Rollup merge of #91847 - BoxyUwU:generic_arg_infer_fixme, r=lcnr

Fix FIXME for `generic_arg_infer` in `create_substs_for_ast_path`

Fixes a FIXME, does some general refactoring of this fn, and also fixes a bug where we would use a const params defaults instead of an inference var ([playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=19456f65ea5dc3fcaa9b696f842ab380))
(lot of stuff in one PR but it was all so close together...)

r? `@lcnr`

Fixes #91614

src/test/ui/const-generics/generic_arg_infer/dont-use-defaults.rs

@@ -0,0 +1,15 @@
+// run-pass
+#![feature(generic_arg_infer)]
+
+// test that we dont use defaults to aide in type inference
+
+struct Foo<const N: usize = 2>;
+impl<const N: usize> Foo<N> {
+    fn make_arr() -> [(); N] {
+        [(); N]
+    }
+}
+
+fn main() {
+    let [(), (), ()] = Foo::<_>::make_arr();
+}

src/test/ui/const-generics/generic_arg_infer/issue-91614.rs

@@ -0,0 +1,8 @@
+#![feature(portable_simd)]
+#![feature(generic_arg_infer)]
+use std::simd::Mask;
+
+fn main() {
+    let y = Mask::<_, _>::splat(false);
+    //~^ error: type annotations needed for `Mask<_, {_: usize}>`
+}

src/test/ui/const-generics/generic_arg_infer/issue-91614.stderr

@@ -0,0 +1,18 @@
+error[E0283]: type annotations needed for `Mask<_, {_: usize}>`
+  --> $DIR/issue-91614.rs:6:13
+   |
+LL |     let y = Mask::<_, _>::splat(false);
+   |         -   ^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T`
+   |         |
+   |         consider giving `y` the explicit type `Mask<_, LANES>`, where the type parameter `T` is specified
+   |
+   = note: cannot satisfy `_: MaskElement`
+note: required by a bound in `Mask::<T, LANES>::splat`
+  --> $SRC_DIR/core/src/../../portable-simd/crates/core_simd/src/masks.rs:LL:COL
+   |
+LL |     T: MaskElement,
+   |        ^^^^^^^^^^^ required by this bound in `Mask::<T, LANES>::splat`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0283`.