Rollup merge of #103478 - SpanishPear:spanishpear/issue_103366_fix, r=TaKO8Ki

 Suggest fix for misplaced generic params on fn item #103366

fixes #103366

This still has some work to go, but works for 2/3 of the initial base cases described in #1033366

simple fn:
```
error: expected identifier, found `<`
 --> shreys/test_1.rs:1:3
  |
1 | fn<T> id(x: T) -> T { x }
  |   ^ expected identifier
  |
help: help: place the generic parameter list after the function name:
  |
1 | fn id<T>(x: T) -> T { x }
  |    ~~~~

```

Complicated bounds
```
error: expected identifier, found `<`
 --> spanishpear/test_2.rs:1:3
  |
1 | fn<'a, B: 'a + std::ops::Add<Output = u32>> f(_x: B) { }
  |   ^ expected identifier
  |
help: help: place the generic parameter list after the function name:
  |
1 | fn f<'a, B: 'a + std::ops::Add<Output = u32>>(_x: B) { }
  |    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Opening a draft PR for comments on approach, particularly I have the following questions:
 -  [x]  Is it okay to be using `err.span_suggestion` over struct derives? I struggled to get the initial implementation (particularly the correct suggestion message) on struct derives, although I think given what I've learned since starting, I could attempt re-doing it with that approach.
  -  [x] in the case where the snippet cannot be obtained from a span, is the `help` but no suggestion okay? I think yes (also, when does this case occur?)
  -  [x] are there any red flags for the generalisation of this work for relevant item kinds (i.e. `struct`, `enum`, `trait`, and `union`). My basic testing indicates it does work for those types except the help tip is currently hardcoded to `after the function name` - which should change dependent on the item.
  - [x] I am planning to not show the suggestion if there is already a `<` after the item identifier, (i.e. if there are already generics, as after a function name per the original issue). Any major objections?
  - [x] Is the style of error okay? I wasn't sure if there was a way to make it display nicer, or if thats handled by span_suggestion

These aren't blocking questions, and I will keep working on:
  - check if there is a `<` after the ident (and if so, not showing the suggestion)
  - generalize the help message
  - figuring out how to write/run/etc ui tests (including reading the docs for them)
  - logic cleanups

22 files changed, 231 insertions, 0 deletions

diff --git a/tests/ui/parser/suggest_misplaced_generics/enum.fixed b/tests/ui/parser/suggest_misplaced_generics/enum.fixed
new file mode 100644
index 00000000000..3332118a1e7
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/enum.fixed
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+enum Foo<T> { Variant(T) }
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the enum name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/enum.rs b/tests/ui/parser/suggest_misplaced_generics/enum.rs
new file mode 100644
index 00000000000..5a2289c5c5a
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/enum.rs
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+enum<T> Foo { Variant(T) }
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the enum name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/enum.stderr b/tests/ui/parser/suggest_misplaced_generics/enum.stderr
new file mode 100644
index 00000000000..5f5947627ee
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/enum.stderr
@@ -0,0 +1,14 @@
+error: expected identifier, found `<`
+  --> $DIR/enum.rs:5:5
+   |
+LL | enum<T> Foo { Variant(T) }
+   |     ^ expected identifier
+   |
+help: place the generic parameter name after the enum name
+   |
+LL - enum<T> Foo { Variant(T) }
+LL + enum Foo<T> { Variant(T) }
+   |
+
+error: aborting due to previous error
+
diff --git a/tests/ui/parser/suggest_misplaced_generics/existing_generics.rs b/tests/ui/parser/suggest_misplaced_generics/existing_generics.rs
new file mode 100644
index 00000000000..1dc182398d8
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/existing_generics.rs
@@ -0,0 +1,9 @@
+// Issue: 103366
+// there is already an existing generic on f, so don't show a suggestion
+
+#[allow(unused)]
+fn<'a, B: 'a + std::ops::Add<Output = u32>> f<T>(_x: B) { }
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the fn name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/existing_generics.stderr b/tests/ui/parser/suggest_misplaced_generics/existing_generics.stderr
