stage2: implement error notes and regress -femit-zir

 * Implement error notes
   - note: other symbol exported here
   - note: previous else prong is here
   - note: previous '_' prong is here
 * Add Compilation.CObject.ErrorMsg. This object properly converts to
   AllErrors.Message when the time comes.
 * Add Compilation.CObject.failure_retryable. Properly handles
   out-of-memory and other transient failures.
 * Introduce Module.SrcLoc which has not only a byte offset but also
   references the file which the byte offset applies to.
 * Scope.Block now contains both a pointer to the "owner" Decl and the
   "source" Decl. As an example, during inline function call, the
   "owner" will be the Decl of the caller and the "source" will be the
   Decl of the callee.
 * Module.ErrorMsg now sports a `file_scope` field so that notes can
   refer to source locations in a file other than the parent error
   message.
 * Some instances where a `*Scope` was stored, now store a
   `*Scope.Container`.
 * Some methods in the `Scope` namespace were moved to the more specific
   type, since there was only an implementation for one particular tag.
   - `removeDecl` moved to `Scope.Container`
   - `destroy` moved to `Scope.File`
 * Two kinds of Scope deleted:
   - zir_module
   - decl
 * astgen: properly use DeclVal / DeclRef. DeclVal was incorrectly
   changed to be a reference; this commit fixes it. Fewer ZIR
   instructions processed as a result.
   - declval_in_module is renamed to declval
   - previous declval ZIR instruction is deleted; it was only for .zir
     files.
 * Test harness: friendlier diagnostics when an unexpected set of errors
   is encountered.
 * zir_sema: fix analyzeInstBlockFlat by properly calling resolvingInst
   on the last zir instruction in the block.

Compile log implementation:
 * Write to a buffer rather than directly to stderr.
 * Only keep track of 1 callsite per Decl.
 * No longer mutate the ZIR Inst struct data.
 * "Compile log statement found" errors are only emitted when there are
   no other compile errors.

-femit-zir and support for .zir source files is regressed. If we wanted
to support this again, outputting .zir would need to be done as yet
another backend rather than in the haphazard way it was previously
implemented.

For parsing .zir, it was implemented previously in a way that was not
helpful for debugging. We need tighter integration with the test harness
for it to be useful; so clearly a rewrite is needed. Given that a
rewrite is needed, and it was getting in the way of progress and
organization of the rest of stage2, I regressed the feature.

src/astgen.zig

@@ -318,7 +318,7 @@ pub fn comptimeExpr(mod: *Module, parent_scope: *Scope, rl: ResultLoc, node: *as
     // Make a scope to collect generated instructions in the sub-expression.
     var block_scope: Scope.GenZIR = .{
         .parent = parent_scope,
-        .decl = parent_scope.decl().?,
+        .decl = parent_scope.ownerDecl().?,
         .arena = parent_scope.arena(),
         .instructions = .{},
     };
@@ -474,7 +474,7 @@ fn labeledBlockExpr(
 
     var block_scope: Scope.GenZIR = .{
         .parent = parent_scope,
-        .decl = parent_scope.decl().?,
+        .decl = parent_scope.ownerDecl().?,
         .arena = gen_zir.arena,
         .instructions = .{},
         .break_result_loc = rl,
@@ -899,7 +899,7 @@ fn containerDecl(mod: *Module, scope: *Scope, rl: ResultLoc, node: *ast.Node.Con
 
     var gen_scope: Scope.GenZIR = .{
         .parent = scope,
-        .decl = scope.decl().?,
+        .decl = scope.ownerDecl().?,
         .arena = scope.arena(),
         .instructions = .{},
     };
@@ -1028,7 +1028,13 @@ fn containerDecl(mod: *Module, scope: *Scope, rl: ResultLoc, node: *ast.Node.Con
         .ty = Type.initTag(.type),
         .val = val,
     });
-    return rlWrapPtr(mod, scope, rl, try addZIRInst(mod, scope, src, zir.Inst.DeclValInModule, .{ .decl = decl }, .{}));
+    if (rl == .ref) {
+        return addZIRInst(mod, scope, src, zir.Inst.DeclRef, .{ .decl = decl }, .{});
+    } else {
+        return rlWrap(mod, scope, rl, try addZIRInst(mod, scope, src, zir.Inst.DeclVal, .{
+            .decl = decl,
+        }, .{}));
+    }
 }
 
