Buggy behaviour on EOF spans

[BigWingBeat]

Using a span with an offset just past the end of the source code has buggy behaviour, depending on the exact offset.
I encountered this while copy/pasting example code from the miette and diagnostic macro docs:

fn main() -> miette::Result<()> {
    let offset = 6;
    Err(miette::miette!(
        labels = vec![miette::LabeledSpan::at_offset(offset, "")],
        ""
    )
    .with_source_code("(2 + 2"))
}

• With offset = 5 the output is:

Error:   ×
   ╭────
 1 │ (2 + 2
   ·      ▲
   ·      ╰──
   ╰────

which is expected.

• With offset = 6 the output is:

Error:   ×
   ╭────
 1 │ (2 + 2
   ╰────

which is wrong as the label is silently hidden. This is what the aforementioned doc examples currently do.

• With offset = 7 the output is:

Error:   ×
   ╭─
thread 'main' (942790) panicked at /home/bigwingbeat/.cargo/git/checkouts/miette-4f06557514a5b20a/df7bcfa/src/source_impls.rs:80:19:
range start index 7 out of range for slice of length 6
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Obviously it shouldn't be panicking. The backtrace is as follows:

Details

Error:   ×
   ╭─
thread 'main' (943097) panicked at /home/bigwingbeat/.cargo/git/checkouts/miette-4f06557514a5b20a/df7bcfa/src/source_impls.rs:80:19:
range start index 7 out of range for slice of length 6
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/panicking.rs:689:5
   1: core::panicking::panic_fmt
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/core/src/panicking.rs:80:14
   2: core::slice::index::slice_index_fail
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/core/src/panic.rs:177:9
   3: index<u8>
             at /home/bigwingbeat/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/index.rs:443:13
   4: index<u8, core::ops::range::Range<usize>>
             at /home/bigwingbeat/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/index.rs:19:15
   5: context_info
             at /home/bigwingbeat/.cargo/git/checkouts/miette-4f06557514a5b20a/df7bcfa/src/source_impls.rs:80:19
   6: read_span
             at /home/bigwingbeat/.cargo/git/checkouts/miette-4f06557514a5b20a/df7bcfa/src/source_impls.rs:102:24
   7: render_context<core::fmt::Formatter>
             at /home/bigwingbeat/.cargo/git/checkouts/miette-4f06557514a5b20a/df7bcfa/src/handlers/graphical.rs:678:18
   8: render_snippets<core::fmt::Formatter>
             at /home/bigwingbeat/.cargo/git/checkouts/miette-4f06557514a5b20a/df7bcfa/src/handlers/graphical.rs:606:18
   9: render_report_inner<core::fmt::Formatter>
             at /home/bigwingbeat/.cargo/git/checkouts/miette-4f06557514a5b20a/df7bcfa/src/handlers/graphical.rs:259:14
  10: render_report<core::fmt::Formatter>
             at /home/bigwingbeat/.cargo/git/checkouts/miette-4f06557514a5b20a/df7bcfa/src/handlers/graphical.rs:247:14
  11: debug
             at /home/bigwingbeat/.cargo/git/checkouts/miette-4f06557514a5b20a/df7bcfa/src/handlers/graphical.rs:1369:14
  12: <core::fmt::rt::Argument>::fmt
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/core/src/fmt/rt.rs:152:76
  13: core::fmt::write
  14: std::io::default_write_fmt::<std::io::stdio::StderrLock>
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/io/mod.rs:639:11
  15: <std::io::stdio::StderrLock as std::io::Write>::write_fmt
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/io/mod.rs:1994:13
  16: <&std::io::stdio::Stderr as std::io::Write>::write_fmt
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/io/stdio.rs:1066:21
  17: <std::io::stdio::Stderr as std::io::Write>::write_fmt
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/io/stdio.rs:1040:18
  18: std::io::stdio::attempt_print_to_stderr
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/io/stdio.rs:1192:22
  19: <core::result::Result<T,E> as std::process::Termination>::report
             at /home/bigwingbeat/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/process.rs:2614:17
  20: {closure#0}<core::result::Result<(), miette::eyreish::Report>>
             at /home/bigwingbeat/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:206:76
  21: <&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe as core::ops::function::FnOnce<()>>::call_once
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/core/src/ops/function.rs:287:21
  22: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/panicking.rs:581:40
  23: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe>
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/panicking.rs:544:19
  24: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/panic.rs:359:14
  25: std::rt::lang_start_internal::{closure#0}
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/rt.rs:175:24
  26: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/panicking.rs:581:40
  27: std::panicking::catch_unwind::<isize, std::rt::lang_start_internal::{closure#0}>
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/panicking.rs:544:19
  28: std::panic::catch_unwind::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/panic.rs:359:14
  29: std::rt::lang_start_internal
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/rt.rs:171:5
  30: std::rt::lang_start
             at /home/bigwingbeat/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:205:5
  31: <unknown>
  32: __libc_start_main
  33: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

• With offset = 8 the output is:

Error:   ×
  [Failed to read contents for label `` (offset: 8, length: 0): OutOfBounds]

Finally, this is what the output should be for the smaller offsets too, instead of the current buggy behaviour.

[olivia-fl]

I believe this is a duplicate of #277 (the issue title on that one is a bit misleading, I believe it is specifically with spans that include the EOF and the behavior is especially confusing with empty spans on EOF).