Promotes 5 Thumb-mode bare-metal Arm targets to Tier 2

[cezarbbb]

Proposal

I would like to promote five Thumb-mode bare-metal Arm targets to Tier 2, to join their Arm-mode counterparts which are already Tier 2:

Thumb-mode target (Tier 3 → Tier 2)	Arm-mode counterpart (already Tier 2)
thumbv7a-none-eabi	armv7a-none-eabi
thumbv7a-none-eabihf	armv7a-none-eabihf
thumbv7r-none-eabi	armv7r-none-eabi
thumbv7r-none-eabihf	armv7r-none-eabihf
thumbv8r-none-eabihf	armv8r-none-eabihf

Note: There is no thumbv8r-none-eabi target because the Cortex-R52 processor always includes an FPU, making a soft-float ABI variant unnecessary.

These Thumb-mode targets generate T32 (Thumb-2) code by default, while their Arm-mode counterparts generate A32 code by default. The two instruction sets share the same LLVM backend, the same ABI (EABI/EABIhf), and the same data layout. The only differences in the target specifications are the llvm_target string and the description. Promoting them to Tier 2 will provide pre-compiled core and alloc artifacts via rustup, making it easier for embedded developers to use Thumb-mode without -Zbuild-std.

Many embedded projects prefer Thumb-mode for its smaller code size, and Cortex-R and Cortex-A processors support both modes. Having both modes at Tier 2 ensures parity and removes the current inconsistency where Arm-mode is Tier 2 but Thumb-mode is Tier 3.

The PR is rust-lang/rust#155763

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
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  • Refer to Proposals, Approvals and Stabilization docs for when a second is sufficient, or when a full team FCP is required.
• Once an MCP is seconded, the Final Comment Period begins.
  • Final Comment Period lasts for 10 days after all outstanding concerns are solved.
  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

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cc @rust-lang/compiler