Rollup merge of #134364 - estebank:derive-docs, r=fmease

Use E0665 for missing `#[default]` on enum and update doc

The docs for E0665 when doing `#[derive(Default]` on an `enum` previously didn't mention `#[default]` at all, or made a distinction between unit variants, that can be annotated, and tuple or struct variants, which cannot.

E0665 was not being emitted, we now use it for the same error it belonged to before.

```
error[E0665]: `#[derive(Default)]` on enum with no `#[default]`
  --> $DIR/macros-nonfatal-errors.rs:42:10
   |
LL |   #[derive(Default)]
   |            ^^^^^^^
LL | / enum NoDeclaredDefault {
LL | |     Foo,
LL | |     Bar,
LL | | }
   | |_- this enum needs a unit variant marked with `#[default]`
   |
   = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
help: make this unit variant default by placing `#[default]` on it
   |
LL |     #[default] Foo,
   |     ++++++++++
help: make this unit variant default by placing `#[default]` on it
   |
LL |     #[default] Bar,
   |     ++++++++++
```

6 files changed, 95 insertions, 52 deletions

diff --git a/compiler/rustc_builtin_macros/messages.ftl b/compiler/rustc_builtin_macros/messages.ftl
index 87d3d288013..7a31d2a2239 100644
--- a/compiler/rustc_builtin_macros/messages.ftl
+++ b/compiler/rustc_builtin_macros/messages.ftl
@@ -249,9 +249,9 @@ builtin_macros_naked_functions_testing_attribute =
     .label = function marked with testing attribute here
     .naked_attribute = `#[naked]` is incompatible with testing attributes
 
-builtin_macros_no_default_variant = no default declared
-    .help = make a unit variant default by placing `#[default]` above it
-    .suggestion = make `{$ident}` default
+builtin_macros_no_default_variant = `#[derive(Default)]` on enum with no `#[default]`
+    .label = this enum needs a unit variant marked with `#[default]`
+    .suggestion = make this unit variant default by placing `#[default]` on it
 
 builtin_macros_non_abi = at least one abi must be provided as an argument to `clobber_abi`
 
diff --git a/compiler/rustc_builtin_macros/src/deriving/default.rs b/compiler/rustc_builtin_macros/src/deriving/default.rs
index a7d9f608cbd..3c7bebd0f19 100644
--- a/compiler/rustc_builtin_macros/src/deriving/default.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/default.rs
@@ -42,7 +42,9 @@ pub(crate) fn expand_deriving_default(
                     StaticStruct(_, fields) => {
                         default_struct_substructure(cx, trait_span, substr, fields)
                     }
-                    StaticEnum(enum_def, _) => default_enum_substructure(cx, trait_span, enum_def),
+                    StaticEnum(enum_def, _) => {
+                        default_enum_substructure(cx, trait_span, enum_def, item.span())
+                    }
                     _ => cx.dcx().span_bug(trait_span, "method in `derive(Default)`"),
                 }
             })),
@@ -96,9 +98,10 @@ fn default_enum_substructure(
     cx: &ExtCtxt<'_>,
     trait_span: Span,
     enum_def: &EnumDef,
+    item_span: Span,
 ) -> BlockOrExpr {
     let expr = match try {
-        let default_variant = extract_default_variant(cx, enum_def, trait_span)?;
+        let default_variant = extract_default_variant(cx, enum_def, trait_span, item_span)?;
         validate_default_attribute(cx, default_variant)?;
         default_variant
     } {
@@ -146,6 +149,7 @@ fn extract_default_variant<'a>(
     cx: &ExtCtxt<'_>,
     enum_def: &'a EnumDef,
     trait_span: Span,
+    item_span: Span,
 ) -> Result<&'a rustc_ast::Variant, ErrorGuaranteed> {
     let default_variants: SmallVec<[_; 1]> = enum_def
         .variants
@@ -163,9 +167,10 @@ fn extract_default_variant<'a>(
                 .filter(|variant| !attr::contains_name(&variant.attrs, sym::non_exhaustive));
 
             let suggs = possible_defaults
-                .map(|v| errors::NoDefaultVariantSugg { span: v.span, ident: v.ident })
+                .map(|v| errors::NoDefaultVariantSugg { span: v.span.shrink_to_lo() })
                 .collect();
-            let guar = cx.dcx().emit_err(errors::NoDefaultVariant { span: trait_span, suggs });
+            let guar =
+                cx.dcx().emit_err(errors::NoDefaultVariant { span: trait_span, item_span, suggs });
 
             return Err(guar);
         }
diff --git a/compiler/rustc_builtin_macros/src/errors.rs b/compiler/rustc_builtin_macros/src/errors.rs
index b3198e7743d..1abdfdb9c65 100644
--- a/compiler/rustc_builtin_macros/src/errors.rs
+++ b/compiler/rustc_builtin_macros/src/errors.rs
@@ -369,26 +369,21 @@ pub(crate) struct DerivePathArgsValue {
 }
 