new file mode 100644
index 00000000000..89716e6f1ed
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/existing_generics.stderr
@@ -0,0 +1,10 @@
+error: expected identifier, found `<`
+  --> $DIR/existing_generics.rs:5:3
+   |
+LL | fn<'a, B: 'a + std::ops::Add<Output = u32>> f<T>(_x: B) { }
+   |   ^ expected identifier
+   |
+   = help: place the generic parameter name after the fn name
+
+error: aborting due to previous error
+
diff --git a/tests/ui/parser/suggest_misplaced_generics/fn-complex-generics.fixed b/tests/ui/parser/suggest_misplaced_generics/fn-complex-generics.fixed
new file mode 100644
index 00000000000..84bf64bd63c
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/fn-complex-generics.fixed
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+fn f<'a, B: 'a + std::ops::Add<Output = u32>>(_x: B) { }
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the fn name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/fn-complex-generics.rs b/tests/ui/parser/suggest_misplaced_generics/fn-complex-generics.rs
new file mode 100644
index 00000000000..d0684397e74
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/fn-complex-generics.rs
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+fn<'a, B: 'a + std::ops::Add<Output = u32>> f(_x: B) { }
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the fn name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/fn-complex-generics.stderr b/tests/ui/parser/suggest_misplaced_generics/fn-complex-generics.stderr
new file mode 100644
index 00000000000..061d0910a74
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/fn-complex-generics.stderr
@@ -0,0 +1,14 @@
+error: expected identifier, found `<`
+  --> $DIR/fn-complex-generics.rs:5:3
+   |
+LL | fn<'a, B: 'a + std::ops::Add<Output = u32>> f(_x: B) { }
+   |   ^ expected identifier
+   |
+help: place the generic parameter name after the fn name
+   |
+LL - fn<'a, B: 'a + std::ops::Add<Output = u32>> f(_x: B) { }
+LL + fn f<'a, B: 'a + std::ops::Add<Output = u32>>(_x: B) { }
+   |
+
+error: aborting due to previous error
+
diff --git a/tests/ui/parser/suggest_misplaced_generics/fn-invalid-generics.rs b/tests/ui/parser/suggest_misplaced_generics/fn-invalid-generics.rs
new file mode 100644
index 00000000000..7fcb6a82ce4
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/fn-invalid-generics.rs
@@ -0,0 +1,8 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// The generics fail to parse here, so don't make any suggestions/help
+
+#[allow(unused)]
+fn<~>()> id(x: T) -> T { x }
+//~^ ERROR expected identifier, found `<`
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/fn-invalid-generics.stderr b/tests/ui/parser/suggest_misplaced_generics/fn-invalid-generics.stderr
new file mode 100644
index 00000000000..47e12016938
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/fn-invalid-generics.stderr
@@ -0,0 +1,8 @@
+error: expected identifier, found `<`
+  --> $DIR/fn-invalid-generics.rs:5:3
+   |
+LL | fn<~>()> id(x: T) -> T { x }
+   |   ^ expected identifier
+
+error: aborting due to previous error
+
diff --git a/tests/ui/parser/suggest_misplaced_generics/fn-simple.fixed b/tests/ui/parser/suggest_misplaced_generics/fn-simple.fixed
new file mode 100644
index 00000000000..cbfd5f2d39c
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/fn-simple.fixed
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+fn id<T>(x: T) -> T { x }
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the fn name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/fn-simple.rs b/tests/ui/parser/suggest_misplaced_generics/fn-simple.rs
new file mode 100644
index 00000000000..b207cf70d85
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/fn-simple.rs
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+fn<T> id(x: T) -> T { x }
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the fn name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/fn-simple.stderr b/tests/ui/parser/suggest_misplaced_generics/fn-simple.stderr
new file mode 100644
index 00000000000..e749f1a0d00
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/fn-simple.stderr
@@ -0,0 +1,14 @@
+error: expected identifier, found `<`
+  --> $DIR/fn-simple.rs:5:3
+   |
+LL | fn<T> id(x: T) -> T { x }
+   |   ^ expected identifier
+   |
