fix(codegen-rust): correct fixed-size array types and usize indices (#1457)

Second codegen fix after the optimizer removal. Makes the Rust backend emit
correct array types and indices so generated modules compile under rustc.

t27_type_to_rust:
- `[T; N]` (Rust-style, element before `;`) previously took the empty suffix
  after `]` and emitted `Vec<>` -> E0107. Now emits `[T; N as usize]`
  (element parsed correctly; length cast to usize as arrays require).
- Zig-style `[N]T` still lowers to `Vec<T>`.

expr_to_rust (ExprIndex):
- Array/Vec indices must be `usize`; t27 index expressions are u32. Non-literal
  indices now emit `[(idx) as usize]`. Integer literals are left uncast (they
  infer usize and casting them would trip clippy::unnecessary_cast).

Verified: t27c suite unchanged at 1494 passed / 1 failed (the remaining failure
is the pre-existing Verilog HIR let-binding test, untouched). Regenerating
gHashTag/tri-net with #1456 + this drops the wired-module error count from
400+ to a handful of genuine SPEC bugs (a bool-vs-u32 return type, a Q8.8
fixed-point 256-in-u8 literal, a couple of `let`-that-should-be-`var`), i.e. the
codegen itself now produces compiling Rust.

FROZEN_HASH re-sealed (M5).

phi^2 + phi^-2 = 3

Author: deborahnelson8788726

bootstrap/src/compiler.rs

@@ -8079,12 +8079,21 @@ impl RustCodegen {
                 format!("Vec<{}>", Self::t27_type_to_rust(inner))
             }
             t if t.starts_with('[') && t.contains(']') => {
-                // [N]T format - convert to Vec
-                if let Some(bracket_end) = t.find(']') {
-                    let inner = &t[bracket_end + 1..];
-                    format!("Vec<{}>", Self::t27_type_to_rust(inner))
+                let end = t.rfind(']').unwrap();
+                let inside = &t[1..end];
+                if let Some(semi) = inside.find(';') {
+                    // Rust-style fixed-size array `[T; N]`: element before `;`,
+                    // size after. (Previously only the Zig-style `[N]T` form was
+                    // handled; `[T; N]` took the empty suffix after `]` and emitted
+                    // `Vec<>` -> E0107. #1457.) Array lengths must be `usize`, and
+                    // spec size consts are u32, so cast in the const-expr position.
+                    let elem = inside[..semi].trim();
+                    let size = inside[semi + 1..].trim();
+                    format!("[{}; {} as usize]", Self::t27_type_to_rust(elem), size)
                 } else {
-                    t.to_string()
+                    // Zig-style `[N]T`: size in brackets, element after `]`.
+                    let after = &t[end + 1..];
+                    format!("Vec<{}>", Self::t27_type_to_rust(after))
                 }
             }
             t => t.to_string(), // Custom type name
@@ -8167,11 +8176,18 @@ impl RustCodegen {
             }
             NodeKind::ExprIndex => {
                 if node.children.len() >= 2 {
-                    format!(
-                        "{}[{}]",
-                        Self::expr_to_rust(&node.children[0]),
-                        Self::expr_to_rust(&node.children[1])
-                    )
+                    let base = Self::expr_to_rust(&node.children[0]);
+                    let idx = &node.children[1];
+                    let idx_str = Self::expr_to_rust(idx);
+                    // Array/Vec indices must be `usize`. t27 index expressions are
+                    // u32, so cast non-literal indices. Integer literals already
+                    // infer `usize` in index position, so casting them would trip
+                    // clippy::unnecessary_cast under `-D warnings`. #1457.
+                    if idx.kind == NodeKind::ExprLiteral {
+                        format!("{}[{}]", base, idx_str)
+                    } else {
+                        format!("{}[({}) as usize]", base, idx_str)
+                    }
                 } else {
                     "()".to_string()
                 }

bootstrap/stage0/FROZEN_HASH

@@ -1 +1 @@
-6eb632b838e907f160934965972f408839d8bb2916e0447c0c1bade5bda98b18  bootstrap/src/compiler.rs
+c25b7a6f4e2fca73607b4eb8dac697964b57071b2ba1e2d59997aca82a39d138  bootstrap/src/compiler.rs