thread 'main' panicked at src/util.rs:95:9: "...%s" is not a valid Ident

I'm working on patching and fixing up the SVD for the LPC11U1x/2x/3x series of MCUs. One of the modifications I made was to use derivedFrom in more places (so that the types used could also be used everywhere), and another was to add a generic set of IOCON registers. Part of the SVD in question

<field derivedFrom="IOCON.PIO0[%s].MODE">, however, causes svd2rust to panic with pio0%s is not a valid Ident. I attempted to resolve this on master by doing a patch similar to the following

diff --git a/src/generate/register.rs b/src/generate/register.rs
index 21fb174..d0fe56b 100644
--- a/src/generate/register.rs
+++ b/src/generate/register.rs
@@ -1462,7 +1462,7 @@ fn base_syn_path(
     } else if base.register().block == fpath.register().block {
         let mut segments = Punctuated::new();
         segments.push(path_segment(Ident::new("super", span)));
-        segments.push(path_segment(base.register().name.to_snake_case_ident(span)));
+        segments.push(path_segment(replace_suffix(&base.register().name, "").to_snake_case_ident(span)));
         segments.push(path_segment(base_ident.clone()));
         type_path(segments)
     } else {

This leads to svd2rust not panicking, though it does produce invalid code for the IOCON RegisterBlock (return type of PIO10 instead of PIO1 for the pio1 method, &PIO10 instead of &PIO1 for pio1_iter). I did a similar patch to case-defaults (#805), and this resulted in what appeared to be correct codegen.

RUST_BACKTRACE=1 panic:

thread 'main' panicked at src/util.rs:95:9:
"pio0%s" is not a valid Ident
stack backtrace:
   0: rust_begin_unwind
             at /rustc/f688dd684faca5b31b156fac2c6e0ae81fc9bc90/library/std/src/panicking.rs:645:5
   1: core::panicking::panic_fmt
             at /rustc/f688dd684faca5b31b156fac2c6e0ae81fc9bc90/library/core/src/panicking.rs:72:14
   2: proc_macro2::fallback::validate_ident
             at /home/masterr3c0rd/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.71/src/fallback.rs:828:9
   3: proc_macro2::fallback::Ident::new_checked
             at /home/masterr3c0rd/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.71/src/fallback.rs:760:9
   4: proc_macro2::imp::Ident::new_checked
             at /home/masterr3c0rd/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.71/src/wrapper.rs:646:50
   5: proc_macro2::Ident::new
             at /home/masterr3c0rd/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.71/src/lib.rs:956:21
   6: svd2rust::util::ToSanitizedCase::to_snake_case_ident
             at /mnt/projects/Personal/svd2rust/src/util.rs:95:9
   7: svd2rust::generate::register::base_syn_path
             at /mnt/projects/Personal/svd2rust/src/generate/register.rs:1465:36
   8: svd2rust::generate::register::fields
             at /mnt/projects/Personal/svd2rust/src/generate/register.rs:883:37
   9: svd2rust::generate::register::render_register_mod
             at /mnt/projects/Personal/svd2rust/src/generate/register.rs:283:17
  10: svd2rust::generate::register::render
             at /mnt/projects/Personal/svd2rust/src/generate/register.rs:112:25
  11: svd2rust::generate::peripheral::render_ercs
             at /mnt/projects/Personal/svd2rust/src/generate/peripheral.rs:1355:36
  12: svd2rust::generate::peripheral::render
             at /mnt/projects/Personal/svd2rust/src/generate/peripheral.rs:240:21
  13: svd2rust::generate::device::render
             at /mnt/projects/Personal/svd2rust/src/generate/device.rs:204:22
  14: svd2rust::run
             at /mnt/projects/Personal/svd2rust/src/main.rs:272:17
  15: svd2rust::main
             at /mnt/projects/Personal/svd2rust/src/main.rs:363:25
  16: core::ops::function::FnOnce::call_once
             at /rustc/f688dd684faca5b31b156fac2c6e0ae81fc9bc90/library/core/src/ops/function.rs:250:5

Start with src/generate/register.rs:1462 and src/util.rs:95, then reproduce the panic using the LPC11U1x/2x/3x SVD fragment linked in the issue. Trace how derivedFrom names containing %s become identifiers and inspect the related case-defaults change in #805. Done means generation no longer panics and produces correct IOCON RegisterBlock references for pio1 and pio1_iter.