 fn errorSetDecl(mod: *Module, scope: *Scope, rl: ResultLoc, node: *ast.Node.ErrorSetDecl) InnerError!*zir.Inst {
@@ -1084,7 +1090,7 @@ fn orelseCatchExpr(
 
     var block_scope: Scope.GenZIR = .{
         .parent = scope,
-        .decl = scope.decl().?,
+        .decl = scope.ownerDecl().?,
         .arena = scope.arena(),
         .instructions = .{},
     };
@@ -1266,7 +1272,7 @@ fn boolBinOp(
 
     var block_scope: Scope.GenZIR = .{
         .parent = scope,
-        .decl = scope.decl().?,
+        .decl = scope.ownerDecl().?,
         .arena = scope.arena(),
         .instructions = .{},
     };
@@ -1412,7 +1418,7 @@ fn ifExpr(mod: *Module, scope: *Scope, rl: ResultLoc, if_node: *ast.Node.If) Inn
     }
     var block_scope: Scope.GenZIR = .{
         .parent = scope,
-        .decl = scope.decl().?,
+        .decl = scope.ownerDecl().?,
         .arena = scope.arena(),
         .instructions = .{},
     };
@@ -1513,7 +1519,7 @@ fn whileExpr(mod: *Module, scope: *Scope, rl: ResultLoc, while_node: *ast.Node.W
 
     var expr_scope: Scope.GenZIR = .{
         .parent = scope,
-        .decl = scope.decl().?,
+        .decl = scope.ownerDecl().?,
         .arena = scope.arena(),
         .instructions = .{},
     };
@@ -1649,7 +1655,7 @@ fn forExpr(mod: *Module, scope: *Scope, rl: ResultLoc, for_node: *ast.Node.For)
 
     var for_scope: Scope.GenZIR = .{
         .parent = scope,
-        .decl = scope.decl().?,
+        .decl = scope.ownerDecl().?,
         .arena = scope.arena(),
         .instructions = .{},
     };
@@ -1843,7 +1849,7 @@ fn getRangeNode(node: *ast.Node) ?*ast.Node.SimpleInfixOp {
 fn switchExpr(mod: *Module, scope: *Scope, rl: ResultLoc, switch_node: *ast.Node.Switch) InnerError!*zir.Inst {
     var block_scope: Scope.GenZIR = .{
         .parent = scope,
-        .decl = scope.decl().?,
+        .decl = scope.ownerDecl().?,
         .arena = scope.arena(),
         .instructions = .{},
     };
@@ -1885,7 +1891,7 @@ fn switchExpr(mod: *Module, scope: *Scope, rl: ResultLoc, switch_node: *ast.Node
 
     var item_scope: Scope.GenZIR = .{
         .parent = scope,
-        .decl = scope.decl().?,
+        .decl = scope.ownerDecl().?,
         .arena = scope.arena(),
         .instructions = .{},
     };
@@ -1922,8 +1928,15 @@ fn switchExpr(mod: *Module, scope: *Scope, rl: ResultLoc, switch_node: *ast.Node
         // Check for else/_ prong, those are handled last.
         if (case.items_len == 1 and case.items()[0].tag == .SwitchElse) {
             if (else_src) |src| {
-                return mod.fail(scope, case_src, "multiple else prongs in switch expression", .{});
-                // TODO notes "previous else prong is here"
+                const msg = try mod.errMsg(
+                    scope,
+                    case_src,
+                    "multiple else prongs in switch expression",
+                    .{},
+                );
+                errdefer msg.destroy(mod.gpa);
+                try mod.errNote(scope, src, msg, "previous else prong is here", .{});
+                return mod.failWithOwnedErrorMsg(scope, msg);
             }
             else_src = case_src;
             special_case = case;
@@ -1932,8 +1945,15 @@ fn switchExpr(mod: *Module, scope: *Scope, rl: ResultLoc, switch_node: *ast.Node
             mem.eql(u8, tree.tokenSlice(case.items()[0].firstToken()), "_"))
         {
             if (underscore_src) |src| {
-                return mod.fail(scope, case_src, "multiple '_' prongs in switch expression", .{});
-                // TODO notes "previous '_' prong is here"
+                const msg = try mod.errMsg(
+                    scope,
+                    case_src,
+                    "multiple '_' prongs in switch expression",
+                    .{},
+                );
+                errdefer msg.destroy(mod.gpa);
+                try mod.errNote(scope, src, msg, "previous '_' prong is here", .{});
+                return mod.failWithOwnedErrorMsg(scope, msg);
             }
             underscore_src = case_src;
             special_case = case;
@@ -1942,9 +1962,16 @@ fn switchExpr(mod: *Module, scope: *Scope, rl: ResultLoc, switch_node: *ast.Node
 
