Add `-Z span-debug` to allow for easier debugging of proc macros

Currently, the `Debug` impl for `proc_macro::Span` just prints out
the byte range. This can make debugging proc macros (either as a crate
author or as a compiler developer) very frustrating, since neither the
actual filename nor the `SyntaxContext` is displayed.

This commit adds a perma-unstable flag `-Z span-debug`. When enabled,
the `Debug` impl for `proc_macro::Span` simply forwards directly to
`rustc_span::Span`. Once #72618 is merged, this will start displaying
actual line numbers.

While `Debug` impls are not subject to Rust's normal stability
guarnatees, we probably shouldn't expose any additional information on
stable until `#![feature(proc_macro_span)]` is stabilized. Otherwise,
we would be providing a 'backdoor' way to access information that's
supposed be behind unstable APIs.

7 files changed, 220 insertions, 1 deletions

diff --git a/src/librustc_expand/expand.rs b/src/librustc_expand/expand.rs
index 5214ca0dc7f..4e41bd4bbfa 100644
--- a/src/librustc_expand/expand.rs
+++ b/src/librustc_expand/expand.rs
@@ -1789,6 +1789,7 @@ pub struct ExpansionConfig<'feat> {
     pub trace_mac: bool,
     pub should_test: bool, // If false, strip `#[test]` nodes
     pub keep_macs: bool,
+    pub span_debug: bool, // If true, use verbose debugging for `proc_macro::Span`
 }
 
 impl<'feat> ExpansionConfig<'feat> {
@@ -1800,6 +1801,7 @@ impl<'feat> ExpansionConfig<'feat> {
             trace_mac: false,
             should_test: false,
             keep_macs: false,
+            span_debug: false,
         }
     }
 
diff --git a/src/librustc_expand/proc_macro_server.rs b/src/librustc_expand/proc_macro_server.rs
index 36af83711f7..79fa091ba18 100644
--- a/src/librustc_expand/proc_macro_server.rs
+++ b/src/librustc_expand/proc_macro_server.rs
@@ -352,6 +352,7 @@ pub(crate) struct Rustc<'a> {
     def_site: Span,
     call_site: Span,
     mixed_site: Span,
+    span_debug: bool,
 }
 
 impl<'a> Rustc<'a> {
@@ -362,6 +363,7 @@ impl<'a> Rustc<'a> {
             def_site: cx.with_def_site_ctxt(expn_data.def_site),
             call_site: cx.with_call_site_ctxt(expn_data.call_site),
             mixed_site: cx.with_mixed_site_ctxt(expn_data.call_site),
+            span_debug: cx.ecfg.span_debug,
         }
     }
 
@@ -646,7 +648,11 @@ impl server::Diagnostic for Rustc<'_> {
 
 impl server::Span for Rustc<'_> {
     fn debug(&mut self, span: Self::Span) -> String {
-        format!("{:?} bytes({}..{})", span.ctxt(), span.lo().0, span.hi().0)
+        if self.span_debug {
+            format!("{:?}", span)
+        } else {
+            format!("{:?} bytes({}..{})", span.ctxt(), span.lo().0, span.hi().0)
+        }
     }
     fn def_site(&mut self) -> Self::Span {
         self.def_site
diff --git a/src/librustc_interface/passes.rs b/src/librustc_interface/passes.rs
index a01170db807..9a60e74d94d 100644
--- a/src/librustc_interface/passes.rs
+++ b/src/librustc_interface/passes.rs
@@ -291,6 +291,7 @@ fn configure_and_expand_inner<'a>(
             recursion_limit: sess.recursion_limit(),
             trace_mac: sess.opts.debugging_opts.trace_macros,
             should_test: sess.opts.test,
+            span_debug: sess.opts.debugging_opts.span_debug,
             ..rustc_expand::expand::ExpansionConfig::default(crate_name.to_string())
         };
 
