rustc silently ignores invalid -C target-feature names

Compiling a simple Hello, World program:

$ rustc -V
rustc 1.20.0 (f3d6973f4 2017-08-27)

# Note the trailing "zzzz"
$ RUSTFLAGS=-Ctarget-feature=+crt-staticzzzz cargo build -v
   Compiling rust-test v0.1.0 (rust-test)
     Running `rustc --crate-name rust_test src/main.rs --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=aafe1708c049ad76 -C extra-filename=-aafe1708c049ad76 --out-dir rust-test/target/debug/deps -L dependency=rust-test/target/debug/deps '-Ctarget-feature=+crt-staticzzzz'`
    Finished dev [unoptimized + debuginfo] target(s) in 0.25 secs

Should RUSTFLAGS=-Ctarget-feature=+crt-staticzzzz be silently accepted?

If I pass RUSTFLAGS=-Ctarget-feature=+batman, a warning is printed, as expected.

'+batman' is not a recognized feature for this target (ignoring feature)

Why is trailing misspelling silently allowed?

This is happening inside LLVM and plausibly should be filed with that project.

crt-static is not passed to LLVM (and shouldn't be). It seems as if rustc consumes not just crt-static but also other target features that have it as a prefix? This filtering looks suspicious:

rust/src/librustc_trans/back/write.rs

Lines 119 to 121 in ded38db

    
           let llvm_features = requested_features.filter(|f| { 
        
               !rustc_features.iter().any(|s| f.contains(s)) 
        
           });

Also rustc:

doesn't canonicalize the feature list like clang does - there may be duplicates, same features with both + and -. I tried to find places in LLVM where this could break things, but found a couple at best. Memory sanitizer perhaps? https://github.com/rust-lang/llvm-project/blob/rustc/9.0-2019-12-19/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp#L3738. The main thing (subtarget creation) is done correctly.
There's at least one such place in rustc ("+atomics" check).
accepts features without + or -. I have no idea why the assert in MCSubtargetInfo.cpp/ApplyFeatureFlag doesn't fire on this.

	let llvm_features = requested_features.filter(\|f\| {
	!rustc_features.iter().any(\|s\| f.contains(s))
	});