gen-rust: a local array passed to a mutable slice parameter lowers to &mut &(w), which rustc refuses #10785

Goal

t27c gen-rust must lower a call that passes a local array to a mutable slice parameter as &mut w, not &mut &(w). Today rustc refuses the generated code, so any spec reached from tri through gen-rust cannot call such a function, and a caller has to copy the function instead of using it (see #10766 / PR #10781, where spice_convert.t27 re-states linear.t27's MNA stamp as pair_add / src_add for this reason).

Repro (t27c 0.5.2 at master 2ee0cb6)

specs/analog/linear.t27 line 397 declares pub fn stamp_pair(w: []f64, off: usize, a: u8, b: u8, y: f64), and stamp (line 414) calls it on a local var w : [55]f64 as stamp_pair(&w, 0, a[e], b[e], 1.0 / val[e]);.

t27c gen-rust specs/analog/linear.t27 emits:

pub fn stamp_pair(w: &mut [f64], off: usize, a: u8, b: u8, y: f64) -> () {
...
            stamp_pair(&mut &(w), 0, a[(e) as usize], b[(e) as usize], (1.0 / val[(e) as usize]));

&mut &(w) is &mut &[f64; 55], which does not coerce to &mut [f64].

Requirements

• FR-001: &x passed to a []T parameter that the callee writes lowers to &mut x (or &mut x[..]); a read-only []const T parameter keeps &x.
• FR-002: The fix lives in the gen-rust lowering (its .t27 home if the pass is ported), with a regression fixture.

Scenarios

• Given linear.t27, When gen-rust runs, Then the emitted file compiles with rustc and its tests match t27c test-report.
• Given a read-only slice parameter, When gen-rust runs, Then the call still lowers to a shared borrow.

Boundary

• In scope: the gen-rust call-argument lowering for slice parameters.
• Out of scope: other backends; the copy in spice_convert.t27 is removed in a follow-up after this lands.

Success Criteria

• t27c gen-rust specs/analog/linear.t27 output compiles with rustc.
• A fixture with a mutable and a read-only slice parameter lowers each correctly.
• spice_convert.t27 can use analog::linear::{stamp} and drop pair_add / src_add (follow-up).

See #5980 for the gen-rust self-hosting context.

Success Criteria

• t27c gen-rust specs/analog/linear.t27 | rustc --edition 2021 -o /dev/null - exit 0
• t27c test specs/analog/linear.t27 exit 0
• t27c gen-rust specs/tri/collections/array.t27 | rustc --edition 2021 -o /dev/null - exit 0