rust-lang · bors · Jun 19, 2020 · Jun 15, 2020
diff --git a/src/libcore/convert/mod.rs b/src/libcore/convert/mod.rs
@@ -374,6 +374,7 @@ pub trait Into<T>: Sized {
 /// [`Into`]: trait.Into.html
 /// [`from`]: trait.From.html#tymethod.from
 /// [book]: ../../book/ch09-00-error-handling.html
+#[rustc_diagnostic_item = "from_trait"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_on_unimplemented(on(
     all(_Self = "&str", T = "std::string::String"),

diff --git a/src/librustc_parse/parser/diagnostics.rs b/src/librustc_parse/parser/diagnostics.rs
@@ -961,7 +961,7 @@ impl<'a> Parser<'a> {
             self.bump();
             let sp = self.prev_token.span;
             self.struct_span_err(sp, &msg)
-                .span_suggestion(sp, "change this to `;`", ";".to_string(), appl)
+                .span_suggestion_short(sp, "change this to `;`", ";".to_string(), appl)
                 .emit();
             return Ok(());
         } else if self.look_ahead(0, |t| {

diff --git a/src/librustc_span/symbol.rs b/src/librustc_span/symbol.rs
@@ -343,6 +343,7 @@ symbols! {
         from_method,
         from_ok,
         from_usize,
+        from_trait,
         fundamental,
         future,
         Future,

diff --git a/src/librustc_typeck/check/cast.rs b/src/librustc_typeck/check/cast.rs
@@ -43,6 +43,7 @@ use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, Ty, TypeAndMut, TypeFoldable};
 use rustc_session::lint;
 use rustc_session::Session;
+use rustc_span::symbol::sym;
 use rustc_span::Span;
 use rustc_trait_selection::traits;
 use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
@@ -333,23 +334,87 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     "only `u8` can be cast as `char`, not `{}`",
                     self.expr_ty
                 )
+                .span_label(self.span, "invalid cast")
                 .emit();
             }
             CastError::NonScalar => {
-                type_error_struct!(
+                let mut err = type_error_struct!(
                     fcx.tcx.sess,
                     self.span,
                     self.expr_ty,
                     E0605,
                     "non-primitive cast: `{}` as `{}`",
                     self.expr_ty,
                     fcx.ty_to_string(self.cast_ty)
-                )
-                .note(
-                    "an `as` expression can only be used to convert between \
-                                         primitive types. Consider using the `From` trait",
-                )
-                .emit();
+                );
+                let mut sugg = None;
+                if let ty::Ref(reg, _, mutbl) = self.cast_ty.kind {
+                    if fcx
+                        .try_coerce(
+                            self.expr,
+                            fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
+                            self.cast_ty,
+                            AllowTwoPhase::No,
+                        )
+                        .is_ok()
+                    {
+                        sugg = Some(format!("&{}", mutbl.prefix_str()));
+                    }
+                }
+                if let Some(sugg) = sugg {
+                    err.span_label(self.span, "invalid cast");
+                    err.span_suggestion_verbose(
+                        self.expr.span.shrink_to_lo(),
+                        "borrow the value for the cast to be valid",
+                        sugg,
+                        Applicability::MachineApplicable,
+                    );
+                } else if !matches!(
+                    self.cast_ty.kind,
+                    ty::FnDef(..) | ty::FnPtr(..) | ty::Closure(..)
+                ) {
+                    let mut label = true;
+                    // Check `impl From<self.expr_ty> for self.cast_ty {}` for accurate suggestion:
+                    if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
+                        if let Some(from_trait) = fcx.tcx.get_diagnostic_item(sym::from_trait) {
+                            let ty = fcx.resolve_vars_if_possible(&self.cast_ty);
+                            // Erase regions to avoid panic in `prove_value` when calling
+                            // `type_implements_trait`.
+                            let ty = fcx.tcx.erase_regions(&ty);
+                            let expr_ty = fcx.resolve_vars_if_possible(&self.expr_ty);
+                            let expr_ty = fcx.tcx.erase_regions(&expr_ty);
+                            let ty_params = fcx.tcx.mk_substs_trait(expr_ty, &[]);
+                            // Check for infer types because cases like `Option<{integer}>` would
+                            // panic otherwise.
+                            if !expr_ty.has_infer_types()
+                                && fcx.tcx.type_implements_trait((
+                                    from_trait,
+                                    ty,
+                                    ty_params,
+                                    fcx.param_env,
+                                ))
+                            {
+                                label = false;
+                                err.span_suggestion(
+                                    self.span,
+                                    "consider using the `From` trait instead",
+                                    format!("{}::from({})", self.cast_ty, snippet),
+                                    Applicability::MaybeIncorrect,
+                                );
+                            }
+                        }
+                    }
+                    let msg = "an `as` expression can only be used to convert between primitive \
+                               types or to coerce to a specific trait object";
+                    if label {
+                        err.span_label(self.span, msg);
+                    } else {
+                        err.note(msg);
+                    }
+                } else {
+                    err.span_label(self.span, "invalid cast");
+                }
+                err.emit();
             }
             CastError::SizedUnsizedCast => {
                 use crate::structured_errors::{SizedUnsizedCastError, StructuredDiagnostic};
@@ -370,21 +435,22 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 };
                 let mut err = struct_span_err!(
                     fcx.tcx.sess,
-                    self.span,
+                    if unknown_cast_to { self.cast_span } else { self.span },
                     E0641,
                     "cannot cast {} a pointer of an unknown kind",
                     if unknown_cast_to { "to" } else { "from" }
                 );
-                err.note(
-                    "the type information given here is insufficient to check whether \
-                          the pointer cast is valid",
-                );
                 if unknown_cast_to {
-                    err.span_suggestion_short(
-                        self.cast_span,
-                        "consider giving more type information",
-                        String::new(),
-                        Applicability::Unspecified,
+                    err.span_label(self.cast_span, "needs more type information");
+                    err.note(
+                        "the type information given here is insufficient to check whether \
+                        the pointer cast is valid",
+                    );
+                } else {
+                    err.span_label(
+                        self.span,
+                        "the type information given here is insufficient to check whether \
+                        the pointer cast is valid",
                     );
                 }
                 err.emit();
@@ -438,13 +504,16 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     Ok(s) => {
                         err.span_suggestion(
                             self.cast_span,
-                            "try casting to a `Box` instead",
+                            "you can cast to a `Box` instead",
                             format!("Box<{}>", s),
                             Applicability::MachineApplicable,
                         );
                     }
                     Err(_) => {
-                        err.span_help(self.cast_span, &format!("did you mean `Box<{}>`?", tstr));
+                        err.span_help(
+                            self.cast_span,
+                            &format!("you might have meant `Box<{}>`", tstr),
+                        );
                     }
                 }
             }

