Rollup merge of #69041 - petrochenkov:stabmodispan, r=Amanieu

proc_macro: Stabilize `Span::resolved_at` and `Span::located_at`

Introduced in https://github.com/rust-lang/rust/pull/47149.
Part of https://github.com/rust-lang/rust/issues/54725.

Motivation: https://github.com/rust-lang/rust/pull/68716#issuecomment-583918919.
Identifiers in proc macros may want to inherit span locations for diagnostics from one tokens (e.g. some tokens from the macro input), but resolve those identifiers from some different location (e.g. from the macro's definition site).
This becomes especially important when multiple resolution locations become available with stabilization of [`Span::mixed_site`](https://github.com/rust-lang/rust/pull/68716).

Why I think this is the right API for setting span's location and hygiene - https://github.com/rust-lang/rust/pull/69041#issuecomment-586644778.

r? @dtolnay

src/test/ui/proc-macro/auxiliary/resolved-located-at.rs

@@ -0,0 +1,32 @@
+// force-host
+// no-prefer-dynamic
+
+#![feature(proc_macro_def_site)]
+#![feature(proc_macro_diagnostic)]
+#![feature(proc_macro_hygiene)]
+#![feature(proc_macro_quote)]
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+use proc_macro::*;
+
+#[proc_macro]
+pub fn resolve_located_at(input: TokenStream) -> TokenStream {
+    match &*input.into_iter().collect::<Vec<_>>() {
+        [a, b, ..] => {
+            // The error is reported at input `a`.
+            let mut diag = Diagnostic::new(Level::Error, "expected error");
+            diag.set_spans(Span::def_site().located_at(a.span()));
+            diag.emit();
+
+            // Resolves to `struct S;` at def site, but the error is reported at input `b`.
+            let s = TokenTree::Ident(Ident::new("S", b.span().resolved_at(Span::def_site())));
+            quote!({
+                struct S;
+
+                $s
+            })
+        }
+        _ => panic!("unexpected input"),
+    }
+}

src/test/ui/proc-macro/resolved-located-at.rs

@@ -0,0 +1,12 @@
+// aux-build:resolved-located-at.rs
+
+#![feature(proc_macro_hygiene)]
+
+#[macro_use]
+extern crate resolved_located_at;
+
+fn main() {
+    resolve_located_at!(a b)
+    //~^ ERROR expected error
+    //~| ERROR mismatched types
+}

src/test/ui/proc-macro/resolved-located-at.stderr

@@ -0,0 +1,21 @@
+error: expected error
+  --> $DIR/resolved-located-at.rs:9:25
+   |
+LL |     resolve_located_at!(a b)
+   |                         ^
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> $DIR/resolved-located-at.rs:9:27
+   |
+LL | fn main() {
+   |           - expected `()` because of default return type
+LL |     resolve_located_at!(a b)
+   |                           ^ expected `()`, found struct `main::S`
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.