Rollup merge of rust-lang#114834 - compiler-errors:try_coerce-side-effects, r=lcnr

Avoid side-effects from `try_coerce` when suggesting borrowing LHS of cast

The name `try_coerce` is a bit misleading -- it has side-effects, so when it's used in diagnostics code, it sometimes causes spurious obligations to be registered which cause other errors to occur that really make no sense in context.

Addendum: let's just rename `try_coerce` to `coerce` -- the `try_` part doesn't really add much, imo.

Author: matthiaskrgr

compiler/rustc_hir_typeck/src/cast.rs

@@ -389,65 +389,49 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 if let ty::Ref(reg, cast_ty, mutbl) = *self.cast_ty.kind() {
                     if let ty::RawPtr(TypeAndMut { ty: expr_ty, .. }) = *self.expr_ty.kind()
                         && fcx
-                            .try_coerce(
-                                self.expr,
+                            .can_coerce(
                                 Ty::new_ref(fcx.tcx,
                                     fcx.tcx.lifetimes.re_erased,
                                     TypeAndMut { ty: expr_ty, mutbl },
                                 ),
                                 self.cast_ty,
-                                AllowTwoPhase::No,
-                                None,
                             )
-                            .is_ok()
                     {
                         sugg = Some((format!("&{}*", mutbl.prefix_str()), cast_ty == expr_ty));
                     } else if let ty::Ref(expr_reg, expr_ty, expr_mutbl) = *self.expr_ty.kind()
                         && expr_mutbl == Mutability::Not
                         && mutbl == Mutability::Mut
                         && fcx
-                            .try_coerce(
-                                self.expr,
+                            .can_coerce(
                                 Ty::new_ref(fcx.tcx,
                                     expr_reg,
                                     TypeAndMut { ty: expr_ty, mutbl: Mutability::Mut },
                                 ),
                                 self.cast_ty,
-                                AllowTwoPhase::No,
-                                None,
                             )
-                            .is_ok()
                     {
                         sugg_mutref = true;
                     }
 
                     if !sugg_mutref
                         && sugg == None
                         && fcx
-                            .try_coerce(
-                                self.expr,
+                            .can_coerce(
                                 Ty::new_ref(fcx.tcx,reg, TypeAndMut { ty: self.expr_ty, mutbl }),
                                 self.cast_ty,
-                                AllowTwoPhase::No,
-                                None,
                             )
-                            .is_ok()
                     {
                         sugg = Some((format!("&{}", mutbl.prefix_str()), false));
                     }
                 } else if let ty::RawPtr(TypeAndMut { mutbl, .. }) = *self.cast_ty.kind()
                     && fcx
-                        .try_coerce(
-                            self.expr,
+                        .can_coerce(
                             Ty::new_ref(fcx.tcx,
                                 fcx.tcx.lifetimes.re_erased,
                                 TypeAndMut { ty: self.expr_ty, mutbl },
                             ),
                             self.cast_ty,
-                            AllowTwoPhase::No,
-                            None,
                         )
-                        .is_ok()
                 {
                     sugg = Some((format!("&{}", mutbl.prefix_str()), false));
                 }
@@ -760,7 +744,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     ty::FnDef(..) => {
                         // Attempt a coercion to a fn pointer type.
                         let f = fcx.normalize(self.expr_span, self.expr_ty.fn_sig(fcx.tcx));
-                        let res = fcx.try_coerce(
+                        let res = fcx.coerce(
                             self.expr,
                             self.expr_ty,
                             Ty::new_fn_ptr(fcx.tcx, f),
@@ -860,7 +844,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
 
             (_, DynStar) => {
                 if fcx.tcx.features().dyn_star {
-                    bug!("should be handled by `try_coerce`")
+                    bug!("should be handled by `coerce`")
                 } else {
                     Err(CastError::IllegalCast)
                 }
@@ -956,7 +940,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
 
                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
                 let array_ptr_type = Ty::new_ptr(fcx.tcx, m_expr);
-                fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No, None)
+                fcx.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 {:?})",
@@ -992,7 +976,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
     }
 
     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'tcx>> {
-        match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No, None) {
+        match fcx.coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No, None) {
             Ok(_) => Ok(()),
             Err(err) => Err(err),
         }

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1005,7 +1005,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// adjusted type of the expression, if successful.
     /// Adjustments are only recorded if the coercion succeeded.
     /// The expressions *must not* have any preexisting adjustments.
-    pub fn try_coerce(
+    pub fn coerce(
         &self,
         expr: &hir::Expr<'_>,
         expr_ty: Ty<'tcx>,
@@ -1036,7 +1036,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         })
     }
 
-    /// Same as `try_coerce()`, but without side-effects.
+    /// Same as `coerce()`, but without side-effects.
     ///
     /// Returns false if the coercion creates any obligations that result in
     /// errors.
@@ -1494,7 +1494,7 @@ 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(
+                fcx.coerce(
                     expression,
                     expression_ty,
                     self.expected_ty,

compiler/rustc_hir_typeck/src/demand.rs

@@ -254,7 +254,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>>) {
         let expected = self.resolve_vars_with_obligations(expected);
 
