Auto merge of #89028 - Aaron1011:coercion-cause, r=nagisa

Propagate coercion cause into `try_coerce`

Currently, `coerce_inner` discards its `ObligationCause`
when calling `try_coerce`. This interfers with other
diagnostc improvements I'm working on, since we will lose
the original span by the time the actual coercion occurs.

Additionally, we now use the span of the trailing expression
(rather than the span of the entire function) when performing
a coercion in `check_return_expr`. This currently has no visible
effect on any of the unit tests, but will unblock future
diagnostic improvements.

5 files changed, 38 insertions, 9 deletions

diff --git a/compiler/rustc_typeck/src/check/cast.rs b/compiler/rustc_typeck/src/check/cast.rs
index 14550690e63..4ea7a8694c0 100644
--- a/compiler/rustc_typeck/src/check/cast.rs
+++ b/compiler/rustc_typeck/src/check/cast.rs
@@ -362,6 +362,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                                 ),
                                 self.cast_ty,
                                 AllowTwoPhase::No,
+                                None,
                             )
                             .is_ok()
                         {
@@ -379,6 +380,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                                     ),
                                     self.cast_ty,
                                     AllowTwoPhase::No,
+                                    None,
                                 )
                                 .is_ok()
                         {
@@ -394,6 +396,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                                 fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
                                 self.cast_ty,
                                 AllowTwoPhase::No,
+                                None,
                             )
                             .is_ok()
                     {
@@ -409,6 +412,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                             ),
                             self.cast_ty,
                             AllowTwoPhase::No,
+                            None,
                         )
                         .is_ok()
                     {
@@ -666,6 +670,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                             self.expr_ty,
                             fcx.tcx.mk_fn_ptr(f),
                             AllowTwoPhase::No,
+                            None,
                         );
                         if let Err(TypeError::IntrinsicCast) = res {
                             return Err(CastError::IllegalCast);
@@ -829,7 +834,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
 
                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
-                fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No)
+                fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No, None)
                     .unwrap_or_else(|_| {
                         bug!(
                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
@@ -861,7 +866,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
     }
 
     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'_>> {
-        match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No) {
+        match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No, None) {
             Ok(_) => Ok(()),
             Err(err) => Err(err),
         }
diff --git a/compiler/rustc_typeck/src/check/check.rs b/compiler/rustc_typeck/src/check/check.rs
index 1fd1253e527..54e4eb47688 100644
--- a/compiler/rustc_typeck/src/check/check.rs
+++ b/compiler/rustc_typeck/src/check/check.rs
@@ -214,7 +214,7 @@ pub(super) fn check_fn<'a, 'tcx>(
         fcx.require_type_is_sized(declared_ret_ty, decl.output.span(), traits::SizedReturnType);
     } else {
         fcx.require_type_is_sized(declared_ret_ty, decl.output.span(), traits::SizedReturnType);
-        fcx.check_return_expr(&body.value);
+        fcx.check_return_expr(&body.value, false);
     }
     fcx.in_tail_expr = false;
 
diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index 013aecae586..92d0470bc2f 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -935,11 +935,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr_ty: Ty<'tcx>,
         target: Ty<'tcx>,
         allow_two_phase: AllowTwoPhase,
+        cause: Option<ObligationCause<'tcx>>,
     ) -> RelateResult<'tcx, Ty<'tcx>> {
         let source = self.resolve_vars_with_obligations(expr_ty);
         debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
 
-        let cause = self.cause(expr.span, ObligationCauseCode::ExprAssignable);
+        let cause =
+            cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
         let coerce = Coerce::new(self, cause, allow_two_phase);
         let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
 
@@ -1363,7 +1365,13 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 // Special-case the first expression we are coercing.
                 // To be honest, I'm not entirely sure why we do this.
                 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
-                fcx.try_coerce(expression, expression_ty, self.expected_ty, AllowTwoPhase::No)
+                fcx.try_coerce(
+                    expression,
+                    expression_ty,
+                    self.expected_ty,
+                    AllowTwoPhase::No,
+                    Some(cause.clone()),
+                )
             } else {
                 match self.expressions {
                     Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
diff --git a/compiler/rustc_typeck/src/check/demand.rs b/compiler/rustc_typeck/src/check/demand.rs
index a86db2d31b3..722b110ed61 100644
--- a/compiler/rustc_typeck/src/check/demand.rs
+++ b/compiler/rustc_typeck/src/check/demand.rs
@@ -134,7 +134,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx>>) {
         let expected = self.resolve_vars_with_obligations(expected);
 
-        let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase) {
+        let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase, None) {
             Ok(ty) => return (ty, None),
             Err(e) => e,
         };
diff --git a/compiler/rustc_typeck/src/check/expr.rs b/compiler/rustc_typeck/src/check/expr.rs
index 917adf0e2b9..53c1470c601 100644
--- a/compiler/rustc_typeck/src/check/expr.rs
+++ b/compiler/rustc_typeck/src/check/expr.rs
@@ -765,7 +765,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             if self.ret_coercion_span.get().is_none() {
                 self.ret_coercion_span.set(Some(e.span));
             }
-            self.check_return_expr(e);
+            self.check_return_expr(e, true);
         } else {
             let mut coercion = self.ret_coercion.as_ref().unwrap().borrow_mut();
             if self.ret_coercion_span.get().is_none() {
@@ -794,16 +794,32 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.tcx.types.never
     }
 
-    pub(super) fn check_return_expr(&self, return_expr: &'tcx hir::Expr<'tcx>) {
+    /// `explicit_return` is `true` if we're checkng an explicit `return expr`,
+    /// and `false` if we're checking a trailing expression.
+    pub(super) fn check_return_expr(
+        &self,
+        return_expr: &'tcx hir::Expr<'tcx>,
+        explicit_return: bool,
+    ) {
         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
             span_bug!(return_expr.span, "check_return_expr called outside fn body")
         });
 
         let ret_ty = ret_coercion.borrow().expected_ty();
         let return_expr_ty = self.check_expr_with_hint(return_expr, ret_ty);
+        let mut span = return_expr.span;
+        // Use the span of the trailing expression for our cause,
+        // not the span of the entire function
+        if !explicit_return {
+            if let ExprKind::Block(body, _) = return_expr.kind {
+                if let Some(last_expr) = body.expr {
+                    span = last_expr.span;
+                }
+            }
+        }
         ret_coercion.borrow_mut().coerce(
             self,
-            &self.cause(return_expr.span, ObligationCauseCode::ReturnValue(return_expr.hir_id)),
+            &self.cause(span, ObligationCauseCode::ReturnValue(return_expr.hir_id)),
             return_expr,
             return_expr_ty,
         );