Based on discussion on the IRLO thread Expand targets acceptable to rustc, and on zulip, a few weeks ago I compiled a list of targets that are supported by rustc (at any tier), which are not accepted by config.sub (used by projects that use autotools, including compilers such as gcc, and treated as normative by various projects of mine, such as https://crates.io/crates/target-tuples, and GitHub - LightningCreations/lccc: Lightning Creations Compiler Frontend for various languages). Of the 167 total targets supported by rustc, 52 are not accepted by config.sub (I have not done any idempotency checks, so of the remaining 115, some many not be in canonical form, though I forsee this as less of an issue provided the details of the target are reasonably related to those of the canonical form).
The link to the unsupported target list, as well as the script used to compile the list is: Rustc Targets · GitHub.
This could, potentially, have issues with compatibility, like were raised in Expand targets acceptable to rustc, though from a different angle, where it may not be possible to compile non-Rust code for those platforms if only the rustc target is known, even if the platform does have support in a major C/++ compiler under a different target name. This also could pose issues with non-rustc implementations which either determine the valid targets as autoconf
--targets (which are passed through
config.sub), or that canonicalize targets according to a subset of the config.sub rules. The former may be an issue with gcc-rs (if anyone working on the project could comment as to whether it is or is not, that would be ideal), and the latter with my own implementation, lccc (link above). The most forseeable issue would be that two implementations disagree on the name for a particular target they both otherwise support, and both interior tooling (such as build scripts) and exterior tooling (such as configuration scripts/files) written for one without consideration of the other will not be portable accross implementations (and dealing with such incompatibilities can be difficult).