Rollup merge of rust-lang#125690 - Lokathor:arm-maintainer-reorg, r=ehuss

ARM Target Docs Update

Updates the ARM target docs, drawing more attention to the `arm-none-eabi` target group by placing all targets *within* that group as a sub-list in the Table of Contents.

Also updates the `armv4t-none-eabi` page (maintainer signoff: I'm that target's maintainer) to clarify that the page covers the arm version and the thumb version of the target, but that the target group page has the full info because there's nothing really specific to say for those targets.

Author: jieyouxu

src/doc/rustc/src/SUMMARY.md

@@ -30,10 +30,15 @@
     - [aarch64-nintendo-switch-freestanding](platform-support/aarch64-nintendo-switch-freestanding.md)
     - [armeb-unknown-linux-gnueabi](platform-support/armeb-unknown-linux-gnueabi.md)
     - [arm-none-eabi](platform-support/arm-none-eabi.md)
-    - [armv4t-none-eabi](platform-support/armv4t-none-eabi.md)
-    - [armv5te-none-eabi](platform-support/armv5te-none-eabi.md)
-    - [armv7r-none-eabi](platform-support/armv7r-none-eabi.md)
-    - [armv8r-none-eabihf](platform-support/armv8r-none-eabihf.md)
+      - [armv4t-none-eabi](platform-support/armv4t-none-eabi.md)
+      - [armv5te-none-eabi](platform-support/armv5te-none-eabi.md)
+      - [armv7r-none-eabi](platform-support/armv7r-none-eabi.md)
+      - [armv8r-none-eabihf](platform-support/armv8r-none-eabihf.md)
+      - [thumbv6m-none-eabi](./platform-support/thumbv6m-none-eabi.md)
+      - [thumbv7em-none-eabi\*](./platform-support/thumbv7em-none-eabi.md)
+      - [thumbv7m-none-eabi](./platform-support/thumbv7m-none-eabi.md)
+      - [thumbv8m.base-none-eabi](./platform-support/thumbv8m.base-none-eabi.md)
+      - [thumbv8m.main-none-eabi\*](./platform-support/thumbv8m.main-none-eabi.md)
     - [armv6k-nintendo-3ds](platform-support/armv6k-nintendo-3ds.md)
     - [armv7-sony-vita-newlibeabihf](platform-support/armv7-sony-vita-newlibeabihf.md)
     - [armv7-unknown-linux-uclibceabi](platform-support/armv7-unknown-linux-uclibceabi.md)
@@ -60,11 +65,6 @@
     - [riscv32imac-unknown-xous-elf](platform-support/riscv32imac-unknown-xous-elf.md)
     - [riscv32*-unknown-none-elf](platform-support/riscv32-unknown-none-elf.md)
     - [sparc-unknown-none-elf](./platform-support/sparc-unknown-none-elf.md)
-    - [thumbv6m-none-eabi](./platform-support/thumbv6m-none-eabi.md)
-    - [thumbv7m-none-eabi](./platform-support/thumbv7m-none-eabi.md)
-    - [thumbv7em-none-eabi\*](./platform-support/thumbv7em-none-eabi.md)
-    - [thumbv8m.base-none-eabi](./platform-support/thumbv8m.base-none-eabi.md)
-    - [thumbv8m.main-none-eabi\*](./platform-support/thumbv8m.main-none-eabi.md)
     - [*-pc-windows-gnullvm](platform-support/pc-windows-gnullvm.md)
     - [\*-nto-qnx-\*](platform-support/nto-qnx.md)
     - [*-unikraft-linux-musl](platform-support/unikraft-linux-musl.md)

src/doc/rustc/src/platform-support/arm-none-eabi.md

@@ -1,6 +1,15 @@
 # `{arm,thumb}*-none-eabi(hf)?`
 
-## Tier 2 Target List
+## Common Target Details
+
+This documentation covers details that apply to a range of bare-metal targets
+for 32-bit Arm CPUs. The `arm-none-eabi` flavor of the GNU compiler toolchain is
+often used to assist compilation to these targets.
+
+Details that apply only to only a specific target in this group are covered in
+their own document.
+
+### Tier 2 Target List
 
 - Arm A-Profile Architectures
   - `armv7a-none-eabi`
@@ -16,7 +25,7 @@
 - *Legacy* Arm Architectures
   - None
 
-## Tier 3 Target List
+### Tier 3 Target List
 
 - Arm A-Profile Architectures
   - `armv7a-none-eabihf`
@@ -28,24 +37,21 @@
   - [`armv4t-none-eabi` and `thumbv4t-none-eabi`](armv4t-none-eabi.md)
   - [`armv5te-none-eabi` and `thumbv5te-none-eabi`](armv5te-none-eabi.md)
 
-## Common Target Details
-
-This documentation covers details that apply to a range of bare-metal targets
-for 32-bit Arm CPUs. In addition, target specific details may be covered in
-their own document.
+## Instruction Sets
 
 There are two 32-bit instruction set architectures (ISAs) defined by Arm:
 
 - The [*A32 ISA*][a32-isa], with fixed-width 32-bit instructions. Previously
-  known as the *Arm* ISA, this originated with the original ARM1 of 1985 and has
+  known as the *Arm* ISA, this originated with the original Arm1 of 1985 and has
   been updated by various revisions to the architecture specifications ever
   since.
 - The [*T32 ISA*][t32-isa], with a mix of 16-bit and 32-bit width instructions.
   Note that this term includes both the original 16-bit width *Thumb* ISA
   introduced with the Armv4T architecture in 1994, and the later 16/32-bit sized
-  *Thumb-2* ISA introduced with the Armv6T2 architecture in 2003. Again, these
-  ISAs have been revised by subsequent revisions to the relevant Arm
-  architecture specifications.
+  *Thumb-2* ISA introduced with the Armv6T2 architecture in 2003.
+
+Again, these ISAs have been revised by subsequent revisions to the relevant Arm
+architecture specifications.
 
 There is also a 64-bit ISA with fixed-width 32-bit instructions called the *A64
 ISA*, but targets which implement that instruction set generally start with

src/doc/rustc/src/platform-support/armv4t-none-eabi.md

@@ -1,16 +1,13 @@
-# armv4t-none-eabi
+# armv4t-none-eabi / thumbv4t-none-eabi
 
 Tier 3
 
-Bare-metal target for any cpu in the Armv4T architecture family, supporting
-ARM/Thumb code interworking (aka `A32`/`T32`), with ARM code as the default code
-generation.
+These two targets are part of the [`arm-none-eabi`](arm-none-eabi.md) target
+group, and all the information there applies.
 
-In particular this supports the Game Boy Advance (GBA), but there's nothing
-GBA-specific with this target, so any Armv4T device should work fine.
-
-See [`arm-none-eabi`](arm-none-eabi.md) for information applicable to all
-`arm-none-eabi` targets.
+Both of these targets can be used on the Game Boy Advance (GBA), among other
+things. On the GBA, one should usually use the `thumb` target to get the best
+overall performance.
 
 ## Target Maintainers
 
@@ -23,6 +20,6 @@ This is a cross-compiled target that you will need to emulate during testing.
 Because this is a device-agnostic target, and the exact emulator that you'll
 need depends on the specific device you want to run your code on.
 
-For example, when programming for the Gameboy Advance, the
-[mgba-test-runner](https://github.com/agbrs/agb) program could be used to make a
-normal set of rust tests be run within the `mgba` emulator.
+* When building for the GBA, [mgba-test-runner](https://github.com/agbrs/agb)
+  can be used to make a normal set of rust tests be run within the `mgba`
+  emulator.