diff --git a/src/test/ui/cast/cast-from-nil.stderr b/src/test/ui/cast/cast-from-nil.stderr
@@ -2,9 +2,7 @@ error[E0605]: non-primitive cast: `()` as `u32`
   --> $DIR/cast-from-nil.rs:2:21
    |
 LL | fn main() { let u = (assert!(true) as u32); }
-   |                     ^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |                     ^^^^^^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
 
 error: aborting due to previous error
 

diff --git a/src/test/ui/cast/cast-to-bare-fn.stderr b/src/test/ui/cast/cast-to-bare-fn.stderr
@@ -2,17 +2,13 @@ error[E0605]: non-primitive cast: `fn(isize) {foo}` as `extern "C" fn() -> isize
   --> $DIR/cast-to-bare-fn.rs:5:13
    |
 LL |     let x = foo as extern "C" fn() -> isize;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
 
 error[E0605]: non-primitive cast: `u64` as `fn(isize) -> (isize, isize)`
   --> $DIR/cast-to-bare-fn.rs:7:13
    |
 LL |     let y = v as extern "Rust" fn(isize) -> (isize, isize);
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
 
 error: aborting due to 2 previous errors
 

diff --git a/src/test/ui/cast/cast-to-nil.stderr b/src/test/ui/cast/cast-to-nil.stderr
@@ -2,9 +2,7 @@ error[E0605]: non-primitive cast: `u32` as `()`
   --> $DIR/cast-to-nil.rs:2:21
    |
 LL | fn main() { let u = 0u32 as (); }
-   |                     ^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |                     ^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
 
