Rollup merge of #127814 - folkertdev:c-cmse-nonsecure-call-error-messages, r=oli-obk

`C-cmse-nonsecure-call`: improved error messages

tracking issue: #81391
issue for the error messages (partially implemented by this PR): #81347
related, in that it also deals with CMSE: https://github.com/rust-lang/rust/pull/127766

When using the `C-cmse-nonsecure-call` ABI, both the arguments and return value must be passed via registers. Previously, when violating this constraint, an ugly LLVM error would be shown. Now, the rust compiler itself will print a pretty message and link to more information.

compiler/rustc_error_codes/src/error_codes/E0798.md

@@ -0,0 +1,39 @@
+Functions marked as `C-cmse-nonsecure-call` place restrictions on their
+inputs and outputs.
+
+- inputs must fit in the 4 available 32-bit argument registers. Alignment
+is relevant.
+- outputs must either fit in 4 bytes, or be a foundational type of
+size 8 (`i64`, `u64`, `f64`).
+- no generics can be used in the signature
+
+For more information,
+see [arm's aapcs32](https://github.com/ARM-software/abi-aa/releases).
+
+Erroneous code example:
+
+```ignore (only fails on supported targets)
+#![feature(abi_c_cmse_nonsecure_call)]
+
+#[no_mangle]
+pub fn test(
+    f: extern "C-cmse-nonsecure-call" fn(u32, u32, u32, u32, u32) -> u32,
+) -> u32 {
+    f(1, 2, 3, 4, 5)
+}
+```
+
+Arguments' alignment is respected. In the example below, padding is inserted
+so that the `u64` argument is passed in registers r2 and r3. There is then no
+room left for the final `f32` argument
+
+```ignore (only fails on supported targets)
+#![feature(abi_c_cmse_nonsecure_call)]
+
+#[no_mangle]
+pub fn test(
+    f: extern "C-cmse-nonsecure-call" fn(u32, u64, f32) -> u32,
+) -> u32 {
+    f(1, 2, 3.0)
+}
+```

compiler/rustc_error_codes/src/lib.rs

@@ -536,6 +536,7 @@ E0794: 0794,
 E0795: 0795,
 E0796: 0796,
 E0797: 0797,
+E0798: 0798,
         );
     )
 }