Point at invalid utf-8 span on user's source code

```
error: couldn't read `$DIR/not-utf8-bin-file.rs`: stream did not contain valid UTF-8
  --> $DIR/not-utf8-2.rs:6:5
   |
LL |     include!("not-utf8-bin-file.rs");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: `[193]` is not valid utf-8
  --> $DIR/not-utf8-bin-file.rs:2:14
   |
LL |     let _ = "�|�␂!5�cc␕␂��";
   |              ^
   = note: this error originates in the macro `include` (in Nightly builds, run with -Z macro-backtrace for more info)
```

When we attempt to load a Rust source code file, if there is a OS file failure we try reading the file as bytes. If that succeeds we try to turn it into UTF-8. If *that* fails, we provide additional context about *where* the file has the first invalid UTF-8 character.

Fix #76869.

compiler/rustc_builtin_macros/src/lib.rs

@@ -16,6 +16,7 @@
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_quote)]
 #![feature(rustdoc_internals)]
+#![feature(string_from_utf8_lossy_owned)]
 #![feature(try_blocks)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end

compiler/rustc_builtin_macros/src/source_util.rs

@@ -210,8 +210,34 @@ pub(crate) fn expand_include_str(
                 MacEager::expr(cx.expr_str(cx.with_def_site_ctxt(bsp), interned_src))
             }
             Err(_) => {
-                let guar = cx.dcx().span_err(sp, format!("`{path}` wasn't a utf-8 file"));
-                DummyResult::any(sp, guar)
+                let mut err = cx.dcx().struct_span_err(sp, format!("`{path}` wasn't a utf-8 file"));
+                let path = PathBuf::from(path.as_str());
+                if let Ok(contents) = std::fs::read(&path)
+                    && let Err(utf8err) = String::from_utf8(contents)
+                {
+                    let start = utf8err.utf8_error().valid_up_to();
+                    let note = format!("invalid utf-8 at byte `{start}`");
+                    let msg = if let Some(len) = utf8err.utf8_error().error_len() {
+                        format!(
+                            "`{:?}` is not valid utf-8",
+                            &utf8err.as_bytes()[start..start + len]
+                        )
+                    } else {
+                        note.clone()
+                    };
+                    let contents = utf8err.into_utf8_lossy();
+                    let source = cx.source_map().new_source_file(path.into(), contents);
+                    let span = Span::with_root_ctxt(
+                        source.normalized_byte_pos(start as u32),
+                        source.normalized_byte_pos(start as u32),
+                    );
+                    if span.is_dummy() {
+                        err.note(note);
+                    } else {
+                        err.span_note(span, msg);
+                    }
+                }
+                DummyResult::any(sp, err.emit())
             }
         },
         Err(dummy) => dummy,
@@ -273,7 +299,7 @@ fn load_binary_file(
                     .and_then(|path| path.into_os_string().into_string().ok());
 
                 if let Some(new_path) = new_path {
-                    err.span_suggestion(
+                    err.span_suggestion_verbose(
                         path_span,
                         "there is a file with the same name in a different directory",
                         format!("\"{}\"", new_path.replace('\\', "/").escape_debug()),

compiler/rustc_parse/src/lib.rs

@@ -11,6 +11,7 @@
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
+#![feature(string_from_utf8_lossy_owned)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
@@ -74,8 +75,36 @@ pub fn new_parser_from_file<'a>(
     sp: Option<Span>,
 ) -> Result<Parser<'a>, Vec<Diag<'a>>> {
     let source_file = psess.source_map().load_file(path).unwrap_or_else(|e| {
-        let msg = format!("couldn't read {}: {}", path.display(), e);
+        let msg = format!("couldn't read `{}`: {}", path.display(), e);
         let mut err = psess.dcx().struct_fatal(msg);
+        if let Ok(contents) = std::fs::read(path)
+            && let Err(utf8err) = String::from_utf8(contents)
+        {
+            // The file exists, but it wasn't valid UTF-8.
+            let start = utf8err.utf8_error().valid_up_to();
+            let note = format!("invalid utf-8 at byte `{start}`");
+            let msg = if let Some(len) = utf8err.utf8_error().error_len() {
+                format!("`{:?}` is not valid utf-8", &utf8err.as_bytes()[start..start + len])
+            } else {
+                note.clone()
+            };
+            let contents = utf8err.into_utf8_lossy();
+            let source = psess.source_map().new_source_file(path.to_owned().into(), contents);
+            let span = Span::with_root_ctxt(
+                source.normalized_byte_pos(start as u32),
+                source.normalized_byte_pos(start as u32),
+            );
+            if span.is_dummy() {
+                err.note(note);
+            } else {
+                if sp.is_some() {
+                    err.span_note(span, msg);
+                } else {
+                    err.span(span);
+                    err.span_label(span, msg);
+                }
+            }
+        }
         if let Some(sp) = sp {
             err.span(sp);
         }

src/tools/compiletest/src/errors.rs

@@ -101,6 +101,8 @@ pub fn load_errors(testfile: &Path, revision: Option<&str>) -> Vec<Error> {
 
     rdr.lines()
         .enumerate()
+        // We want to ignore utf-8 failures in tests during collection of annotations.
+        .filter(|(_, line)| line.is_ok())
         .filter_map(|(line_num, line)| {
             parse_expected(last_nonfollow_error, line_num + 1, &line.unwrap(), revision).map(
                 |(which, error)| {

tests/ui/macros/not-utf8-2.rs

@@ -0,0 +1,7 @@
+//@ error-pattern: did not contain valid UTF-8
+//@ reference: input.encoding.utf8
+//@ reference: input.encoding.invalid
+
+fn foo() {
+    include!("not-utf8-bin-file.rs");
+}