 error: aborting due to previous error
 

diff --git a/src/test/ui/cast/cast-to-unsized-trait-object-suggestion.stderr b/src/test/ui/cast/cast-to-unsized-trait-object-suggestion.stderr
@@ -12,7 +12,7 @@ error[E0620]: cast to unsized type: `std::boxed::Box<{integer}>` as `dyn std::ma
 LL |     Box::new(1) as dyn Send;
    |     ^^^^^^^^^^^^^^^--------
    |                    |
-   |                    help: try casting to a `Box` instead: `Box<dyn Send>`
+   |                    help: you can cast to a `Box` instead: `Box<dyn Send>`
 
 error: aborting due to 2 previous errors
 

diff --git a/src/test/ui/closures/closure-no-fn-3.stderr b/src/test/ui/closures/closure-no-fn-3.stderr
@@ -2,9 +2,7 @@ error[E0605]: non-primitive cast: `[closure@$DIR/closure-no-fn-3.rs:6:27: 6:37 b
   --> $DIR/closure-no-fn-3.rs:6:27
    |
 LL |     let baz: fn() -> u8 = (|| { b }) as fn() -> u8;
-   |                           ^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
 
 error: aborting due to previous error
 

diff --git a/src/test/ui/coercion/coerce-to-bang-cast.stderr b/src/test/ui/coercion/coerce-to-bang-cast.stderr
@@ -2,17 +2,13 @@ error[E0605]: non-primitive cast: `i32` as `!`
   --> $DIR/coerce-to-bang-cast.rs:6:13
    |
 LL |     let y = {return; 22} as !;
-   |             ^^^^^^^^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |             ^^^^^^^^^^^^^^^^^ 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: `i32` as `!`
   --> $DIR/coerce-to-bang-cast.rs:11:13
    |
 LL |     let y = 22 as !;
-   |             ^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |             ^^^^^^^ 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
 

diff --git a/src/test/ui/consts/const-eval/const-eval-overflow-4b.stderr b/src/test/ui/consts/const-eval/const-eval-overflow-4b.stderr
@@ -16,7 +16,7 @@ error[E0604]: only `u8` can be cast as `char`, not `i8`
   --> $DIR/const-eval-overflow-4b.rs:25:13
    |
 LL |     : [u32; 5i8 as char as usize]
-   |             ^^^^^^^^^^^
+   |             ^^^^^^^^^^^ invalid cast
 
 error: aborting due to 3 previous errors
 

diff --git a/src/test/ui/error-codes/E0604.stderr b/src/test/ui/error-codes/E0604.stderr
@@ -2,7 +2,7 @@ error[E0604]: only `u8` can be cast as `char`, not `u32`
   --> $DIR/E0604.rs:2:5
    |
 LL |     1u32 as char;
-   |     ^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^ invalid cast
 
 error: aborting due to previous error
 

diff --git a/src/test/ui/error-codes/E0605.stderr b/src/test/ui/error-codes/E0605.stderr
@@ -2,17 +2,13 @@ error[E0605]: non-primitive cast: `u8` as `std::vec::Vec<u8>`
   --> $DIR/E0605.rs:3:5
    |
 LL |     x as Vec<u8>;
-   |     ^^^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |     ^^^^^^^^^^^^ 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: `*const u8` as `&u8`
   --> $DIR/E0605.rs:6:5
    |
 LL |     v as &u8;
-   |     ^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |     ^^^^^^^^ 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
 

diff --git a/src/test/ui/error-festival.stderr b/src/test/ui/error-festival.stderr
@@ -42,15 +42,13 @@ error[E0604]: only `u8` can be cast as `char`, not `u32`
   --> $DIR/error-festival.rs:25:5
    |
 LL |     0u32 as char;
-   |     ^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^ invalid cast
 
 error[E0605]: non-primitive cast: `u8` as `std::vec::Vec<u8>`
   --> $DIR/error-festival.rs:29:5
    |
 LL |     x as Vec<u8>;
-   |     ^^^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |     ^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
 
