delegation: fix ICE with double definition inside block in impl header

[mansiverma897993]

When a function is defined inside a block (such as an anonymous constant or a macro like \core::direct_const_arg!) in an impl header, its parent is the enclosing \Impl\ item. \n_kind\ determines if a function/delegation item is associated or free based on its parent's DefKind. When the parent is \DefKind::Impl, \n_kind\ returned \FnKind::AssocInherentImpl\ or \FnKind::AssocTraitImpl, even though the function is actually block-local (free). This incorrect classification propagates, hitting \unreachable!\ panics in delegation helper functions.

Fix this by returning \FnKind::Free\ immediately if the function has \DefKind::Fn\ (since associated functions always have \DefKind::AssocFn).

Fixes #159127

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[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test [ui] tests/ui/zero-sized/zero-sized-btreemap-insert.rs ... ok

failures:

---- [ui] tests/ui/delegation/hir-crate-items-before-lowering-ices.rs#ice_159127 stdout ----
Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/delegation/hir-crate-items-before-lowering-ices.ice_159127/hir-crate-items-before-lowering-ices.ice_159127.stderr`
diff of stderr:

2   --> $DIR/hir-crate-items-before-lowering-ices.rs:80:13
3    |
4 LL |             reuse foo::<>as bar;
-    |             ------------------- previous definition of the value `bar` here
+    |             -------------------- previous definition of the value `bar` here
6 LL |             reuse bar;
7    |             ^^^^^^^^^^ `bar` redefined here
8    |

11 error: expected type, found `direct_const_arg!()` constant
12   --> $DIR/hir-crate-items-before-lowering-ices.rs:75:9
13    |
---
22 LL | |
23 LL | |
24 LL | |             fn foo() {}

25 LL | |             reuse foo::<>as bar;
- ...
27 LL | |             reuse bar;
28 LL | |         })
-    | |_________^
+    | |__________^
---
The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args delegation/hir-crate-items-before-lowering-ices.rs`

error in revision `ice_159127`: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/delegation/hir-crate-items-before-lowering-ices.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--cfg" "ice_159127" "--check-cfg" "cfg(test,FALSE,ice_155125,ice_155127,ice_155128,ice_155164,ice_155202,ice_159127)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/delegation/hir-crate-items-before-lowering-ices.ice_159127" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0428]: the name `bar` is defined multiple times
##[error]  --> /checkout/tests/ui/delegation/hir-crate-items-before-lowering-ices.rs:80:13
   |
LL |             reuse foo::<>as bar;
   |             -------------------- previous definition of the value `bar` here
LL |             reuse bar; //[ice_159127]~ ERROR: the name `bar` is defined multiple times
   |             ^^^^^^^^^^ `bar` redefined here
   |
   = note: `bar` must be defined only once in the value namespace of this block

error: expected type, found `direct_const_arg!()` constant
##[error]  --> /checkout/tests/ui/delegation/hir-crate-items-before-lowering-ices.rs:75:9
   |
LL | /         core::direct_const_arg!({
LL | |         //[ice_159127]~^ ERROR: expected type, found `direct_const_arg!()` constant
LL | |         //[ice_159127]~| ERROR: complex const arguments must be placed inside of a `const` block
LL | |             fn foo() {}
LL | |             reuse foo::<>as bar;
LL | |             reuse bar; //[ice_159127]~ ERROR: the name `bar` is defined multiple times
LL | |         })
   | |__________^

error: aborting due to 2 previous errors