 #[derive(Diagnostic)]
-#[diag(builtin_macros_no_default_variant)]
-#[help]
+#[diag(builtin_macros_no_default_variant, code = E0665)]
 pub(crate) struct NoDefaultVariant {
     #[primary_span]
     pub(crate) span: Span,
+    #[label]
+    pub(crate) item_span: Span,
     #[subdiagnostic]
     pub(crate) suggs: Vec<NoDefaultVariantSugg>,
 }
 
 #[derive(Subdiagnostic)]
-#[suggestion(
-    builtin_macros_suggestion,
-    code = "#[default] {ident}",
-    applicability = "maybe-incorrect",
-    style = "tool-only"
-)]
+#[suggestion(builtin_macros_suggestion, code = "#[default] ", applicability = "maybe-incorrect")]
 pub(crate) struct NoDefaultVariantSugg {
     #[primary_span]
     pub(crate) span: Span,
-    pub(crate) ident: Ident,
 }
 
 #[derive(Diagnostic)]
diff --git a/compiler/rustc_error_codes/src/error_codes/E0665.md b/compiler/rustc_error_codes/src/error_codes/E0665.md
index ae54d6d1579..caa94423377 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0665.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0665.md
@@ -1,10 +1,9 @@
-#### Note: this error code is no longer emitted by the compiler.
-
-The `Default` trait was derived on an enum.
+The `Default` trait was derived on an enum without specifying the default
+variant.
 
 Erroneous code example:
 
-```compile_fail
+```compile_fail,E0665
 #[derive(Default)]
 enum Food {
     Sweet,
@@ -16,18 +15,30 @@ The `Default` cannot be derived on an enum for the simple reason that the
 compiler doesn't know which value to pick by default whereas it can for a
 struct as long as all its fields implement the `Default` trait as well.
 
-If you still want to implement `Default` on your enum, you'll have to do it "by
-hand":
+For the case where the desired default variant has no payload, you can
+annotate it with `#[default]` to derive it:
 
 ```
+#[derive(Default)]
 enum Food {
+    #[default]
     Sweet,
     Salty,
 }
+```
+
+In the case where the default variant does have a payload, you will have to
+implement `Default` on your enum manually:
+
+```
+enum Food {
+    Sweet(i32),
+    Salty,
+}
 
 impl Default for Food {
     fn default() -> Food {
-        Food::Sweet
+        Food::Sweet(1)
     }
 }
 ```
diff --git a/tests/ui/macros/macros-nonfatal-errors.rs b/tests/ui/macros/macros-nonfatal-errors.rs
index 46e865031ec..658455b1b5b 100644
--- a/tests/ui/macros/macros-nonfatal-errors.rs
+++ b/tests/ui/macros/macros-nonfatal-errors.rs
@@ -39,12 +39,18 @@ enum AttrOnInnerExpression {
     Baz,
 }
 
-#[derive(Default)] //~ ERROR no default declared
+#[derive(Default)] //~ ERROR `#[derive(Default)]` on enum with no `#[default]`
 enum NoDeclaredDefault {
     Foo,
     Bar,
 }
 
+#[derive(Default)] //~ ERROR `#[derive(Default)]` on enum with no `#[default]`
+enum NoDeclaredDefaultWithoutUnitVariant {
+    Foo(i32),
+    Bar(i32),
+}
+
 #[derive(Default)] //~ ERROR multiple declared defaults
 enum MultipleDefaults {
     #[default]
diff --git a/tests/ui/macros/macros-nonfatal-errors.stderr b/tests/ui/macros/macros-nonfatal-errors.stderr
index abf43e2a009..fd5e41986a8 100644
--- a/tests/ui/macros/macros-nonfatal-errors.stderr
+++ b/tests/ui/macros/macros-nonfatal-errors.stderr
@@ -46,18 +46,43 @@ LL |     Bar([u8; #[default] 1]),
    |
    = help: consider a manual implementation of `Default`
 