         if (else_src) |some_else| {
             if (underscore_src) |some_underscore| {
-                return mod.fail(scope, switch_src, "else and '_' prong in switch expression", .{});
-                // TODO notes "else prong is here"
-                // TODO notes "'_' prong is here"
+                const msg = try mod.errMsg(
+                    scope,
+                    switch_src,
+                    "else and '_' prong in switch expression",
+                    .{},
+                );
+                errdefer msg.destroy(mod.gpa);
+                try mod.errNote(scope, some_else, msg, "else prong is here", .{});
+                try mod.errNote(scope, some_underscore, msg, "'_' prong is here", .{});
+                return mod.failWithOwnedErrorMsg(scope, msg);
             }
         }
 
@@ -2162,7 +2189,13 @@ fn identifier(mod: *Module, scope: *Scope, rl: ResultLoc, ident: *ast.Node.OneTo
     }
 
     if (mod.lookupDeclName(scope, ident_name)) |decl| {
-        return rlWrapPtr(mod, scope, rl, try addZIRInst(mod, scope, src, zir.Inst.DeclValInModule, .{ .decl = decl }, .{}));
+        if (rl == .ref) {
+            return addZIRInst(mod, scope, src, zir.Inst.DeclRef, .{ .decl = decl }, .{});
+        } else {
+            return rlWrap(mod, scope, rl, try addZIRInst(mod, scope, src, zir.Inst.DeclVal, .{
+                .decl = decl,
+            }, .{}));
+        }
     }
 
     return mod.failNode(scope, &ident.base, "use of undeclared identifier '{s}'", .{ident_name});
@@ -2927,6 +2960,8 @@ fn rlWrapVoid(mod: *Module, scope: *Scope, rl: ResultLoc, node: *ast.Node, resul
     return rlWrap(mod, scope, rl, void_inst);
 }
 
+/// TODO go over all the callsites and see where we can introduce "by-value" ZIR instructions
+/// to save ZIR memory. For example, see DeclVal vs DeclRef.
 fn rlWrapPtr(mod: *Module, scope: *Scope, rl: ResultLoc, ptr: *zir.Inst) InnerError!*zir.Inst {
     if (rl == .ref) return ptr;
 
@@ -3032,7 +3067,7 @@ pub fn addZIRInstBlock(
     scope: *Scope,
     src: usize,
     tag: zir.Inst.Tag,
-    body: zir.Module.Body,
+    body: zir.Body,
 ) !*zir.Inst.Block {
     const gen_zir = scope.getGenZIR();
     try gen_zir.instructions.ensureCapacity(mod.gpa, gen_zir.instructions.items.len + 1);
@@ -3070,7 +3105,7 @@ pub fn addZIRInstConst(mod: *Module, scope: *Scope, src: usize, typed_value: Typ
 }
 
 /// TODO The existence of this function is a workaround for a bug in stage1.
-pub fn addZIRInstLoop(mod: *Module, scope: *Scope, src: usize, body: zir.Module.Body) !*zir.Inst.Loop {
+pub fn addZIRInstLoop(mod: *Module, scope: *Scope, src: usize, body: zir.Body) !*zir.Inst.Loop {
     const P = std.meta.fieldInfo(zir.Inst.Loop, .positionals).field_type;
     return addZIRInstSpecial(mod, scope, src, zir.Inst.Loop, P{ .body = body }, .{});
 }