-        let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase, None) {
+        let e = match self.coerce(expr, checked_ty, expected, allow_two_phase, None) {
             Ok(ty) => return (ty, None),
             Err(e) => e,
         };
@@ -475,7 +475,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 {
                     let Some(arg_ty) = self.node_ty_opt(arg_expr.hir_id) else { continue; };
                     let arg_ty = arg_ty.fold_with(&mut fudger);
-                    let _ = self.try_coerce(
+                    let _ = self.coerce(
                         arg_expr,
                         arg_ty,
                         *expected_arg_ty,

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -260,9 +260,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // fulfillment error to be more accurate.
             let coerced_ty = self.resolve_vars_with_obligations(coerced_ty);
 
-            let coerce_error = self
-                .try_coerce(provided_arg, checked_ty, coerced_ty, AllowTwoPhase::Yes, None)
-                .err();
+            let coerce_error =
+                self.coerce(provided_arg, checked_ty, coerced_ty, AllowTwoPhase::Yes, None).err();
 
             if coerce_error.is_some() {
                 return Compatibility::Incompatible(coerce_error);

tests/ui/traits/trait-upcasting/type-checking-test-1.current.stderr

@@ -2,22 +2,8 @@ error[E0605]: non-primitive cast: `&dyn Foo` as `&dyn Bar<_>`
   --> $DIR/type-checking-test-1.rs:19:13
    |
 LL |     let _ = x as &dyn Bar<_>; // Ambiguous
-   |             ^^^^^^^^^^^^^^^^ invalid cast
-   |
-help: consider borrowing the value
-   |
-LL |     let _ = &x as &dyn Bar<_>; // Ambiguous
-   |             +
-
-error[E0277]: the trait bound `&dyn Foo: Bar<_>` is not satisfied
-  --> $DIR/type-checking-test-1.rs:19:13
-   |
-LL |     let _ = x as &dyn Bar<_>; // Ambiguous
-   |             ^ the trait `Bar<_>` is not implemented for `&dyn Foo`
-   |
-   = note: required for the cast from `&&dyn Foo` to `&dyn Bar<_>`
+   |             ^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0277, E0605.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0605`.

tests/ui/traits/trait-upcasting/type-checking-test-1.rs

@@ -18,7 +18,6 @@ fn test_specific(x: &dyn Foo) {
 fn test_unknown_version(x: &dyn Foo) {
     let _ = x as &dyn Bar<_>; // Ambiguous
                               //~^ ERROR non-primitive cast
-                              //[current]~^^ ERROR the trait bound `&dyn Foo: Bar<_>` is not satisfied
 }
 
 fn test_infer_version(x: &dyn Foo) {

tests/ui/traits/trait-upcasting/type-checking-test-2.rs

@@ -18,13 +18,11 @@ fn test_specific2(x: &dyn Foo<u32>) {
 fn test_specific3(x: &dyn Foo<i32>) {
     let _ = x as &dyn Bar<u32>; // Error
                                 //~^ ERROR non-primitive cast
-                                //~^^ ERROR the trait bound `&dyn Foo<i32>: Bar<u32>` is not satisfied
 }
 
 fn test_infer_arg(x: &dyn Foo<u32>) {
     let a = x as &dyn Bar<_>; // Ambiguous
                               //~^ ERROR non-primitive cast
-                              //~^^ ERROR the trait bound `&dyn Foo<u32>: Bar<_>` is not satisfied
     let _ = a.bar();
 }
 

tests/ui/traits/trait-upcasting/type-checking-test-2.stderr

@@ -2,41 +2,14 @@ error[E0605]: non-primitive cast: `&dyn Foo<i32>` as `&dyn Bar<u32>`
   --> $DIR/type-checking-test-2.rs:19:13
    |
 LL |     let _ = x as &dyn Bar<u32>; // Error
-   |             ^^^^^^^^^^^^^^^^^^ invalid cast
-   |
-help: consider borrowing the value
-   |
-LL |     let _ = &x as &dyn Bar<u32>; // Error
-   |             +
-
-error[E0277]: the trait bound `&dyn Foo<i32>: Bar<u32>` is not satisfied
-  --> $DIR/type-checking-test-2.rs:19:13
-   |
-LL |     let _ = x as &dyn Bar<u32>; // Error
-   |             ^ the trait `Bar<u32>` is not implemented for `&dyn Foo<i32>`
-   |
-   = note: required for the cast from `&&dyn Foo<i32>` to `&dyn Bar<u32>`
+   |             ^^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
 
 error[E0605]: non-primitive cast: `&dyn Foo<u32>` as `&dyn Bar<_>`
-  --> $DIR/type-checking-test-2.rs:25:13
+  --> $DIR/type-checking-test-2.rs:24:13
    |
 LL |     let a = x as &dyn Bar<_>; // Ambiguous
-   |             ^^^^^^^^^^^^^^^^ invalid cast
-   |
-help: consider borrowing the value
-   |
-LL |     let a = &x as &dyn Bar<_>; // Ambiguous
-   |             +
-
-error[E0277]: the trait bound `&dyn Foo<u32>: Bar<_>` is not satisfied
-  --> $DIR/type-checking-test-2.rs:25:13
-   |
-LL |     let a = x as &dyn Bar<_>; // Ambiguous
-   |             ^ the trait `Bar<_>` is not implemented for `&dyn Foo<u32>`
-   |
-   = note: required for the cast from `&&dyn Foo<u32>` to `&dyn Bar<_>`
+   |             ^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
 
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0277, E0605.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0605`.