-error: no default declared
+error[E0665]: `#[derive(Default)]` on enum with no `#[default]`
   --> $DIR/macros-nonfatal-errors.rs:42:10
    |
-LL | #[derive(Default)]
-   |          ^^^^^^^
+LL |   #[derive(Default)]
+   |            ^^^^^^^
+LL | / enum NoDeclaredDefault {
+LL | |     Foo,
+LL | |     Bar,
+LL | | }
+   | |_- this enum needs a unit variant marked with `#[default]`
    |
-   = help: make a unit variant default by placing `#[default]` above it
    = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: make this unit variant default by placing `#[default]` on it
+   |
+LL |     #[default] Foo,
+   |     ++++++++++
+help: make this unit variant default by placing `#[default]` on it
+   |
+LL |     #[default] Bar,
+   |     ++++++++++
 
-error: multiple declared defaults
+error[E0665]: `#[derive(Default)]` on enum with no `#[default]`
   --> $DIR/macros-nonfatal-errors.rs:48:10
    |
+LL |   #[derive(Default)]
+   |            ^^^^^^^
+LL | / enum NoDeclaredDefaultWithoutUnitVariant {
+LL | |     Foo(i32),
+LL | |     Bar(i32),
+LL | | }
+   | |_- this enum needs a unit variant marked with `#[default]`
+   |
+   = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: multiple declared defaults
+  --> $DIR/macros-nonfatal-errors.rs:54:10
+   |
 LL | #[derive(Default)]
    |          ^^^^^^^
 ...
@@ -74,7 +99,7 @@ LL |     Baz,
    = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: `#[default]` attribute does not accept a value
-  --> $DIR/macros-nonfatal-errors.rs:60:5
+  --> $DIR/macros-nonfatal-errors.rs:66:5
    |
 LL |     #[default = 1]
    |     ^^^^^^^^^^^^^^
@@ -82,7 +107,7 @@ LL |     #[default = 1]
    = help: try using `#[default]`
 
 error: multiple `#[default]` attributes
-  --> $DIR/macros-nonfatal-errors.rs:68:5
+  --> $DIR/macros-nonfatal-errors.rs:74:5
    |
 LL |     #[default]
    |     ---------- `#[default]` used here
@@ -93,13 +118,13 @@ LL |     Foo,
    |
    = note: only one `#[default]` attribute is needed
 help: try removing this
-  --> $DIR/macros-nonfatal-errors.rs:67:5
+  --> $DIR/macros-nonfatal-errors.rs:73:5
    |
 LL |     #[default]
    |     ^^^^^^^^^^
 
 error: multiple `#[default]` attributes
-  --> $DIR/macros-nonfatal-errors.rs:78:5
+  --> $DIR/macros-nonfatal-errors.rs:84:5
    |
 LL |     #[default]
    |     ---------- `#[default]` used here
@@ -111,7 +136,7 @@ LL |     Foo,
    |
    = note: only one `#[default]` attribute is needed
 help: try removing these
-  --> $DIR/macros-nonfatal-errors.rs:75:5
+  --> $DIR/macros-nonfatal-errors.rs:81:5
    |
 LL |     #[default]
    |     ^^^^^^^^^^
@@ -121,7 +146,7 @@ LL |     #[default]
    |     ^^^^^^^^^^
 
 error: the `#[default]` attribute may only be used on unit enum variants
-  --> $DIR/macros-nonfatal-errors.rs:85:5
+  --> $DIR/macros-nonfatal-errors.rs:91:5
    |
 LL |     Foo {},
    |     ^^^
@@ -129,7 +154,7 @@ LL |     Foo {},
    = help: consider a manual implementation of `Default`
 
 error: default variant must be exhaustive
-  --> $DIR/macros-nonfatal-errors.rs:93:5
+  --> $DIR/macros-nonfatal-errors.rs:99:5
    |
 LL |     #[non_exhaustive]
    |     ----------------- declared `#[non_exhaustive]` here
@@ -139,37 +164,37 @@ LL |     Foo,
    = help: consider a manual implementation of `Default`
 
 error: asm template must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:98:10
+  --> $DIR/macros-nonfatal-errors.rs:104:10
    |
 LL |     asm!(invalid);
    |          ^^^^^^^
 
