Auto merge of #42973 - zackmdavis:json_byte_position_to_start_at_top_of_file, r=nrc

make JSON error byte position start at top of file

The `hi` and `lo` offsets in a span are relative to a `CodeMap`, but this
doesn't seem to be terribly useful for tool consumers who don't have the
codemap, but might want the byte offset within an actual file?

I couldn't get @killercup's [example](https://github.com/rust-lang/rust/issues/35164#issuecomment-301436519) to run, perhaps due to the limitations of the merely-stage-1 compiler that I built (error was `libproc_macro-456500c7095d8fbe.so: cannot open shared object file: No such file or directory`)??—but a dummy project confirms that the byte offsets have successfully been changed to be file-relative—

**Before:**

```
$ cargo run --message-format json
   Compiling byte_json v0.1.0 (file:///home/ubuntu/byte_json)
{"message":{"children":[{"children":[],"code":null,"level":"note","message":"#[warn(dead_code)] on by default","rendered":null,"spans":[]}],"code":null,"level":"warning","message":"function is never used: `rah`","rendered":null,"spans":[{"byte_end":100,"byte_start":67,"column_end":2,"column_start":1,"expansion":null,"file_name":"src/foo.rs","is_primary":true,"label":null,"line_end":5,"line_start":3,"suggested_replacement":null,"text":[{"highlight_end":11,"highlight_start":1,"text":"fn rah() {"},{"highlight_end":21,"highlight_start":1,"text":"    println!(\"rah!\")"},{"highlight_end":2,"highlight_start":1,"text":"}"}]}]},"package_id":"byte_json 0.1.0 (path+file:///home/ubuntu/byte_json)","reason":"compiler-message","target":{"crate_types":["bin"],"kind":["bin"],"name":"byte_json","src_path":"/home/ubuntu/byte_json/src/main.rs"}}
{"message":{"children":[{"children":[],"code":null,"level":"note","message":"#[warn(dead_code)] on by default","rendered":null,"spans":[]}],"code":null,"level":"warning","message":"function is never used: `alas`","rendered":null,"spans":[{"byte_end":137,"byte_start":102,"column_end":2,"column_start":1,"expansion":null,"file_name":"src/bar.rs","is_primary":true,"label":null,"line_end":3,"line_start":1,"suggested_replacement":null,"text":[{"highlight_end":12,"highlight_start":1,"text":"fn alas() {"},{"highlight_end":22,"highlight_start":1,"text":"    println!(\"alas\");"},{"highlight_end":2,"highlight_start":1,"text":"}"}]}]},"package_id":"byte_json 0.1.0 (path+file:///home/ubuntu/byte_json)","reason":"compiler-message","target":{"crate_types":["bin"],"kind":["bin"],"name":"byte_json","src_path":"/home/ubuntu/byte_json/src/main.rs"}}
{"features":[],"filenames":["/home/ubuntu/byte_json/target/debug/byte_json"],"fresh":false,"package_id":"byte_json 0.1.0 (path+file:///home/ubuntu/byte_json)","profile":{"debug_assertions":true,"debuginfo":2,"opt_level":"0","overflow_checks":true,"test":false},"reason":"compiler-artifact","target":{"crate_types":["bin"],"kind":["bin"],"name":"byte_json","src_path":"/home/ubuntu/byte_json/src/main.rs"}}
    Finished dev [unoptimized + debuginfo] target(s) in 0.36 secs
     Running `target/debug/byte_json`
Hello, world!
```

**After:**

```
$ RUSTC=../rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc cargo run --message-format json
   Compiling byte_json v0.1.0 (file:///home/ubuntu/byte_json)
{"message":{"children":[{"children":[],"code":null,"level":"note","message":"#[warn(dead_code)] on by default","rendered":null,"spans":[]}],"code":null,"level":"warning","message":"function is never used: `rah`","rendered":null,"spans":[{"byte_end":35,"byte_start":2,"column_end":2,"column_start":1,"expansion":null,"file_name":"src/foo.rs","is_primary":true,"label":null,"line_end":5,"line_start":3,"suggested_replacement":null,"text":[{"highlight_end":11,"highlight_start":1,"text":"fn rah() {"},{"highlight_end":21,"highlight_start":1,"text":"    println!(\"rah!\")"},{"highlight_end":2,"highlight_start":1,"text":"}"}]}]},"package_id":"byte_json 0.1.0 (path+file:///home/ubuntu/byte_json)","reason":"compiler-message","target":{"crate_types":["bin"],"kind":["bin"],"name":"byte_json","src_path":"/home/ubuntu/byte_json/src/main.rs"}}
{"message":{"children":[{"children":[],"code":null,"level":"note","message":"#[warn(dead_code)] on by default","rendered":null,"spans":[]}],"code":null,"level":"warning","message":"function is never used: `alas`","rendered":null,"spans":[{"byte_end":35,"byte_start":0,"column_end":2,"column_start":1,"expansion":null,"file_name":"src/bar.rs","is_primary":true,"label":null,"line_end":3,"line_start":1,"suggested_replacement":null,"text":[{"highlight_end":12,"highlight_start":1,"text":"fn alas() {"},{"highlight_end":22,"highlight_start":1,"text":"    println!(\"alas\");"},{"highlight_end":2,"highlight_start":1,"text":"}"}]}]},"package_id":"byte_json 0.1.0 (path+file:///home/ubuntu/byte_json)","reason":"compiler-message","target":{"crate_types":["bin"],"kind":["bin"],"name":"byte_json","src_path":"/home/ubuntu/byte_json/src/main.rs"}}
{"features":[],"filenames":["/home/ubuntu/byte_json/target/debug/byte_json"],"fresh":false,"package_id":"byte_json 0.1.0 (path+file:///home/ubuntu/byte_json)","profile":{"debug_assertions":true,"debuginfo":2,"opt_level":"0","overflow_checks":true,"test":false},"reason":"compiler-artifact","target":{"crate_types":["bin"],"kind":["bin"],"name":"byte_json","src_path":"/home/ubuntu/byte_json/src/main.rs"}}
    Finished dev [unoptimized + debuginfo] target(s) in 2.59 secs
     Running `target/debug/byte_json`
Hello, world!
```

Resolves #35164.

r? @jonathandturner

src/libsyntax/json.rs

@@ -256,8 +256,8 @@ impl DiagnosticSpan {
         });
         DiagnosticSpan {
             file_name: start.file.name.clone(),
-            byte_start: span.lo.0,
-            byte_end: span.hi.0,
+            byte_start: span.lo.0 - start.file.start_pos.0,
+            byte_end: span.hi.0 - start.file.start_pos.0,
             line_start: start.line,
             line_end: end.line,
             column_start: start.col.0 + 1,
@@ -362,4 +362,3 @@ impl JsonEmitter {
         suggestion.splice_lines(&*self.cm).iter().map(|line| line.0.to_owned()).collect()
     }
 }
-

src/test/run-make/issue-35164/Makefile

@@ -0,0 +1,4 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) main.rs --error-format json 2>&1 | grep -q '"byte_start":490.*"byte_end":496'

src/test/run-make/issue-35164/main.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+mod submodule;
+
+fn main() {
+    submodule::foo();
+}

src/test/run-make/issue-35164/submodule/mod.rs

@@ -0,0 +1,13 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub fn foo() {
+    let _MyFoo = 2;
+}