Add support for embedding pretty printers via the #[debugger_visualizer] attribute. Add tests for embedding pretty printers and update documentation.

Ensure all error checking for `#[debugger_visualizer]` is done up front and not when the `debugger_visualizer` query is run.

Clean up potential ODR violations when embedding pretty printers into the `__rustc_debug_gdb_scripts_section__` section.

Respond to PR comments and update documentation.

compiler/rustc_codegen_llvm/src/debuginfo/gdb.rs

@@ -5,11 +5,14 @@ use crate::llvm;
 use crate::builder::Builder;
 use crate::common::CodegenCx;
 use crate::value::Value;
+use rustc_codegen_ssa::base::collect_debugger_visualizers_transitive;
 use rustc_codegen_ssa::traits::*;
+use rustc_hir::def_id::LOCAL_CRATE;
 use rustc_middle::bug;
-use rustc_session::config::DebugInfo;
+use rustc_session::config::{CrateType, DebugInfo};
 
 use rustc_span::symbol::sym;
+use rustc_span::DebuggerVisualizerType;
 
 /// Inserts a side-effect free instruction sequence that makes sure that the
 /// .debug_gdb_scripts global is referenced, so it isn't removed by the linker.
@@ -37,9 +40,33 @@ pub fn get_or_insert_gdb_debug_scripts_section_global<'ll>(cx: &CodegenCx<'ll, '
 
     section_var.unwrap_or_else(|| {
         let section_name = b".debug_gdb_scripts\0";
-        let section_contents = b"\x01gdb_load_rust_pretty_printers.py\0";
+        let mut section_contents = Vec::new();
+
+        // Add the pretty printers for the standard library first.
+        section_contents.extend_from_slice(b"\x01gdb_load_rust_pretty_printers.py\0");
+
+        // Next, add the pretty printers that were specified via the `#[debugger_visualizer]` attribute.
+        let visualizers = collect_debugger_visualizers_transitive(
+            cx.tcx,
+            DebuggerVisualizerType::GdbPrettyPrinter,
+        );
+        let crate_name = cx.tcx.crate_name(LOCAL_CRATE);
+        for (index, visualizer) in visualizers.iter().enumerate() {
+            // The initial byte `4` instructs GDB that the following pretty printer
+            // is defined inline as opposed to in a file standalone file.
+            section_contents.extend_from_slice(b"\x04");
+            let vis_name = format!("pretty-printer-{}-{}\n", crate_name.as_str(), index);
+            section_contents.extend_from_slice(vis_name.as_bytes());
+            section_contents.extend_from_slice(&visualizer.src);
+
+            // The final byte `0` tells GDB that the pretty printer has been
+            // fully defined and can continue searching for additional
+            // pretty printers.
+            section_contents.extend_from_slice(b"\0");
+        }
 
         unsafe {
+            let section_contents = section_contents.as_slice();
             let llvm_type = cx.type_array(cx.type_i8(), section_contents.len() as u64);
 
             let section_var = cx
@@ -62,7 +89,32 @@ pub fn needs_gdb_debug_scripts_section(cx: &CodegenCx<'_, '_>) -> bool {
     let omit_gdb_pretty_printer_section =
         cx.tcx.sess.contains_name(cx.tcx.hir().krate_attrs(), sym::omit_gdb_pretty_printer_section);
 
+    // To ensure the section `__rustc_debug_gdb_scripts_section__` will not create
+    // ODR violations at link time, this section will not be emitted for rlibs since
+    // each rlib could produce a different set of visualizers that would be embedded
+    // in the `.debug_gdb_scripts` section. For that reason, we make sure that the
+    // section is only emitted for leaf crates.
+    let embed_visualizers = cx.sess().crate_types().iter().any(|&crate_type| match crate_type {
+        CrateType::Executable | CrateType::Dylib | CrateType::Cdylib | CrateType::Staticlib => {
+            // These are crate types for which we will embed pretty printers since they
+            // are treated as leaf crates.
+            true
+        }
+        CrateType::ProcMacro => {
+            // We could embed pretty printers for proc macro crates too but it does not
+            // seem like a good default, since this is a rare use case and we don't
+            // want to slow down the common case.
+            false
+        }
+        CrateType::Rlib => {
+            // As per the above description, embedding visualizers for rlibs could
+            // lead to ODR violations so we skip this crate type as well.
+            false
+        }
+    });
+
     !omit_gdb_pretty_printer_section
         && cx.sess().opts.debuginfo != DebugInfo::None
         && cx.sess().target.emit_debug_gdb_scripts
+        && embed_visualizers
 }