add rust error message for CMSE stack spill

when the `C-cmse-nonsecure-call` ABI is used, arguments and return values must be passed via registers. Failing to do so (i.e. spilling to the stack) causes an LLVM error down the line, but now rustc will properly emit an error a bit earlier in the chain

2 files changed, 37 insertions, 0 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0798.md b/compiler/rustc_error_codes/src/error_codes/E0798.md
new file mode 100644
index 00000000000..79ed041004e
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0798.md
@@ -0,0 +1,36 @@
+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`).
+
+For more information,
+see [arm's aapcs32](https://github.com/ARM-software/abi-aa/releases).
+
+Erroneous code example:
+
+```compile_fail,E0798
+#![feature(abi_c_cmse_nonsecure_call)]
+
+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
+
+```compile_fail,E0798
+#![feature(abi_c_cmse_nonsecure_call)]
+
+fn test(
+    f: extern "C-cmse-nonsecure-call" fn(u32, u64, f32) -> u32,
+) -> u32 {
+    f(1, 2, 3.0)
+}
+```
diff --git a/compiler/rustc_error_codes/src/lib.rs b/compiler/rustc_error_codes/src/lib.rs
index d13d5e1bca2..2a7bc2501c0 100644
--- a/compiler/rustc_error_codes/src/lib.rs
+++ b/compiler/rustc_error_codes/src/lib.rs
@@ -536,6 +536,7 @@ E0794: 0794,
 E0795: 0795,
 E0796: 0796,
 E0797: 0797,
+E0798: 0798,
         );
     )
 }