tests/ui/macros/not-utf8-2.stderr

@@ -0,0 +1,15 @@
+error: couldn't read `$DIR/not-utf8-bin-file.rs`: stream did not contain valid UTF-8
+  --> $DIR/not-utf8-2.rs:6:5
+   |
+LL |     include!("not-utf8-bin-file.rs");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: `[193]` is not valid utf-8
+  --> $DIR/not-utf8-bin-file.rs:2:14
+   |
+LL |     let _ = "�|�␂!5�cc␕␂��";
+   |              ^
+   = note: this error originates in the macro `include` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+

tests/ui/macros/not-utf8-bin-file.rs

@@ -0,0 +1,4 @@
+fn main() {
+    let _ = "Á|Õ!5¢ccŒÓ";
+    //~^ ERROR stream did not contain valid UTF-8
+}

tests/ui/macros/not-utf8-bin-file.stderr

@@ -0,0 +1,8 @@
+error: couldn't read `$DIR/not-utf8-bin-file.rs`: stream did not contain valid UTF-8
+  --> $DIR/not-utf8-bin-file.rs:2:14
+   |
+LL |     let _ = "�|�␂!5�cc␕␂��";
+   |              ^ `[193]` is not valid utf-8
+
+error: aborting due to 1 previous error
+

tests/ui/macros/not-utf8.rs

@@ -3,5 +3,5 @@
 //@ reference: input.encoding.invalid
 
 fn foo() {
-    include!("not-utf8.bin")
+    include!("not-utf8.bin");
 }

tests/ui/macros/not-utf8.stderr

@@ -1,9 +1,14 @@
-error: couldn't read $DIR/not-utf8.bin: stream did not contain valid UTF-8
+error: couldn't read `$DIR/not-utf8.bin`: stream did not contain valid UTF-8
   --> $DIR/not-utf8.rs:6:5
    |
-LL |     include!("not-utf8.bin")
+LL |     include!("not-utf8.bin");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
    |
+note: `[193]` is not valid utf-8
+  --> $DIR/not-utf8.bin:1:1
+   |
+LL | �|�␂!5�cc␕␂�Ӻi��WWj�ȥ�'�}�␒�J�ȉ��W�␞O�@����␜w�V���LO����␔[ ␃_�'���SQ�~ذ��ų&��-    ��lN~��!@␌ _#���kQ��h�␝�:�...
+   | ^
    = note: this error originates in the macro `include` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error

tests/ui/modules/path-no-file-name.rs

@@ -1,4 +1,4 @@
-//@ normalize-stderr: "\.:.*\(" -> ".: $$ACCESS_DENIED_MSG ("
+//@ normalize-stderr: "\.`:.*\(" -> ".`: $$ACCESS_DENIED_MSG ("
 //@ normalize-stderr: "os error \d+" -> "os error $$ACCESS_DENIED_CODE"
 
 #[path = "."]

tests/ui/modules/path-no-file-name.stderr

@@ -1,4 +1,4 @@
-error: couldn't read $DIR/.: $ACCESS_DENIED_MSG (os error $ACCESS_DENIED_CODE)
+error: couldn't read `$DIR/.`: $ACCESS_DENIED_MSG (os error $ACCESS_DENIED_CODE)
   --> $DIR/path-no-file-name.rs:5:1
    |
 LL | mod m;

tests/ui/parser/issues/issue-5806.rs

@@ -1,4 +1,4 @@
-//@ normalize-stderr: "parser:.*\(" -> "parser: $$ACCESS_DENIED_MSG ("
+//@ normalize-stderr: "parser`:.*\(" -> "parser`: $$ACCESS_DENIED_MSG ("
 //@ normalize-stderr: "os error \d+" -> "os error $$ACCESS_DENIED_CODE"
 
 #[path = "../parser"]

tests/ui/parser/issues/issue-5806.stderr

@@ -1,4 +1,4 @@
-error: couldn't read $DIR/../parser: $ACCESS_DENIED_MSG (os error $ACCESS_DENIED_CODE)
+error: couldn't read `$DIR/../parser`: $ACCESS_DENIED_MSG (os error $ACCESS_DENIED_CODE)
   --> $DIR/issue-5806.rs:5:1
    |
 LL | mod foo;

tests/ui/parser/mod_file_with_path_attr.rs

@@ -1,4 +1,4 @@
-//@ normalize-stderr: "not_a_real_file.rs:.*\(" -> "not_a_real_file.rs: $$FILE_NOT_FOUND_MSG ("
+//@ normalize-stderr: "not_a_real_file.rs`:.*\(" -> "not_a_real_file.rs`: $$FILE_NOT_FOUND_MSG ("
 
 #[path = "not_a_real_file.rs"]
 mod m; //~ ERROR not_a_real_file.rs

tests/ui/parser/mod_file_with_path_attr.stderr

@@ -1,4 +1,4 @@
-error: couldn't read $DIR/not_a_real_file.rs: $FILE_NOT_FOUND_MSG (os error 2)
+error: couldn't read `$DIR/not_a_real_file.rs`: $FILE_NOT_FOUND_MSG (os error 2)
   --> $DIR/mod_file_with_path_attr.rs:4:1
    |
 LL | mod m;

tests/ui/unpretty/staged-api-invalid-path-108697.stderr

@@ -1,4 +1,4 @@
-error: couldn't read $DIR/lol: No such file or directory (os error 2)
+error: couldn't read `$DIR/lol`: No such file or directory (os error 2)
   --> $DIR/staged-api-invalid-path-108697.rs:8:1
    |
 LL | mod foo;