core: rustc: improve compile message for unsized types

[liigo]

Currently, the compiler only tell us there is unsized type, but it's not clear how to fix the error, especially for a newbie.

t.rs:5:17: 5:18 error: the trait `core::marker::Sized` is not implemented for the type `MyTrait` [E0277]
t.rs:5 fn do_something(t: MyTrait) {
                       ^
t.rs:5:17: 5:18 note: `MyTrait` does not have a constant size known at compile-time
t.rs:5 fn do_something(t: MyTrait) {
                       ^
error: aborting due to previous error

[rust-highfive]

r? @aturon

(rust_highfive has picked a reviewer for you, use r? to override)

[aturon]

Hi @liigo, sorry this fell through my inbox!

This looks like a good change, but I'm not sure about the last clause about generic types. I suspect in most cases this comes up you've got roughly the right type in mind, but need to put it behind a pointer -- so maybe just stick with the first two suggestions?

Otherwise LGTM.

[liigo]

Sure! I'm sorry for replying this too late. I'm on holiday and will be at
work in about one or two days.
On Aug 4, 2015 22:50, "Aaron Turon" notifications@github.com wrote:

Hi @liigo https://github.com/liigo, sorry this fell through my inbox!

This looks like a good change, but I'm not sure about the last clause
about generic types. I suspect in most cases this comes up you've got
roughly the right type in mind, but need to put it behind a pointer -- so
maybe just stick with the first two suggestions?

Otherwise LGTM.

—
Reply to this email directly or view it on GitHub
#26617 (comment).

[liigo]

@aturon r? I've addressed your comment, and rebased.

[arielb1]

This will make #23286 even worse, and even if we print the cause:

test.rs:3:6: 3:7 error: the trait `core::marker::Sized` is not implemented for the type `[u8]` [E0277]
test.rs:3 fn f(p: Path) {}
               ^
test.rs:3:6: 3:7 help: run `rustc --explain E0277` to see a detailed explanation
test.rs:3:6: 3:7 note: `[u8]` does not have a constant size known at compile-time. Try to use `&[u8]` or `Box<[u8]>`?
test.rs:3 fn f(p: Path) {}
               ^
test.rs:3:6: 3:7 note: required because it appears within the type `std::sys::os_str::Slice`
test.rs:3 fn f(p: Path) {}
               ^
test.rs:3:6: 3:7 note: required because it appears within the type `std::ffi::os_str::OsStr`
test.rs:3 fn f(p: Path) {}
               ^
test.rs:3:6: 3:7 note: required because it appears within the type `std::path::Path`
test.rs:3 fn f(p: Path) {}
               ^
test.rs:3:6: 3:7 note: all local variables must have a statically known size
test.rs:3 fn f(p: Path) {}
               ^

Maybe have #[rustc_on_unimplemented_help="maybe put it behind a pointer?"] (bikeshed) that appears after the note?

[liigo]

Ok, I'm closing this.