Fix span calculation in format strings

Author: FabianWolff

Diff for: compiler/rustc_builtin_macros/src/format.rs

@@ -939,6 +939,7 @@ pub fn expand_preparsed_format_args(
 
     let msg = "format argument must be a string literal";
     let fmt_sp = efmt.span;
+    let efmt_kind_is_lit: bool = matches!(efmt.kind, ast::ExprKind::Lit(_));
     let (fmt_str, fmt_style, fmt_span) = match expr_to_spanned_string(ecx, efmt, msg) {
         Ok(mut fmt) if append_newline => {
             fmt.0 = Symbol::intern(&format!("{}\n", fmt.0));
@@ -989,7 +990,19 @@ pub fn expand_preparsed_format_args(
 
     if !parser.errors.is_empty() {
         let err = parser.errors.remove(0);
-        let sp = fmt_span.from_inner(err.span);
+        let sp = if efmt_kind_is_lit {
+            fmt_span.from_inner(err.span)
+        } else {
+            // The format string could be another macro invocation, e.g.:
+            //     format!(concat!("abc", "{}"), 4);
+            // However, `err.span` is an inner span relative to the *result* of
+            // the macro invocation, which is why we would get a nonsensical
+            // result calling `fmt_span.from_inner(err.span)` as above, and
+            // might even end up inside a multibyte character (issue #86085).
+            // Therefore, we conservatively report the error for the entire
+            // argument span here.
+            fmt_span
+        };
         let mut e = ecx.struct_span_err(sp, &format!("invalid format string: {}", err.description));
         e.span_label(sp, err.label + " in format string");
         if let Some(note) = err.note {

Diff for: src/test/ui/fmt/format-concat-span.rs

@@ -0,0 +1,15 @@
+// If the format string is another macro invocation, rustc would previously
+// compute nonsensical spans, such as:
+//
+//   error: invalid format string: unmatched `}` found
+//    --> test.rs:2:17
+//     |
+//   2 |     format!(concat!("abc}"));
+//     |                 ^ unmatched `}` in format string
+//
+// This test checks that this behavior has been fixed.
+
+fn main() {
+    format!(concat!("abc}"));
+    //~^ ERROR: invalid format string: unmatched `}` found
+}

Diff for: src/test/ui/fmt/format-concat-span.stderr

@@ -0,0 +1,11 @@
+error: invalid format string: unmatched `}` found
+  --> $DIR/format-concat-span.rs:13:13
+   |
+LL |     format!(concat!("abc}"));
+   |             ^^^^^^^^^^^^^^^ unmatched `}` in format string
+   |
+   = note: if you intended to print `}`, you can escape it using `}}`
+   = note: this error originates in the macro `concat` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+

Diff for: src/test/ui/fmt/issue-86085.rs

@@ -0,0 +1,6 @@
+// Tests for an ICE with the fuzzed input below.
+
+fn main ( ) {
+format ! ( concat ! ( r#"lJ𐏿Æ�.𐏿�"# , "r} {}" )     ) ;
+//~^ ERROR: invalid format string: unmatched `}` found
+}

Diff for: src/test/ui/fmt/issue-86085.stderr

@@ -0,0 +1,11 @@
+error: invalid format string: unmatched `}` found
+  --> $DIR/issue-86085.rs:4:12
+   |
+LL | format ! ( concat ! ( r#"lJ𐏿Æ�.𐏿�"# , "r} {}" )     ) ;
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unmatched `}` in format string
+   |
+   = note: if you intended to print `}`, you can escape it using `}}`
+   = note: this error originates in the macro `concat` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+