Standard library function `<(dyn core::any::Any + core::marker::Send + 'static)>::downcast_ref` does not codegen

[danielsn]

            // thread 'rustc' panicked at 'Can't cast
            // Expr { value: Symbol { identifier: "_ZN54_$LT$dyn$u20$core..any..Any$u2b$core..marker..Send$GT$12downcast_ref17hfd95d38a01cb23d4E::1::var_3" }, typ: StructTag("tag-dyn core::any::Any + core::marker::Send"), location: None }
            // Pointer { typ: Empty }', compiler/rustc_codegen_llvm/src/gotoc/cbmc/goto_program/expr.rs:408:9

related: #109

[avanhatt]

I can probably look into this one

[avanhatt]

Minimal test that has almost the same trace (just without the 'static):

use std::any::Any;

fn cast(s: &(dyn Any + Send)) {
    let _c = s as &dyn Any;
}

fn main() {
    cast(&"".to_string());
}

Trace:

thread 'rustc' panicked at 'Can't cast

Expr { value: Symbol { identifier: "_RNvCs21hi0yVfW1J_8any_cast4cast::1::var_4" }, typ: StructTag("tag-dyn std::any::Any + std::marker::Send"), location: None }

Pointer { typ: Empty }', compiler/rustc_codegen_llvm/src/gotoc/cbmc/goto_program/expr.rs:408:9

stack backtrace:

```rust stack backtrace: 0: rust_begin_unwind at ./library/std/src/panicking.rs:515:5 1: std::panicking::begin_panic_fmt at ./library/std/src/panicking.rs:457:5 2: rustc_codegen_llvm::gotoc::cbmc::goto_program::expr::Expr::cast_to 3: rustc_codegen_llvm::gotoc::rvalue::::cast_sized_pointer_to_fat_pointer 4: rustc_codegen_llvm::gotoc::rvalue::::cast_sized_expr_to_unsized_expr 5: rustc_codegen_llvm::gotoc::rvalue::::codegen_pointer_cast 6: rustc_codegen_llvm::gotoc::rvalue::::codegen_rvalue 7: rustc_codegen_llvm::gotoc::statement::::codegen_statement 8: rustc_codegen_llvm::gotoc::::codegen_block 9: as core::iter::traits::iterator::Iterator>::fold 10: rustc_codegen_llvm::gotoc::::codegen_function 11: ::codegen_crate 12: rustc_session::utils::::time 13: rustc_interface::passes::start_codegen 14: rustc_interface::passes::QueryContext::enter 15: rustc_interface::queries::Query::compute 16: rustc_interface::queries::Queries::ongoing_codegen 17: rustc_driver::run_compiler::{{closure}}::{{closure}} at ./compiler/rustc_driver/src/lib.rs:414:13 18: rustc_interface::queries::::enter at ./compiler/rustc_interface/src/queries.rs:435:19 19: rustc_driver::run_compiler::{{closure}} at ./compiler/rustc_driver/src/lib.rs:313:22 20: rustc_interface::interface::create_compiler_and_run::{{closure}} at ./compiler/rustc_interface/src/interface.rs:208:13 21: rustc_span::with_source_map at ./compiler/rustc_span/src/lib.rs:873:5 22: rustc_interface::interface::create_compiler_and_run at ./compiler/rustc_interface/src/interface.rs:202:5 23: rustc_interface::interface::run_compiler::{{closure}} at ./compiler/rustc_interface/src/interface.rs:224:12 24: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}}::{{closure}} at ./compiler/rustc_interface/src/util.rs:158:13 25: scoped_tls::ScopedKey::set at /Users/avanhatt/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137:9 26: rustc_span::with_session_globals at ./compiler/rustc_span/src/lib.rs:104:5 27: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}} at ./compiler/rustc_interface/src/util.rs:156:9 28: rustc_interface::util::scoped_thread::{{closure}} at ./compiler/rustc_interface/src/util.rs:131:24 note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.55.0-dev running on x86_64-apple-darwin

note: compiler flags: -Z codegen-backend=gotoc -Z symbol-mangling-version=v0

query stack during panic:
end of query stack

</details>

[avanhatt]

I think the problem is here: we may be assuming any pointer cast is from a sized pointer to either a sized or unsized pointer, but when auto traits are in play, pointer casts can be from an unsized pointer as well.

rvalue.rs

            PointerCast::Unsize => {
                let src_goto_expr = self.codegen_operand(o);
                let src_mir_type = self.operand_ty(o);
                let dst_mir_type = t;
                let dst_goto_expr = self.cast_sized_expr_to_unsized_expr( // no cast_unsized variant?
                    src_goto_expr.clone(),
                    src_mir_type,
                    dst_mir_type,
                );
                dst_goto_expr.unwrap_or(src_goto_expr)
            }

[adpaco-aws]

Related to #111