Better message for invalid keyword placement in fn

After this commit, `unsafe async fn ...` now suggests the `async unsafe` fix
instead of misunderstanding the issue.

This is not perfect for repeated keywords (`const async const`) and for
keywords that are misplaced after `extern "some abi"` because of the way
`check_fn_font_matter` works, but changing it breaks so many tests and
diagnostics it has been judged too high a cost for this PR.

src/test/ui/async-await/no-async-const.stderr

@@ -2,7 +2,12 @@ error: expected one of `extern`, `fn`, or `unsafe`, found keyword `const`
   --> $DIR/no-async-const.rs:4:11
    |
 LL | pub async const fn x() {}
-   |           ^^^^^ expected one of `extern`, `fn`, or `unsafe`
+   |     ------^^^^^
+   |     |     |
+   |     |     expected one of `extern`, `fn`, or `unsafe`
+   |     help: `const` must come before `async`: `const async`
+   |
+   = note: keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`
 
 error: aborting due to previous error
 

src/test/ui/async-await/no-unsafe-async.stderr

@@ -5,15 +5,25 @@ LL | impl S {
    |        - while parsing this item list starting here
 LL |     #[cfg(FALSE)]
 LL |     unsafe async fn g() {}
-   |            ^^^^^ expected one of `extern` or `fn`
+   |     -------^^^^^
+   |     |      |
+   |     |      expected one of `extern` or `fn`
+   |     help: `async` must come before `unsafe`: `async unsafe`
 LL | }
    | - the item list ends here
+   |
+   = note: keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`
 
 error: expected one of `extern` or `fn`, found keyword `async`
   --> $DIR/no-unsafe-async.rs:11:8
    |
 LL | unsafe async fn f() {}
-   |        ^^^^^ expected one of `extern` or `fn`
+   | -------^^^^^
+   | |      |
+   | |      expected one of `extern` or `fn`
+   | help: `async` must come before `unsafe`: `async unsafe`
+   |
+   = note: keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`
 
 error: aborting due to 2 previous errors
 

src/test/ui/parser/issue-87217-keyword-order/const-async-const.rs

@@ -0,0 +1,11 @@
+// edition:2018
+
+// Test that even when `const` is already present, the proposed fix is `const const async`,
+// like for `pub pub`.
+
+const async const fn test() {}
+//~^ ERROR expected one of `extern`, `fn`, or `unsafe`, found keyword `const`
+//~| NOTE expected one of `extern`, `fn`, or `unsafe`
+//~| HELP `const` must come before `async`
+//~| SUGGESTION const async
+//~| NOTE keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`

src/test/ui/parser/issue-87217-keyword-order/const-async-const.stderr

@@ -0,0 +1,13 @@
+error: expected one of `extern`, `fn`, or `unsafe`, found keyword `const`
+  --> $DIR/const-async-const.rs:6:13
+   |
+LL | const async const fn test() {}
+   |       ------^^^^^
+   |       |     |
+   |       |     expected one of `extern`, `fn`, or `unsafe`
+   |       help: `const` must come before `async`: `const async`
+   |
+   = note: keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`
+
+error: aborting due to previous error
+

src/test/ui/parser/issue-87217-keyword-order/several-kw-jump.rs

@@ -0,0 +1,14 @@
+// edition:2018
+
+// There is an order to respect for keywords before a function:
+// `<visibility>, const, async, unsafe, extern, "<ABI>"`
+//
+// This test ensures the compiler is helpful about them being misplaced.
+// Visibilities are tested elsewhere.
+
+async unsafe const fn test() {}
+//~^ ERROR expected one of `extern` or `fn`, found keyword `const`
+//~| NOTE expected one of `extern` or `fn`
+//~| HELP `const` must come before `async unsafe`
+//~| SUGGESTION const async unsafe
+//~| NOTE keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`

