Sema: implement switch expressions

The logic for putting ranges into the else prong is moved from AstGen to
Sema. However, logic to emit multi-items the same as single-items cannot
be done until TZIR supports mapping multiple items to the same block of
code. This will be simple to represent when we do the upcoming TZIR memory
layout changes.

Not yet implemented in this commit is the validation of duplicate
values. The trick is going to be emitting error messages with accurate
source locations, without adding extra source nodes to the ZIR
switch instruction.

This will be done by computing the respective AST node based on the
switch node (which we do have available), only when a compile error
occurs and we need to know the source location to attach the message to.

1 files changed, 21 insertions, 2 deletions

diff --git a/src/Module.zig b/src/Module.zig
index 3309a10b30..91790aa1b6 100644
--- a/src/Module.zig
+++ b/src/Module.zig
@@ -888,7 +888,7 @@ pub const Scope = struct {
         pub fn addSwitchBr(
             block: *Scope.Block,
             src: LazySrcLoc,
-            target: *ir.Inst,
+            operand: *ir.Inst,
             cases: []ir.Inst.SwitchBr.Case,
             else_body: ir.Body,
         ) !*ir.Inst {
@@ -899,7 +899,7 @@ pub const Scope = struct {
                     .ty = Type.initTag(.noreturn),
                     .src = src,
                 },
-                .target = target,
+                .target = operand,
                 .cases = cases,
                 .else_body = else_body,
             };
@@ -1533,6 +1533,8 @@ pub const SrcLoc = struct {
             .node_offset_bin_lhs,
             .node_offset_bin_rhs,
             .node_offset_switch_operand,
+            .node_offset_switch_special_prong,
+            .node_offset_switch_range,
             => src_loc.container.decl.container.file_scope,
         };
     }
@@ -1665,6 +1667,8 @@ pub const SrcLoc = struct {
                 return token_starts[tok_index];
             },
             .node_offset_switch_operand => @panic("TODO"),
+            .node_offset_switch_special_prong => @panic("TODO"),
+            .node_offset_switch_range => @panic("TODO"),
         }
     }
 };
@@ -1802,6 +1806,17 @@ pub const LazySrcLoc = union(enum) {
     /// which points to a switch expression AST node. Next, nagivate to the operand.
     /// The Decl is determined contextually.
     node_offset_switch_operand: i32,
+    /// The source location points to the else/`_` prong of a switch expression, found
+    /// by taking this AST node index offset from the containing Decl AST node,
+    /// which points to a switch expression AST node. Next, nagivate to the else/`_` prong.
+    /// The Decl is determined contextually.
+    node_offset_switch_special_prong: i32,
+    /// The source location points to all the ranges of a switch expression, found
+    /// by taking this AST node index offset from the containing Decl AST node,
+    /// which points to a switch expression AST node. Next, nagivate to any of the
+    /// range nodes. The error applies to all of them.
+    /// The Decl is determined contextually.
+    node_offset_switch_range: i32,
 
     /// Upgrade to a `SrcLoc` based on the `Decl` or file in the provided scope.
     pub fn toSrcLoc(lazy: LazySrcLoc, scope: *Scope) SrcLoc {
@@ -1836,6 +1851,8 @@ pub const LazySrcLoc = union(enum) {
             .node_offset_bin_lhs,
             .node_offset_bin_rhs,
             .node_offset_switch_operand,
+            .node_offset_switch_special_prong,
+            .node_offset_switch_range,
             => .{
                 .container = .{ .decl = scope.srcDecl().? },
                 .lazy = lazy,
@@ -1876,6 +1893,8 @@ pub const LazySrcLoc = union(enum) {
             .node_offset_bin_lhs,
             .node_offset_bin_rhs,
             .node_offset_switch_operand,
+            .node_offset_switch_special_prong,
+            .node_offset_switch_range,
             => .{
                 .container = .{ .decl = decl },
                 .lazy = lazy,