fix: Pavex no longer returns a borrow checker error when a &mut refer…

…ence must be coerced to a shared reference (#401)

libs/pavexc/src/compiler/analyses/call_graph/borrow_checker/multiple_consumers.rs

@@ -19,6 +19,7 @@ use crate::compiler::computation::Computation;
 use crate::diagnostic::{
     AnnotatedSnippet, CompilerDiagnostic, HelpWithSnippet, LocationExt, OptionalSourceSpanExt,
 };
+use crate::language::ResolvedType;
 use crate::rustdoc::CrateCollection;
 use crate::{diagnostic, try_source};
 
@@ -59,8 +60,13 @@ pub(super) fn multiple_consumers(
             })
             .collect();
         if consumer_ids.len() > 1 {
-            if copy_checker.is_copy(&call_graph, node_id, component_db, computation_db) {
-                // You can't have a "used after moved" error for a Copy type.
+            // You can't have a "used after moved" error for a Copy type.
+            if copy_checker.is_copy(&call_graph, node_id, component_db, computation_db)
+            // Even though `&mut` references are not `Copy`, "used after move" is not
+            // the right error message for them.
+            // They belong to the `move_while_borrowed` category. So we skip them here.
+            || is_ref(node_id, &call_graph, component_db, computation_db)
+            {
                 continue;
             }
 
@@ -165,6 +171,29 @@ pub(super) fn multiple_consumers(
     }
 }
 
+fn is_ref(
+    node_id: NodeIndex,
+    call_graph: &RawCallGraph,
+    component_db: &ComponentDb,
+    computation_db: &ComputationDb,
+) -> bool {
+    let _is_ref = |ty_| matches!(ty_, &ResolvedType::Reference(..));
+    match &call_graph[node_id] {
+        CallGraphNode::Compute { component_id, .. } => {
+            let component = component_db.hydrated_component(*component_id, computation_db);
+            let Some(output_type) = component.output_type() else {
+                // `()` is not a reference.
+                return false;
+            };
+            _is_ref(&output_type)
+        }
+        CallGraphNode::MatchBranching => {
+            return false;
+        }
+        CallGraphNode::InputParameter { type_, .. } => _is_ref(type_),
+    }
+}
+
 fn emit_multiple_consumers_error(
     consumed_node_id: NodeIndex,
     consuming_node_ids: BTreeSet<NodeIndex>,

libs/pavexc/src/compiler/codegen_utils.rs

@@ -114,7 +114,28 @@ where
                 }
             }
         };
+
+        // We also register the reference as a shared reference
+        // since we rely on the compiler's deref coercion to convert
+        // `&mut T` to `&T` when needed.
+        if let ResolvedType::Reference(TypeReference {
+            is_mutable: true,
+            lifetime,
+            inner,
+        }) = &type_
+        {
+            dependency_bindings.insert(
+                ResolvedType::Reference(TypeReference {
+                    is_mutable: false,
+                    lifetime: lifetime.to_owned(),
+                    inner: inner.to_owned(),
+                }),
+                tokens.clone(),
+            );
+        }
+
         dependency_bindings.insert(type_, tokens);
+
         if to_be_removed {
             // It won't be needed in the future
             blocks.remove(&dependency_index);

libs/ui_tests/Cargo.toml

@@ -148,6 +148,8 @@ members = [
   "borrow_checker/diamond/diamond_can_be_solved_if_we_can_clone/generated_app",
   "borrow_checker/diamond/diamond_cannot_be_solved_if_we_cannot_clone",
   "borrow_checker/diamond/diamond_cannot_be_solved_if_we_cannot_clone/generated_app",
+  "borrow_checker/mixed_mutability",
+  "borrow_checker/mixed_mutability/generated_app",
   "borrow_checker/multiple_consumers/a_clonable_framework_type_can_be_moved_twice",
   "borrow_checker/multiple_consumers/a_clonable_framework_type_can_be_moved_twice/generated_app",
   "borrow_checker/multiple_consumers/a_clonable_input_type_can_be_moved_twice",

libs/ui_tests/borrow_checker/mixed_mutability/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "app_7c2a5f14"
+version = "0.1.0"
+edition = "2021"
+
+[lints.rust.unexpected_cfgs]
+level = "allow"
+check-cfg = ["cfg(pavex_ide_hint)"]
+
+[dependencies.pavex]
+workspace = true
+
+[dependencies.pavex_cli_client]
+workspace = true
+
+[dependencies.workspace_hack]
+workspace = true

