pub trait Trait {
    fn function(&mut self)
    where
        Self: Sized;
}

impl Trait for Type {
    fn function(&mut self) {}
}

fn main() {
    (&mut Type as &mut dyn Trait).function();
}
```

the problem is Sized, not the mutability of self. Thus don't emit the
"you need &T instead of &mut T" note, or the other way around, as all
it does is just invert the mutability of whatever was supplied.

Fixes #103622.
---
 compiler/rustc_hir_typeck/src/method/mod.rs   |  2 +-
 .../mutability-mismatch.rs                    | 34 +++++++++++++++++++
 .../mutability-mismatch.stderr                | 24 +++++++++++++
 src/test/ui/illegal-sized-bound/regular.rs    | 32 +++++++++++++++++
 .../ui/illegal-sized-bound/regular.stderr     | 20 +++++++++++
 5 files changed, 111 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/illegal-sized-bound/mutability-mismatch.rs
 create mode 100644 src/test/ui/illegal-sized-bound/mutability-mismatch.stderr
 create mode 100644 src/test/ui/illegal-sized-bound/regular.rs
 create mode 100644 src/test/ui/illegal-sized-bound/regular.stderr

diff --git a/compiler/rustc_hir_typeck/src/method/mod.rs b/compiler/rustc_hir_typeck/src/method/mod.rs
index a2ca5c3b7b749..5d76e17b9f4d0 100644
--- a/compiler/rustc_hir_typeck/src/method/mod.rs
+++ b/compiler/rustc_hir_typeck/src/method/mod.rs
@@ -210,7 +210,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     ProbeScope::TraitsInScope,
                 ) {
                     Ok(ref new_pick) if pick.differs_from(new_pick) => {
-                        needs_mut = true;
+                        needs_mut = new_pick.self_ty.ref_mutability() != self_ty.ref_mutability();
                     }
                     _ => {}
                 }
diff --git a/src/test/ui/illegal-sized-bound/mutability-mismatch.rs b/src/test/ui/illegal-sized-bound/mutability-mismatch.rs
new file mode 100644
index 0000000000000..deb84f6fe97cf
--- /dev/null
+++ b/src/test/ui/illegal-sized-bound/mutability-mismatch.rs
@@ -0,0 +1,34 @@
+struct MutType;
+
+pub trait MutTrait {
+    fn function(&mut self)
+    where
+        Self: Sized;
+    //~^ this has a `Sized` requirement
+}
+
+impl MutTrait for MutType {
+    fn function(&mut self) {}
+}
+
+struct Type;
+
+pub trait Trait {
+    fn function(&self)
+    where
+        Self: Sized;
+    //~^ this has a `Sized` requirement
+}
+
+impl Trait for Type {
+    fn function(&self) {}
+}
+
+fn main() {
+    (&MutType as &dyn MutTrait).function();
+    //~^ ERROR the `function` method cannot be invoked on a trait object
+    //~| NOTE you need `&mut dyn MutTrait` instead of `&dyn MutTrait`
+    (&mut Type as &mut dyn Trait).function();
+    //~^ ERROR the `function` method cannot be invoked on a trait object
+    //~| NOTE you need `&dyn Trait` instead of `&mut dyn Trait`
+}
diff --git a/src/test/ui/illegal-sized-bound/mutability-mismatch.stderr b/src/test/ui/illegal-sized-bound/mutability-mismatch.stderr
new file mode 100644
index 0000000000000..dbbf79a4f1a03
--- /dev/null
+++ b/src/test/ui/illegal-sized-bound/mutability-mismatch.stderr
@@ -0,0 +1,24 @@
+error: the `function` method cannot be invoked on a trait object
+  --> $DIR/mutability-mismatch.rs:28:33
+   |
+LL |         Self: Sized;
+   |               ----- this has a `Sized` requirement
+...
+LL |     (&MutType as &dyn MutTrait).function();
+   |                                 ^^^^^^^^
+   |
+   = note: you need `&mut dyn MutTrait` instead of `&dyn MutTrait`
+
+error: the `function` method cannot be invoked on a trait object
+  --> $DIR/mutability-mismatch.rs:31:35
+   |
+LL |         Self: Sized;
+   |               ----- this has a `Sized` requirement
+...
+LL |     (&mut Type as &mut dyn Trait).function();
+   |                                   ^^^^^^^^
+   |
+   = note: you need `&dyn Trait` instead of `&mut dyn Trait`
+
+error: aborting due to 2 previous errors
+
diff --git a/src/test/ui/illegal-sized-bound/regular.rs b/src/test/ui/illegal-sized-bound/regular.rs
new file mode 100644
index 0000000000000..7abd27ef98318
--- /dev/null
+++ b/src/test/ui/illegal-sized-bound/regular.rs
@@ -0,0 +1,32 @@
+struct MutType;
+
+pub trait MutTrait {
+    fn function(&mut self)
+    where
+        Self: Sized;
+    //~^ this has a `Sized` requirement
+}
+
+impl MutTrait for MutType {
+    fn function(&mut self) {}
+}
+
+struct Type;
+
+pub trait Trait {
+    fn function(&self)
+    where
+        Self: Sized;
+    //~^ this has a `Sized` requirement
+}
+
+impl Trait for Type {
+    fn function(&self) {}
+}
+
+fn main() {
+    (&mut MutType as &mut dyn MutTrait).function();
+    //~^ ERROR the `function` method cannot be invoked on a trait object
+    (&Type as &dyn Trait).function();
+    //~^ ERROR the `function` method cannot be invoked on a trait object
+}
diff --git a/src/test/ui/illegal-sized-bound/regular.stderr b/src/test/ui/illegal-sized-bound/regular.stderr
new file mode 100644
index 0000000000000..7f3744145d927
--- /dev/null
+++ b/src/test/ui/illegal-sized-bound/regular.stderr
@@ -0,0 +1,20 @@
+error: the `function` method cannot be invoked on a trait object
+  --> $DIR/regular.rs:28:41
+   |
+LL |         Self: Sized;
+   |               ----- this has a `Sized` requirement
+...
+LL |     (&mut MutType as &mut dyn MutTrait).function();
+   |                                         ^^^^^^^^
+
+error: the `function` method cannot be invoked on a trait object
+  --> $DIR/regular.rs:30:27
+   |
+LL |         Self: Sized;
+   |               ----- this has a `Sized` requirement
+...
+LL |     (&Type as &dyn Trait).function();
+   |                           ^^^^^^^^
+
+error: aborting due to 2 previous errors
+