From 9ae43567a3ef14a815482ccaa2a5b412a716743e Mon Sep 17 00:00:00 2001
From: r00ster91 <r00ster91@proton.me>
Date: Fri, 21 Jul 2023 03:22:59 +0200
Subject: [PATCH] Sema: improve error set/union discard/ignore errors

Previously the error had a note suggesting to use `try`, `catch`, or
`if`, even for error sets where none of those work.
Instead, in case of an error set the way you can handle the error
depends very much on the specific case. For example you might be in a
`catch` where you are discarding or ignoring the error set capture
value, in which case one way to handle the error might be to `return`
the error.
So, in that case, we do not attach that error note.

Additionally, this makes the error tell you what kind of an error it is:
is it an error set or an error union? This distinction is very relevant
in how to handle the error.
---
 src/Sema.zig                                       | 10 ++++++----
 .../compile_errors/discarding_error_value.zig      |  9 +++++++--
 .../ignored_deferred_function_call.zig             | 10 +++++++++-
 .../ignored_expression_in_while_continuation.zig   | 14 +++++++++++---
 4 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/src/Sema.zig b/src/Sema.zig
index f286b6385f..c55531d924 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -3568,9 +3568,10 @@ fn ensureResultUsed(
     const mod = sema.mod;
     switch (ty.zigTypeTag(mod)) {
         .Void, .NoReturn => return,
-        .ErrorSet, .ErrorUnion => {
+        .ErrorSet => return sema.fail(block, src, "error set is ignored", .{}),
+        .ErrorUnion => {
             const msg = msg: {
-                const msg = try sema.errMsg(block, src, "error is ignored", .{});
+                const msg = try sema.errMsg(block, src, "error union is ignored", .{});
                 errdefer msg.destroy(sema.gpa);
                 try sema.errNote(block, src, msg, "consider using 'try', 'catch', or 'if'", .{});
                 break :msg msg;
@@ -3600,9 +3601,10 @@ fn zirEnsureResultNonError(sema: *Sema, block: *Block, inst: Zir.Inst.Index) Com
     const src = inst_data.src();
     const operand_ty = sema.typeOf(operand);
     switch (operand_ty.zigTypeTag(mod)) {
-        .ErrorSet, .ErrorUnion => {
+        .ErrorSet => return sema.fail(block, src, "error set is discarded", .{}),
+        .ErrorUnion => {
             const msg = msg: {
-                const msg = try sema.errMsg(block, src, "error is discarded", .{});
+                const msg = try sema.errMsg(block, src, "error union is discarded", .{});
                 errdefer msg.destroy(sema.gpa);
                 try sema.errNote(block, src, msg, "consider using 'try', 'catch', or 'if'", .{});
                 break :msg msg;
diff --git a/test/cases/compile_errors/discarding_error_value.zig b/test/cases/compile_errors/discarding_error_value.zig
index c24d517d3e..eab3ecaf98 100644
--- a/test/cases/compile_errors/discarding_error_value.zig
+++ b/test/cases/compile_errors/discarding_error_value.zig
@@ -1,13 +1,18 @@
-export fn entry() void {
+export fn entry1() void {
     _ = foo();
 }
 fn foo() !void {
     return error.OutOfMemory;
 }
+export fn entry2() void {
+    const x: error{a} = undefined;
+    _ = x;
+}
 
 // error
 // backend=stage2
 // target=native
 //
-// :2:12: error: error is discarded
+// :2:12: error: error union is discarded
 // :2:12: note: consider using 'try', 'catch', or 'if'
+// :9:9: error: error set is discarded
diff --git a/test/cases/compile_errors/ignored_deferred_function_call.zig b/test/cases/compile_errors/ignored_deferred_function_call.zig
index 9537255d33..90c978b40f 100644
--- a/test/cases/compile_errors/ignored_deferred_function_call.zig
+++ b/test/cases/compile_errors/ignored_deferred_function_call.zig
@@ -5,9 +5,17 @@ fn bar() anyerror!i32 {
     return 0;
 }
 
+export fn foo2() void {
+    defer bar2();
+}
+fn bar2() anyerror {
+    return error.a;
+}
+
 // error
 // backend=stage2
 // target=native
 //
-// :2:14: error: error is ignored
+// :2:14: error: error union is ignored
 // :2:14: note: consider using 'try', 'catch', or 'if'
+// :9:15: error: error set is ignored
diff --git a/test/cases/compile_errors/ignored_expression_in_while_continuation.zig b/test/cases/compile_errors/ignored_expression_in_while_continuation.zig
index 4fa396190a..ca1b60da5e 100644
--- a/test/cases/compile_errors/ignored_expression_in_while_continuation.zig
+++ b/test/cases/compile_errors/ignored_expression_in_while_continuation.zig
@@ -15,13 +15,21 @@ fn bad() anyerror!void {
     return error.Bad;
 }
 
+export fn d() void {
+    while (true) : (bad2()) {}
+}
+fn bad2() anyerror {
+    return error.Bad;
+}
+
 // error
 // backend=stage2
 // target=native
 //
-// :2:24: error: error is ignored
+// :2:24: error: error union is ignored
 // :2:24: note: consider using 'try', 'catch', or 'if'
-// :7:25: error: error is ignored
+// :7:25: error: error union is ignored
 // :7:25: note: consider using 'try', 'catch', or 'if'
-// :12:25: error: error is ignored
+// :12:25: error: error union is ignored
 // :12:25: note: consider using 'try', 'catch', or 'if'
+// :19:25: error: error set is ignored