+help: place the generic parameter name after the fn name
+   |
+LL - fn<T> id(x: T) -> T { x }
+LL + fn id<T>(x: T) -> T { x }
+   |
+
+error: aborting due to previous error
+
diff --git a/tests/ui/parser/suggest_misplaced_generics/struct.fixed b/tests/ui/parser/suggest_misplaced_generics/struct.fixed
new file mode 100644
index 00000000000..fec05bdeca1
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/struct.fixed
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+struct Foo<T> { x: T }
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the struct name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/struct.rs b/tests/ui/parser/suggest_misplaced_generics/struct.rs
new file mode 100644
index 00000000000..6b80150d546
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/struct.rs
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+struct<T> Foo { x: T }
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the struct name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/struct.stderr b/tests/ui/parser/suggest_misplaced_generics/struct.stderr
new file mode 100644
index 00000000000..2b650907092
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/struct.stderr
@@ -0,0 +1,14 @@
+error: expected identifier, found `<`
+  --> $DIR/struct.rs:5:7
+   |
+LL | struct<T> Foo { x: T }
+   |       ^ expected identifier
+   |
+help: place the generic parameter name after the struct name
+   |
+LL - struct<T> Foo { x: T }
+LL + struct Foo<T> { x: T }
+   |
+
+error: aborting due to previous error
+
diff --git a/tests/ui/parser/suggest_misplaced_generics/trait.fixed b/tests/ui/parser/suggest_misplaced_generics/trait.fixed
new file mode 100644
index 00000000000..a471a078af1
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/trait.fixed
@@ -0,0 +1,11 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+trait Foo<T> {
+    //~^ ERROR expected identifier, found `<`
+    //~| HELP place the generic parameter name after the trait name
+}
+
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/trait.rs b/tests/ui/parser/suggest_misplaced_generics/trait.rs
new file mode 100644
index 00000000000..55355f451f9
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/trait.rs
@@ -0,0 +1,11 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+trait<T> Foo {
+    //~^ ERROR expected identifier, found `<`
+    //~| HELP place the generic parameter name after the trait name
+}
+
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/trait.stderr b/tests/ui/parser/suggest_misplaced_generics/trait.stderr
new file mode 100644
index 00000000000..ac86cfa4697
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/trait.stderr
@@ -0,0 +1,14 @@
+error: expected identifier, found `<`
+  --> $DIR/trait.rs:5:6
+   |
+LL | trait<T> Foo {
+   |      ^ expected identifier
+   |
+help: place the generic parameter name after the trait name
+   |
+LL - trait<T> Foo {
+LL + trait Foo<T> {
+   |
+
+error: aborting due to previous error
+
diff --git a/tests/ui/parser/suggest_misplaced_generics/type.fixed b/tests/ui/parser/suggest_misplaced_generics/type.fixed
new file mode 100644
index 00000000000..a97b9e66d0b
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/type.fixed
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+type Foo<T> = T;
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the type name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/type.rs b/tests/ui/parser/suggest_misplaced_generics/type.rs
new file mode 100644
index 00000000000..17e200536fa
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/type.rs
@@ -0,0 +1,9 @@
+// Issue: 103366 , Suggest fix for misplaced generic params
+// run-rustfix
+
+#[allow(unused)]
+type<T> Foo = T;
+//~^ ERROR expected identifier, found `<`
+//~| HELP place the generic parameter name after the type name
+
+fn main() {}
diff --git a/tests/ui/parser/suggest_misplaced_generics/type.stderr b/tests/ui/parser/suggest_misplaced_generics/type.stderr
new file mode 100644
index 00000000000..22744f6cf37
--- /dev/null
+++ b/tests/ui/parser/suggest_misplaced_generics/type.stderr
@@ -0,0 +1,14 @@
+error: expected identifier, found `<`
+  --> $DIR/type.rs:5:5
+   |
+LL | type<T> Foo = T;
+   |     ^ expected identifier
+   |
+help: place the generic parameter name after the type name
+   |
+LL - type<T> Foo = T;
+LL + type Foo<T> = T;
+   |
+
+error: aborting due to previous error
+