 error[E0054]: cannot cast as `bool`
   --> $DIR/error-festival.rs:33:24

diff --git a/src/test/ui/fat-ptr-cast.stderr b/src/test/ui/fat-ptr-cast.stderr
@@ -34,9 +34,7 @@ error[E0605]: non-primitive cast: `std::boxed::Box<[i32]>` as `usize`
   --> $DIR/fat-ptr-cast.rs:14:5
    |
 LL |     b as usize;
-   |     ^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |     ^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
 
 error[E0606]: casting `*const [i32]` as `usize` is invalid
   --> $DIR/fat-ptr-cast.rs:15:5

diff --git a/src/test/ui/issues/issue-10991.stderr b/src/test/ui/issues/issue-10991.stderr
@@ -2,9 +2,7 @@ error[E0605]: non-primitive cast: `()` as `usize`
   --> $DIR/issue-10991.rs:3:14
    |
 LL |     let _t = nil as usize;
-   |              ^^^^^^^^^^^^
-   |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   |              ^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
 
 error: aborting due to previous error
 

diff --git a/src/test/ui/issues/issue-16048.rs b/src/test/ui/issues/issue-16048.rs
@@ -18,12 +18,12 @@ impl<'a> Test<'a> for Foo<'a> {
 }
 
 impl<'a> NoLifetime for Foo<'a> {
-    fn get<'p, T : Test<'a>>(&self) -> T {
+    fn get<'p, T: Test<'a> + From<Foo<'a>>>(&self) -> T {
     //~^ ERROR E0195
     //~| NOTE lifetimes do not match method in trait
         return *self as T;
         //~^ ERROR non-primitive cast: `Foo<'a>` as `T`
-        //~| NOTE an `as` expression can only be used to convert between primitive types.
+        //~| NOTE an `as` expression can only be used to convert between primitive types
     }
 }
 

diff --git a/src/test/ui/issues/issue-16048.stderr b/src/test/ui/issues/issue-16048.stderr
@@ -4,16 +4,16 @@ error[E0195]: lifetime parameters or bounds on method `get` do not match the tra
 LL |     fn get<'p, T : Test<'p>>(&self) -> T;
    |           ------------------ lifetimes in impl do not match this method in trait
 ...
-LL |     fn get<'p, T : Test<'a>>(&self) -> T {
-   |           ^^^^^^^^^^^^^^^^^^ lifetimes do not match method in trait
+LL |     fn get<'p, T: Test<'a> + From<Foo<'a>>>(&self) -> T {
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetimes do not match method in trait
 
 error[E0605]: non-primitive cast: `Foo<'a>` as `T`
   --> $DIR/issue-16048.rs:24:16
    |
 LL |         return *self as T;
-   |                ^^^^^^^^^^
+   |                ^^^^^^^^^^ help: consider using the `From` trait instead: `T::from(*self)`
    |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+   = note: 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
 

diff --git a/src/test/ui/issues/issue-17441.stderr b/src/test/ui/issues/issue-17441.stderr
@@ -16,7 +16,7 @@ error[E0620]: cast to unsized type: `std::boxed::Box<usize>` as `dyn std::fmt::D
 LL |     let _bar = Box::new(1_usize) as dyn std::fmt::Debug;
    |                ^^^^^^^^^^^^^^^^^^^^^-------------------
    |                                     |
-   |                                     help: try casting to a `Box` instead: `Box<dyn std::fmt::Debug>`
+   |                                     help: you can cast to a `Box` instead: `Box<dyn std::fmt::Debug>`
 
 error[E0620]: cast to unsized type: `usize` as `dyn std::fmt::Debug`
   --> $DIR/issue-17441.rs:8:16

diff --git a/src/test/ui/issues/issue-22289.stderr b/src/test/ui/issues/issue-22289.stderr
@@ -2,9 +2,12 @@ error[E0605]: non-primitive cast: `i32` as `&(dyn std::any::Any + 'static)`
   --> $DIR/issue-22289.rs:2:5
    |
 LL |     0 as &dyn std::any::Any;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
    |
-   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
+help: borrow the value for the cast to be valid
+   |
+LL |     &0 as &dyn std::any::Any;
+   |     ^
 
 error: aborting due to previous error