tint/sem: Add const to StructMember 'type'.

This should be immutable once the sem tree is built.
Add a hacky const_cast in Resolver for now, as this is preferable to
having some things non-const in sem.

Bug: tint:745
Change-Id: I67ebce76730347c9543875ab8e1c21a47d71fd56
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/97584
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/resolver/const_eval.cc

@@ -361,7 +361,7 @@ const Constant* ZeroValue(ProgramBuilder& builder, const sem::Type* type) {
             return nullptr;
         },
         [&](const sem::Struct* s) -> const Constant* {
-            std::unordered_map<sem::Type*, const Constant*> zero_by_type;
+            std::unordered_map<const sem::Type*, const Constant*> zero_by_type;
             utils::Vector<const sem::Constant*, 4> zeros;
             zeros.Reserve(s->Members().size());
             for (auto* member : s->Members()) {

src/tint/resolver/resolver.cc

@@ -2803,7 +2803,7 @@ bool Resolver::ApplyStorageClassUsageToType(ast::StorageClass sc,
         str->AddUsage(sc);
 
         for (auto* member : str->Members()) {
-            if (!ApplyStorageClassUsageToType(sc, member->Type(), usage)) {
+            if (!ApplyStorageClassUsageToType(sc, const_cast<sem::Type*>(member->Type()), usage)) {
                 std::stringstream err;
                 err << "while analysing structure member " << sem_.TypeNameOf(str) << "."
                     << builder_->Symbols().NameFor(member->Declaration()->symbol);

src/tint/sem/struct.cc

@@ -157,7 +157,7 @@ bool Struct::IsConstructible() const {
 
 StructMember::StructMember(const ast::StructMember* declaration,
                            Symbol name,
-                           sem::Type* type,
+                           const sem::Type* type,
                            uint32_t index,
                            uint32_t offset,
                            uint32_t align,

src/tint/sem/struct.h

@@ -182,7 +182,7 @@ class StructMember final : public Castable<StructMember, Node> {
     /// @param size the byte size of the member
     StructMember(const ast::StructMember* declaration,
                  Symbol name,
-                 sem::Type* type,
+                 const sem::Type* type,
                  uint32_t index,
                  uint32_t offset,
                  uint32_t align,
@@ -205,7 +205,7 @@ class StructMember final : public Castable<StructMember, Node> {
     const sem::Struct* Struct() const { return struct_; }
 
     /// @returns the type of the member
-    sem::Type* Type() const { return type_; }
+    const sem::Type* Type() const { return type_; }
 
     /// @returns the member index
     uint32_t Index() const { return index_; }
@@ -223,7 +223,7 @@ class StructMember final : public Castable<StructMember, Node> {
     const ast::StructMember* const declaration_;
     const Symbol name_;
     const sem::Struct* struct_;
-    sem::Type* const type_;
+    const sem::Type* type_;
     const uint32_t index_;
     const uint32_t offset_;
     const uint32_t align_;

src/tint/transform/decompose_strided_matrix.cc

@@ -105,10 +105,11 @@ DecomposeStridedMatrix::~DecomposeStridedMatrix() = default;
 
 bool DecomposeStridedMatrix::ShouldRun(const Program* program, const DataMap&) const {
     bool should_run = false;
-    GatherCustomStrideMatrixMembers(program, [&](const sem::StructMember*, sem::Matrix*, uint32_t) {
-        should_run = true;
-        return GatherResult::kStop;
-    });
+    GatherCustomStrideMatrixMembers(program,
+                                    [&](const sem::StructMember*, const sem::Matrix*, uint32_t) {
+                                        should_run = true;
+                                        return GatherResult::kStop;
+                                    });
     return should_run;
 }
 
@@ -118,7 +119,7 @@ void DecomposeStridedMatrix::Run(CloneContext& ctx, const DataMap&, DataMap&) co
     // and populate the `decomposed` map with the members that have been replaced.
     std::unordered_map<const ast::StructMember*, MatrixInfo> decomposed;
     GatherCustomStrideMatrixMembers(
-        ctx.src, [&](const sem::StructMember* member, sem::Matrix* matrix, uint32_t stride) {
+        ctx.src, [&](const sem::StructMember* member, const sem::Matrix* matrix, uint32_t stride) {
             // We've got ourselves a struct member of a matrix type with a custom
             // stride. Replace this with an array of column vectors.
             MatrixInfo info{stride, matrix};