src/test/ui/parser/issue-87217-keyword-order/several-kw-jump.stderr

@@ -0,0 +1,13 @@
+error: expected one of `extern` or `fn`, found keyword `const`
+  --> $DIR/several-kw-jump.rs:9:14
+   |
+LL | async unsafe const fn test() {}
+   | -------------^^^^^
+   | |            |
+   | |            expected one of `extern` or `fn`
+   | help: `const` must come before `async unsafe`: `const async unsafe`
+   |
+   = note: keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`
+
+error: aborting due to previous error
+

src/test/ui/parser/issue-87217-keyword-order/wrong-async.rs

@@ -0,0 +1,14 @@
+// edition:2018
+
+// There is an order to respect for keywords before a function:
+// `<visibility>, const, async, unsafe, extern, "<ABI>"`
+//
+// This test ensures the compiler is helpful about them being misplaced.
+// Visibilities are tested elsewhere.
+
+unsafe async fn test() {}
+//~^ ERROR expected one of `extern` or `fn`, found keyword `async`
+//~| NOTE expected one of `extern` or `fn`
+//~| HELP `async` must come before `unsafe`
+//~| SUGGESTION async unsafe
+//~| NOTE keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`

src/test/ui/parser/issue-87217-keyword-order/wrong-async.stderr

@@ -0,0 +1,13 @@
+error: expected one of `extern` or `fn`, found keyword `async`
+  --> $DIR/wrong-async.rs:9:8
+   |
+LL | unsafe async fn test() {}
+   | -------^^^^^
+   | |      |
+   | |      expected one of `extern` or `fn`
+   | help: `async` must come before `unsafe`: `async unsafe`
+   |
+   = note: keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`
+
+error: aborting due to previous error
+

src/test/ui/parser/issue-87217-keyword-order/wrong-const.rs

@@ -0,0 +1,14 @@
+// edition:2018
+
+// There is an order to respect for keywords before a function:
+// `<visibility>, const, async, unsafe, extern, "<ABI>"`
+//
+// This test ensures the compiler is helpful about them being misplaced.
+// Visibilities are tested elsewhere.
+
+unsafe const fn test() {}
+//~^ ERROR expected one of `extern` or `fn`, found keyword `const`
+//~| NOTE expected one of `extern` or `fn`
+//~| HELP `const` must come before `unsafe`
+//~| SUGGESTION const unsafe
+//~| NOTE keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`

src/test/ui/parser/issue-87217-keyword-order/wrong-const.stderr

@@ -0,0 +1,13 @@
+error: expected one of `extern` or `fn`, found keyword `const`
+  --> $DIR/wrong-const.rs:9:8
+   |
+LL | unsafe const fn test() {}
+   | -------^^^^^
+   | |      |
+   | |      expected one of `extern` or `fn`
+   | help: `const` must come before `unsafe`: `const unsafe`
+   |
+   = note: keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`
+
+error: aborting due to previous error
+

src/test/ui/parser/issue-87217-keyword-order/wrong-unsafe.rs

@@ -0,0 +1,14 @@
+// edition:2018
+
+// There is an order to respect for keywords before a function:
+// `<visibility>, const, async, unsafe, extern, "<ABI>"`
+//
+// This test ensures the compiler is helpful about them being misplaced.
+// Visibilities are tested elsewhere.
+
+extern unsafe fn test() {}
+//~^ ERROR expected `fn`, found keyword `unsafe`
+//~| NOTE expected `fn`
+//~| HELP `unsafe` must come before `extern`
+//~| SUGGESTION unsafe extern
+//~| NOTE keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`

src/test/ui/parser/issue-87217-keyword-order/wrong-unsafe.stderr

@@ -0,0 +1,13 @@
+error: expected `fn`, found keyword `unsafe`
+  --> $DIR/wrong-unsafe.rs:9:8
+   |
+LL | extern unsafe fn test() {}
+   | -------^^^^^^
+   | |      |
+   | |      expected `fn`
+   | help: `unsafe` must come before `extern`: `unsafe extern`
+   |
+   = note: keyword order for functions declaration is `default`, `pub`, `const`, `async`, `unsafe`, `extern`
+
+error: aborting due to previous error
+