Use Span's start, end, line, column methods on stable 1.88+

Author: dtolnay

build.rs

@@ -20,6 +20,7 @@ fn main() {
         println!("cargo:rustc-check-cfg=cfg(no_literal_c_string)");
         println!("cargo:rustc-check-cfg=cfg(no_source_text)");
         println!("cargo:rustc-check-cfg=cfg(proc_macro_span)");
+        println!("cargo:rustc-check-cfg=cfg(proc_macro_span_location)");
         println!("cargo:rustc-check-cfg=cfg(procmacro2_backtrace)");
         println!("cargo:rustc-check-cfg=cfg(procmacro2_build_probe)");
         println!("cargo:rustc-check-cfg=cfg(procmacro2_nightly_testing)");
@@ -68,18 +69,16 @@ fn main() {
         return;
     }
 
-    println!("cargo:rerun-if-changed=src/probe/proc_macro_span.rs");
-
     let proc_macro_span;
     let consider_rustc_bootstrap;
-    if compile_probe(false) {
+    if compile_probe_unstable("proc_macro_span", false) {
         // This is a nightly or dev compiler, so it supports unstable features
         // regardless of RUSTC_BOOTSTRAP. No need to rerun build script if
         // RUSTC_BOOTSTRAP is changed.
         proc_macro_span = true;
         consider_rustc_bootstrap = false;
     } else if let Some(rustc_bootstrap) = env::var_os("RUSTC_BOOTSTRAP") {
-        if compile_probe(true) {
+        if compile_probe_unstable("proc_macro_span", true) {
             // This is a stable or beta compiler for which the user has set
             // RUSTC_BOOTSTRAP to turn on unstable features. Rerun build script
             // if they change it.
@@ -116,13 +115,19 @@ fn main() {
     }
 
     if proc_macro_span {
-        // Enable non-dummy behavior of Span::start and Span::end methods which
-        // requires an unstable compiler feature. Enabled when building with
-        // nightly, unless `-Z allow-feature` in RUSTFLAGS disallows unstable
-        // features.
+        // Enable non-dummy behavior of Span::byte_range and Span::join methods
+        // which requires an unstable compiler feature. Enabled when building
+        // with nightly, unless `-Z allow-feature` in RUSTFLAGS disallows
+        // unstable features.
         println!("cargo:rustc-cfg=proc_macro_span");
     }
 
+    if proc_macro_span || (rustc >= 88 && compile_probe_stable("proc_macro_span_location")) {
+        // Enable non-dummy behavior of Span::start and Span::end methods on
+        // Rust 1.88+.
+        println!("cargo:rustc-cfg=proc_macro_span_location");
+    }
+
     if semver_exempt && proc_macro_span {
         // Implement the semver exempt API in terms of the nightly-only
         // proc_macro API.
@@ -134,22 +139,31 @@ fn main() {
     }
 }
 
-fn compile_probe(rustc_bootstrap: bool) -> bool {
-    if env::var_os("RUSTC_STAGE").is_some() {
-        // We are running inside rustc bootstrap. This is a highly non-standard
-        // environment with issues such as:
-        //
-        //     https://github.com/rust-lang/cargo/issues/11138
-        //     https://github.com/rust-lang/rust/issues/114839
-        //
-        // Let's just not use nightly features here.
-        return false;
-    }
+fn compile_probe_unstable(feature: &str, rustc_bootstrap: bool) -> bool {
+    // RUSTC_STAGE indicates that this crate is being compiled as a dependency
+    // of a multistage rustc bootstrap. This environment uses Cargo in a highly
+    // non-standard way with issues such as:
+    //
+    //     https://github.com/rust-lang/cargo/issues/11138
+    //     https://github.com/rust-lang/rust/issues/114839
+    //
+    env::var_os("RUSTC_STAGE").is_none() && do_compile_probe(feature, rustc_bootstrap)
+}
+
+fn compile_probe_stable(feature: &str) -> bool {
+    env::var_os("RUSTC_STAGE").is_some() || do_compile_probe(feature, true)
+}
+
+fn do_compile_probe(feature: &str, rustc_bootstrap: bool) -> bool {
+    println!("cargo:rerun-if-changed=src/probe/{}.rs", feature);
 
     let rustc = cargo_env_var("RUSTC");
     let out_dir = cargo_env_var("OUT_DIR");
     let out_subdir = Path::new(&out_dir).join("probe");
-    let probefile = Path::new("src").join("probe").join("proc_macro_span.rs");
+    let probefile = Path::new("src")
+        .join("probe")
+        .join(feature)
+        .with_extension("rs");
 
     if let Err(err) = fs::create_dir(&out_subdir) {
         if err.kind() != ErrorKind::AlreadyExists {

src/probe.rs

@@ -2,3 +2,6 @@
 
 #[cfg(proc_macro_span)]
 pub(crate) mod proc_macro_span;
+
+#[cfg(proc_macro_span_location)]
+pub(crate) mod proc_macro_span_location;

src/probe/proc_macro_span_location.rs

@@ -0,0 +1,21 @@
+// The subset of Span's API stabilized in Rust 1.88.
+
+extern crate proc_macro;
+
+use proc_macro::Span;
+
+pub fn start(this: &Span) -> Span {
+    this.start()
+}
+
+pub fn end(this: &Span) -> Span {
+    this.end()
+}
+
+pub fn line(this: &Span) -> usize {
+    this.line()
+}
+
+pub fn column(this: &Span) -> usize {
+    this.column()
+}

src/wrapper.rs

@@ -4,6 +4,8 @@ use crate::fallback::{self, FromStr2 as _};
 use crate::location::LineColumn;
 #[cfg(proc_macro_span)]
 use crate::probe::proc_macro_span;
+#[cfg(all(span_locations, proc_macro_span_location))]
+use crate::probe::proc_macro_span_location;
 use crate::{Delimiter, Punct, Spacing, TokenTree};
 use core::fmt::{self, Debug, Display};
 #[cfg(span_locations)]
@@ -433,12 +435,12 @@ impl Span {
     #[cfg(span_locations)]
     pub(crate) fn start(&self) -> LineColumn {
         match self {
-            #[cfg(proc_macro_span)]
+            #[cfg(proc_macro_span_location)]
             Span::Compiler(s) => LineColumn {
-                line: proc_macro_span::line(s),
-                column: proc_macro_span::column(s).saturating_sub(1),
+                line: proc_macro_span_location::line(s),
+                column: proc_macro_span_location::column(s).saturating_sub(1),
             },
-            #[cfg(not(proc_macro_span))]
+            #[cfg(not(proc_macro_span_location))]
             Span::Compiler(_) => LineColumn { line: 0, column: 0 },
             Span::Fallback(s) => s.start(),
         }
@@ -447,15 +449,15 @@ impl Span {
     #[cfg(span_locations)]
     pub(crate) fn end(&self) -> LineColumn {
         match self {
-            #[cfg(proc_macro_span)]
+            #[cfg(proc_macro_span_location)]
             Span::Compiler(s) => {
-                let end = proc_macro_span::end(s);
+                let end = proc_macro_span_location::end(s);
                 LineColumn {
-                    line: proc_macro_span::line(&end),
-                    column: proc_macro_span::column(&end).saturating_sub(1),
+                    line: proc_macro_span_location::line(&end),
+                    column: proc_macro_span_location::column(&end).saturating_sub(1),
                 }
             }
-            #[cfg(not(proc_macro_span))]
+            #[cfg(not(proc_macro_span_location))]
             Span::Compiler(_) => LineColumn { line: 0, column: 0 },
             Span::Fallback(s) => s.end(),
         }