Gate async fn trait bound modifier on async_trait_bounds

[compiler-errors]

This PR moves async Fn() trait bounds into a new feature gate: feature(async_trait_bounds). The general vibe is that we will most likely stabilize the feature(async_closure) without the async Fn() trait bound modifier, so we need to gate that separately.

We're trying to work on the general vision of async trait bound modifier general in: rust-lang/rfcs#3710, however that RFC still needs more time for consensus to converge, and we've decided that the value that users get from calling the bound async Fn() is not really worth blocking landing async closures in general.

This is blocked on #132611, since moving these tests to AsyncFn is annoying without a prelude import.

r? @ghost

[rust-log-analyzer]

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#16 exporting to docker image format
#16 sending tarball 27.9s done
#16 DONE 34.7s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
...

failures:

---- [coverage-map] tests/coverage/async_closure.rs stdout ----
Saved the actual cov-map to "/checkout/obj/build/x86_64-unknown-linux-gnu/test/coverage/async_closure.coverage-map/async_closure.cov-map"
diff of cov-map:
1 Function name: async_closure::call_once::<async_closure::main::{closure#0}>
1 Function name: async_closure::call_once::<async_closure::main::{closure#0}>
- Raw bytes (9): 0x[01, 01, 00, 01, 01, 07, 01, 00, 2c]
+ Raw bytes (9): 0x[01, 01, 00, 01, 01, 07, 01, 00, 2b]
4 - file 0 => global file 1
5 Number of expressions: 0

6 Number of file 0 mappings: 1
6 Number of file 0 mappings: 1
- - Code(Counter(0)) at (prev + 7, 1) to (start + 0, 44)
+ - Code(Counter(0)) at (prev + 7, 1) to (start + 0, 43)
8 Highest counter ID seen: c0
10 Function name: async_closure::call_once::<async_closure::main::{closure#0}>::{closure#0}


- Raw bytes (14): 0x[01, 01, 00, 02, 01, 07, 2c, 01, 0e, 05, 02, 01, 00, 02]
+ Raw bytes (14): 0x[01, 01, 00, 02, 01, 07, 2b, 01, 0e, 05, 02, 01, 00, 02]
13 - file 0 => global file 1
14 Number of expressions: 0

15 Number of file 0 mappings: 2
15 Number of file 0 mappings: 2
- - Code(Counter(0)) at (prev + 7, 44) to (start + 1, 14)
+ - Code(Counter(0)) at (prev + 7, 43) to (start + 1, 14)
17 - Code(Counter(1)) at (prev + 2, 1) to (start + 0, 2)
18 Highest counter ID seen: c1


The actual cov-map differed from the expected cov-map.


error: 1 errors occurred comparing coverage output.
status: exit status: 0
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/coverage-dump" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/coverage/async_closure.coverage-map/async_closure.ll"
Function name: async_closure::call_once::<async_closure::main::{closure#0}>
Function name: async_closure::call_once::<async_closure::main::{closure#0}>
Raw bytes (9): 0x[01, 01, 00, 01, 01, 07, 01, 00, 2b]
- file 0 => global file 1
Number of expressions: 0
Number of file 0 mappings: 1
Number of file 0 mappings: 1
- Code(Counter(0)) at (prev + 7, 1) to (start + 0, 43)
Highest counter ID seen: c0
Function name: async_closure::call_once::<async_closure::main::{closure#0}>::{closure#0}
Function name: async_closure::call_once::<async_closure::main::{closure#0}>::{closure#0}
Raw bytes (14): 0x[01, 01, 00, 02, 01, 07, 2b, 01, 0e, 05, 02, 01, 00, 02]
- file 0 => global file 1
Number of expressions: 0
Number of file 0 mappings: 2
Number of file 0 mappings: 2
- Code(Counter(0)) at (prev + 7, 43) to (start + 1, 14)
- Code(Counter(1)) at (prev + 2, 1) to (start + 0, 2)
Highest counter ID seen: c1
Function name: async_closure::main
Function name: async_closure::main
Raw bytes (14): 0x[01, 01, 00, 02, 01, 0b, 01, 01, 16, 01, 02, 05, 02, 02]
- file 0 => global file 1
Number of expressions: 0
Number of file 0 mappings: 2
Number of file 0 mappings: 2
- Code(Counter(0)) at (prev + 11, 1) to (start + 1, 22)
- Code(Counter(0)) at (prev + 2, 5) to (start + 2, 2)
Highest counter ID seen: c0
Function name: async_closure::main::{closure#0}
Function name: async_closure::main::{closure#0}
Raw bytes (9): 0x[01, 01, 00, 01, 01, 0c, 23, 00, 24]
- file 0 => global file 1
Number of expressions: 0
Number of file 0 mappings: 1
Number of file 0 mappings: 1
- Code(Counter(0)) at (prev + 12, 35) to (start + 0, 36)
Highest counter ID seen: c0
Function name: async_closure::main::{closure#0}::{closure#0}::<i16>
Function name: async_closure::main::{closure#0}::{closure#0}::<i16>
Raw bytes (9): 0x[01, 01, 00, 01, 01, 0c, 22, 00, 24]
- file 0 => global file 1
Number of expressions: 0
Number of file 0 mappings: 1
Number of file 0 mappings: 1
- Code(Counter(0)) at (prev + 12, 34) to (start + 0, 36)
Highest counter ID seen: c0
Function name: async_closure::main::{closure#0}::{closure#1}::<i32>
Function name: async_closure::main::{closure#0}::{closure#1}::<i32>
Raw bytes (9): 0x[01, 01, 00, 01, 01, 0c, 23, 00, 24]
- file 0 => global file 1
Number of expressions: 0
Number of file 0 mappings: 1
Number of file 0 mappings: 1
- Code(Counter(0)) at (prev + 12, 35) to (start + 0, 36)
Highest counter ID seen: c0
stderr: none