 error: `concat_idents!()` requires ident args
-  --> $DIR/macros-nonfatal-errors.rs:101:5
+  --> $DIR/macros-nonfatal-errors.rs:107:5
    |
 LL |     concat_idents!("not", "idents");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:103:17
+  --> $DIR/macros-nonfatal-errors.rs:109:17
    |
 LL |     option_env!(invalid);
    |                 ^^^^^^^
 
 error: expected string literal
-  --> $DIR/macros-nonfatal-errors.rs:104:10
+  --> $DIR/macros-nonfatal-errors.rs:110:10
    |
 LL |     env!(invalid);
    |          ^^^^^^^
 
 error: `env!()` takes 1 or 2 arguments
-  --> $DIR/macros-nonfatal-errors.rs:105:5
+  --> $DIR/macros-nonfatal-errors.rs:111:5
    |
 LL |     env!(foo, abr, baz);
    |     ^^^^^^^^^^^^^^^^^^^
 
 error: environment variable `RUST_HOPEFULLY_THIS_DOESNT_EXIST` not defined at compile time
-  --> $DIR/macros-nonfatal-errors.rs:106:5
+  --> $DIR/macros-nonfatal-errors.rs:112:5
    |
 LL |     env!("RUST_HOPEFULLY_THIS_DOESNT_EXIST");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -178,7 +203,7 @@ LL |     env!("RUST_HOPEFULLY_THIS_DOESNT_EXIST");
    = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: format argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:108:13
+  --> $DIR/macros-nonfatal-errors.rs:114:13
    |
 LL |     format!(invalid);
    |             ^^^^^^^
@@ -189,19 +214,19 @@ LL |     format!("{}", invalid);
    |             +++++
 
 error: argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:110:14
+  --> $DIR/macros-nonfatal-errors.rs:116:14
    |
 LL |     include!(invalid);
    |              ^^^^^^^
 
 error: argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:112:18
+  --> $DIR/macros-nonfatal-errors.rs:118:18
    |
 LL |     include_str!(invalid);
    |                  ^^^^^^^
 
 error: couldn't read `$DIR/i'd be quite surprised if a file with this name existed`: $FILE_NOT_FOUND_MSG
-  --> $DIR/macros-nonfatal-errors.rs:113:5
+  --> $DIR/macros-nonfatal-errors.rs:119:5
    |
 LL |     include_str!("i'd be quite surprised if a file with this name existed");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -209,13 +234,13 @@ LL |     include_str!("i'd be quite surprised if a file with this name existed")
    = note: this error originates in the macro `include_str` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:114:20
+  --> $DIR/macros-nonfatal-errors.rs:120:20
    |
 LL |     include_bytes!(invalid);
    |                    ^^^^^^^
 
 error: couldn't read `$DIR/i'd be quite surprised if a file with this name existed`: $FILE_NOT_FOUND_MSG
-  --> $DIR/macros-nonfatal-errors.rs:115:5
+  --> $DIR/macros-nonfatal-errors.rs:121:5
    |
 LL |     include_bytes!("i'd be quite surprised if a file with this name existed");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -223,13 +248,13 @@ LL |     include_bytes!("i'd be quite surprised if a file with this name existed
    = note: this error originates in the macro `include_bytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: trace_macros! accepts only `true` or `false`
-  --> $DIR/macros-nonfatal-errors.rs:117:5
+  --> $DIR/macros-nonfatal-errors.rs:123:5
    |
 LL |     trace_macros!(invalid);
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
 error: default variant must be exhaustive
-  --> $DIR/macros-nonfatal-errors.rs:127:9
+  --> $DIR/macros-nonfatal-errors.rs:133:9
    |
 LL |         #[non_exhaustive]
    |         ----------------- declared `#[non_exhaustive]` here
@@ -239,10 +264,11 @@ LL |         Foo,
    = help: consider a manual implementation of `Default`
 
 error: cannot find macro `llvm_asm` in this scope
-  --> $DIR/macros-nonfatal-errors.rs:99:5
+  --> $DIR/macros-nonfatal-errors.rs:105:5
    |
 LL |     llvm_asm!(invalid);
    |     ^^^^^^^^
 
-error: aborting due to 28 previous errors
+error: aborting due to 29 previous errors
 
+For more information about this error, try `rustc --explain E0665`.