diff --git a/src/librustc_interface/tests.rs b/src/librustc_interface/tests.rs
index 18cbea858d4..87647f3b0b0 100644
--- a/src/librustc_interface/tests.rs
+++ b/src/librustc_interface/tests.rs
@@ -506,6 +506,7 @@ fn test_debugging_options_tracking_hash() {
     untracked!(save_analysis, true);
     untracked!(self_profile, SwitchWithOptPath::Enabled(None));
     untracked!(self_profile_events, Some(vec![String::new()]));
+    untracked!(span_debug, true);
     untracked!(span_free_formats, true);
     untracked!(strip, Strip::None);
     untracked!(terminal_width, Some(80));
diff --git a/src/librustc_session/options.rs b/src/librustc_session/options.rs
index a38e7f063d7..d22c6ec9d7d 100644
--- a/src/librustc_session/options.rs
+++ b/src/librustc_session/options.rs
@@ -996,6 +996,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "make the current crate share its generic instantiations"),
     show_span: Option<String> = (None, parse_opt_string, [TRACKED],
         "show spans for compiler debugging (expr|pat|ty)"),
+    span_debug: bool = (false, parse_bool, [UNTRACKED],
+        "forward proc_macro::Span's `Debug` impl to `Span`"),
     // o/w tests have closure@path
     span_free_formats: bool = (false, parse_bool, [UNTRACKED],
         "exclude spans when debug-printing compiler state (default: no)"),
diff --git a/src/test/ui/proc-macro/debug/dump-debug-span-debug.rs b/src/test/ui/proc-macro/debug/dump-debug-span-debug.rs
new file mode 100644
index 00000000000..fd34eb974c0
--- /dev/null
+++ b/src/test/ui/proc-macro/debug/dump-debug-span-debug.rs
@@ -0,0 +1,41 @@
+// run-pass
+// aux-build:macro-dump-debug.rs
+// compile-flags: -Z span-debug
+
+extern crate macro_dump_debug;
+use macro_dump_debug::dump_debug;
+
+dump_debug! {
+    ident   // ident
+    r#ident // raw ident
+    ,       // alone punct
+    ==>     // joint punct
+    ()      // empty group
+    [_]     // nonempty group
+
+    // unsuffixed literals
+    0
+    1.0
+    "S"
+    b"B"
+    r"R"
+    r##"R"##
+    br"BR"
+    br##"BR"##
+    'C'
+    b'B'
+
+    // suffixed literals
+    0q
+    1.0q
+    "S"q
+    b"B"q
+    r"R"q
+    r##"R"##q
+    br"BR"q
+    br##"BR"##q
+    'C'q
+    b'B'q
+}
+
+fn main() {}
diff --git a/src/test/ui/proc-macro/debug/dump-debug-span-debug.stderr b/src/test/ui/proc-macro/debug/dump-debug-span-debug.stderr
new file mode 100644
index 00000000000..163a2c9f44c
--- /dev/null
+++ b/src/test/ui/proc-macro/debug/dump-debug-span-debug.stderr
@@ -0,0 +1,166 @@
+TokenStream [Ident { ident: "ident", span: $DIR/dump-debug-span-debug.rs:9:5: 9:10 }, Ident { ident: "r#ident", span: $DIR/dump-debug-span-debug.rs:10:5: 10:12 }, Punct { ch: ',', spacing: Alone, span: $DIR/dump-debug-span-debug.rs:11:5: 11:6 }, Punct { ch: '=', spacing: Joint, span: $DIR/dump-debug-span-debug.rs:12:5: 12:7 }, Punct { ch: '=', spacing: Joint, span: $DIR/dump-debug-span-debug.rs:12:5: 12:7 }, Punct { ch: '>', spacing: Alone, span: $DIR/dump-debug-span-debug.rs:12:7: 12:8 }, Group { delimiter: Parenthesis, stream: TokenStream [], span: $DIR/dump-debug-span-debug.rs:13:5: 13:7 }, Group { delimiter: Bracket, stream: TokenStream [Ident { ident: "_", span: $DIR/dump-debug-span-debug.rs:14:6: 14:7 }], span: $DIR/dump-debug-span-debug.rs:14:5: 14:8 }, Literal { kind: Integer, symbol: "0", suffix: None, span: $DIR/dump-debug-span-debug.rs:17:5: 17:6 }, Literal { kind: Float, symbol: "1.0", suffix: None, span: $DIR/dump-debug-span-debug.rs:18:5: 18:8 }, Literal { kind: Str, symbol: "S", suffix: None, span: $DIR/dump-debug-span-debug.rs:19:5: 19:8 }, Literal { kind: ByteStr, symbol: "B", suffix: None, span: $DIR/dump-debug-span-debug.rs:20:5: 20:9 }, Literal { kind: StrRaw(0), symbol: "R", suffix: None, span: $DIR/dump-debug-span-debug.rs:21:5: 21:9 }, Literal { kind: StrRaw(2), symbol: "R", suffix: None, span: $DIR/dump-debug-span-debug.rs:22:5: 22:13 }, Literal { kind: ByteStrRaw(0), symbol: "BR", suffix: None, span: $DIR/dump-debug-span-debug.rs:23:5: 23:11 }, Literal { kind: ByteStrRaw(2), symbol: "BR", suffix: None, span: $DIR/dump-debug-span-debug.rs:24:5: 24:15 }, Literal { kind: Char, symbol: "C", suffix: None, span: $DIR/dump-debug-span-debug.rs:25:5: 25:8 }, Literal { kind: Byte, symbol: "B", suffix: None, span: $DIR/dump-debug-span-debug.rs:26:5: 26:9 }, Literal { kind: Integer, symbol: "0", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:29:5: 29:7 }, Literal { kind: Float, symbol: "1.0", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:30:5: 30:9 }, Literal { kind: Str, symbol: "S", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:31:5: 31:9 }, Literal { kind: ByteStr, symbol: "B", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:32:5: 32:10 }, Literal { kind: StrRaw(0), symbol: "R", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:33:5: 33:10 }, Literal { kind: StrRaw(2), symbol: "R", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:34:5: 34:14 }, Literal { kind: ByteStrRaw(0), symbol: "BR", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:35:5: 35:12 }, Literal { kind: ByteStrRaw(2), symbol: "BR", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:36:5: 36:16 }, Literal { kind: Char, symbol: "C", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:37:5: 37:9 }, Literal { kind: Byte, symbol: "B", suffix: Some("q"), span: $DIR/dump-debug-span-debug.rs:38:5: 38:10 }]
+TokenStream [
+    Ident {
+        ident: "ident",
+        span: $DIR/dump-debug-span-debug.rs:9:5: 9:10,
+    },
+    Ident {
+        ident: "r#ident",
+        span: $DIR/dump-debug-span-debug.rs:10:5: 10:12,
+    },
+    Punct {
+        ch: ',',
+        spacing: Alone,
+        span: $DIR/dump-debug-span-debug.rs:11:5: 11:6,
+    },
+    Punct {
+        ch: '=',
+        spacing: Joint,
+        span: $DIR/dump-debug-span-debug.rs:12:5: 12:7,
+    },
+    Punct {
+        ch: '=',
+        spacing: Joint,
+        span: $DIR/dump-debug-span-debug.rs:12:5: 12:7,
+    },
+    Punct {
+        ch: '>',
+        spacing: Alone,
+        span: $DIR/dump-debug-span-debug.rs:12:7: 12:8,
+    },
+    Group {
+        delimiter: Parenthesis,
+        stream: TokenStream [],
+        span: $DIR/dump-debug-span-debug.rs:13:5: 13:7,
+    },
+    Group {
+        delimiter: Bracket,
+        stream: TokenStream [
+            Ident {
+                ident: "_",
+                span: $DIR/dump-debug-span-debug.rs:14:6: 14:7,
+            },
+        ],
+        span: $DIR/dump-debug-span-debug.rs:14:5: 14:8,
+    },
+    Literal {
+        kind: Integer,
+        symbol: "0",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:17:5: 17:6,
+    },
+    Literal {
+        kind: Float,
+        symbol: "1.0",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:18:5: 18:8,
+    },
+    Literal {
+        kind: Str,
+        symbol: "S",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:19:5: 19:8,
+    },
+    Literal {
+        kind: ByteStr,
+        symbol: "B",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:20:5: 20:9,
+    },
+    Literal {
+        kind: StrRaw(0),
+        symbol: "R",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:21:5: 21:9,
+    },
+    Literal {
+        kind: StrRaw(2),
+        symbol: "R",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:22:5: 22:13,
+    },
+    Literal {
+        kind: ByteStrRaw(0),
+        symbol: "BR",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:23:5: 23:11,
+    },
+    Literal {
+        kind: ByteStrRaw(2),
+        symbol: "BR",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:24:5: 24:15,
+    },
+    Literal {
+        kind: Char,
+        symbol: "C",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:25:5: 25:8,
+    },
+    Literal {
+        kind: Byte,
+        symbol: "B",
+        suffix: None,
+        span: $DIR/dump-debug-span-debug.rs:26:5: 26:9,
+    },
+    Literal {
+        kind: Integer,
+        symbol: "0",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:29:5: 29:7,
+    },
+    Literal {
+        kind: Float,
+        symbol: "1.0",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:30:5: 30:9,
+    },
+    Literal {
+        kind: Str,
+        symbol: "S",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:31:5: 31:9,
+    },
+    Literal {
+        kind: ByteStr,
+        symbol: "B",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:32:5: 32:10,
+    },
+    Literal {
+        kind: StrRaw(0),
+        symbol: "R",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:33:5: 33:10,
+    },
+    Literal {
+        kind: StrRaw(2),
+        symbol: "R",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:34:5: 34:14,
+    },
+    Literal {
+        kind: ByteStrRaw(0),
+        symbol: "BR",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:35:5: 35:12,
+    },
+    Literal {
+        kind: ByteStrRaw(2),
+        symbol: "BR",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:36:5: 36:16,
+    },
+    Literal {
+        kind: Char,
+        symbol: "C",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:37:5: 37:9,
+    },
+    Literal {
+        kind: Byte,
+        symbol: "B",
+        suffix: Some("q"),
+        span: $DIR/dump-debug-span-debug.rs:38:5: 38:10,
+    },
+]