libs/ui_tests/borrow_checker/mixed_mutability/diagnostics.dot

@@ -0,0 +1,75 @@
+digraph "GET / - 0" {
+    0 [ label = "2| pavex::middleware::wrap_noop(pavex::middleware::Next<crate::route_0::Next0>) -> pavex::response::Response"]
+    1 [ label = "1| pavex::middleware::Next::new(crate::route_0::Next0) -> pavex::middleware::Next<crate::route_0::Next0>"]
+    2 [ label = "0| crate::route_0::Next0() -> crate::route_0::Next0"]
+    3 [ label = "3| <pavex::response::Response as pavex::response::IntoResponse>::into_response(pavex::response::Response) -> pavex::response::Response"]
+    1 -> 0 [ ]
+    2 -> 1 [ ]
+    0 -> 3 [ ]
+}
+
+digraph "GET / - 1" {
+    0 [ label = "4| app_7c2a5f14::wrapper(pavex::middleware::Next<crate::route_0::Next1<'a>>, app_7c2a5f14::C) -> pavex::response::Response"]
+    1 [ label = "3| pavex::middleware::Next::new(crate::route_0::Next1<'a>) -> pavex::middleware::Next<crate::route_0::Next1<'a>>"]
+    2 [ label = "1| app_7c2a5f14::c(&app_7c2a5f14::A) -> app_7c2a5f14::C"]
+    3 [ label = "0| app_7c2a5f14::a() -> app_7c2a5f14::A"]
+    4 [ label = "2| crate::route_0::Next1(&'a mut app_7c2a5f14::A) -> crate::route_0::Next1<'a>"]
+    5 [ label = "5| <pavex::response::Response as pavex::response::IntoResponse>::into_response(pavex::response::Response) -> pavex::response::Response"]
+    2 -> 0 [ ]
+    3 -> 2 [ label = "&"]
+    1 -> 0 [ ]
+    4 -> 1 [ ]
+    3 -> 4 [ label = "&mut "]
+    0 -> 5 [ ]
+}
+
+digraph "GET / - 2" {
+    0 [ label = "2| app_7c2a5f14::handler(app_7c2a5f14::B, &mut app_7c2a5f14::A) -> pavex::response::Response"]
+    1 [ label = "1| app_7c2a5f14::b(&app_7c2a5f14::A) -> app_7c2a5f14::B"]
+    3 [ label = "3| <pavex::response::Response as pavex::response::IntoResponse>::into_response(pavex::response::Response) -> pavex::response::Response"]
+    4 [ label = "0| &mut app_7c2a5f14::A"]
+    1 -> 0 [ ]
+    0 -> 3 [ ]
+    4 -> 1 [ ]
+    4 -> 0 [ ]
+}
+
+digraph "* * - 0" {
+    0 [ label = "3| pavex::middleware::wrap_noop(pavex::middleware::Next<crate::route_1::Next0<'a>>) -> pavex::response::Response"]
+    1 [ label = "2| pavex::middleware::Next::new(crate::route_1::Next0<'a>) -> pavex::middleware::Next<crate::route_1::Next0<'a>>"]
+    2 [ label = "1| crate::route_1::Next0(&'a pavex::router::AllowedMethods) -> crate::route_1::Next0<'a>"]
+    4 [ label = "4| <pavex::response::Response as pavex::response::IntoResponse>::into_response(pavex::response::Response) -> pavex::response::Response"]
+    5 [ label = "0| &pavex::router::AllowedMethods"]
+    1 -> 0 [ ]
+    2 -> 1 [ ]
+    0 -> 4 [ ]
+    5 -> 2 [ ]
+}
+
+digraph "* * - 1" {
+    0 [ label = "5| app_7c2a5f14::wrapper(pavex::middleware::Next<crate::route_1::Next1<'a>>, app_7c2a5f14::C) -> pavex::response::Response"]
+    1 [ label = "4| pavex::middleware::Next::new(crate::route_1::Next1<'a>) -> pavex::middleware::Next<crate::route_1::Next1<'a>>"]
+    2 [ label = "1| app_7c2a5f14::c(&app_7c2a5f14::A) -> app_7c2a5f14::C"]
+    3 [ label = "0| app_7c2a5f14::a() -> app_7c2a5f14::A"]
+    4 [ label = "3| crate::route_1::Next1(&'a pavex::router::AllowedMethods) -> crate::route_1::Next1<'a>"]
+    6 [ label = "6| <pavex::response::Response as pavex::response::IntoResponse>::into_response(pavex::response::Response) -> pavex::response::Response"]
+    7 [ label = "2| &pavex::router::AllowedMethods"]
+    2 -> 0 [ ]
+    3 -> 2 [ label = "&"]
+    1 -> 0 [ ]
+    4 -> 1 [ ]
+    0 -> 6 [ ]
+    7 -> 4 [ ]
+}
+
+digraph "* * - 2" {
+    0 [ label = "1| pavex::router::default_fallback(&pavex::router::AllowedMethods) -> pavex::response::Response"]
+    2 [ label = "2| <pavex::response::Response as pavex::response::IntoResponse>::into_response(pavex::response::Response) -> pavex::response::Response"]
+    3 [ label = "0| &pavex::router::AllowedMethods"]
+    0 -> 2 [ ]
+    3 -> 0 [ ]
+}
+
+digraph app_state {
+    0 [ label = "0| crate::ApplicationState() -> crate::ApplicationState"]
+}