Rvalues are incorrectly expected to have unsized types.

[eddyb]

This appears to be a long-standing issue with blocks, recently exposed in match by #19769.
Original error by @steveklabnik (source code):

<std macros>:3:22: 3:40 error: mismatched types: expected `error::Error`, found `collections::string::String` (expected trait error::Error, found struct collections::string::String)
<std macros>:3         format_args!(::std::fmt::format, $($arg)*)
                                    ^~~~~~~~~~~~~~~~~~
note: in expansion of format_args!
<std macros>:3:9: 3:51 note: expansion site
<std macros>:1:1: 5:2 note: in expansion of format!
src/build.rs:113:36: 116:34 note: expansion site
src/build.rs:113:32: 116:45 error: the trait `core::kinds::Sized` is not implemented for the type `error::Error`
src/build.rs:113                     return Err(box format!("{}\n{}",
src/build.rs:114                                            String::from_utf8_lossy(output.output[]),
src/build.rs:115                                            String::from_utf8_lossy(output.error[]))
src/build.rs:116                                as Box<Error>);
src/build.rs:113:32: 116:45 note: only sized types can be made into objects
src/build.rs:113                     return Err(box format!("{}\n{}",
src/build.rs:114                                            String::from_utf8_lossy(output.output[]),
src/build.rs:115                                            String::from_utf8_lossy(output.error[]))
src/build.rs:116                                as Box<Error>);
src/build.rs:113:32: 116:45 error: the trait `error::Error` is not implemented for the type `error::Error`
src/build.rs:113                     return Err(box format!("{}\n{}",
src/build.rs:114                                            String::from_utf8_lossy(output.output[]),
src/build.rs:115                                            String::from_utf8_lossy(output.error[]))
src/build.rs:116                                as Box<Error>);
src/build.rs:113:32: 116:45 note: required for the cast to the object type `error::Error`
src/build.rs:113                     return Err(box format!("{}\n{}",
src/build.rs:114                                            String::from_utf8_lossy(output.output[]),
src/build.rs:115                                            String::from_utf8_lossy(output.error[]))
src/build.rs:116                                as Box<Error>);
<std macros>:3:22: 3:40 error: mismatched types: expected `error::Error`, found `collections::string::String` (expected trait error::Error, found struct collections::string::String)
<std macros>:3         format_args!(::std::fmt::format, $($arg)*)
                                    ^~~~~~~~~~~~~~~~~~
note: in expansion of format_args!
<std macros>:3:9: 3:51 note: expansion site
<std macros>:1:1: 5:2 note: in expansion of format!
src/build.rs:120:32: 120:80 note: expansion site
src/build.rs:120:28: 120:91 error: the trait `core::kinds::Sized` is not implemented for the type `error::Error`
src/build.rs:120                 return Err(box format!("Could not execute `rustdoc`: {}", e) as Box<Error>);
                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/build.rs:120:28: 120:91 note: only sized types can be made into objects
src/build.rs:120                 return Err(box format!("Could not execute `rustdoc`: {}", e) as Box<Error>);
                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/build.rs:120:28: 120:91 error: the trait `error::Error` is not implemented for the type `error::Error`
src/build.rs:120                 return Err(box format!("Could not execute `rustdoc`: {}", e) as Box<Error>);
                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/build.rs:120:28: 120:91 note: required for the cast to the object type `error::Error`
src/build.rs:120                 return Err(box format!("Could not execute `rustdoc`: {}", e) as Box<Error>);
                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

My own format_args experiments (#20136) were hit by this:

struct Cell<T>(T);
impl <T: Copy + std::fmt::Show> std::fmt::Show for Cell<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        let dst = &mut *f;
        (|args| dst.write_fmt(args))(&::std::fmt::Arguments::new({
            #[inline]
            #[allow(dead_code)]
            static __STATIC_FMTSTR: &'static [&'static str] = &["Cell { value: ", " }"];
            __STATIC_FMTSTR
        }, &match (&self.0,) {
            (__arg0,) => [::std::fmt::argument(::std::fmt::Show::fmt, __arg0)]
        }))
    }
}

fmt-test0.rs:11:26: 11:79 error: mismatched types: expected `[core::fmt::Argument<'_>]`, found `[core::fmt::Argument<'_>, ..1]` (expected slice, found array of 1 elements)
fmt-test0.rs:11             (__arg0,) => [::std::fmt::argument(::std::fmt::Show::fmt, __arg0)]
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~