Auto merge of #74091 - richkadel:llvm-coverage-map-gen-4, r=tmandry

Generating the coverage map

@tmandry @wesleywiser

rustc now generates the coverage map and can support (limited)
coverage report generation, at the function level.

Example commands to generate a coverage report:
```shell
$ BUILD=$HOME/rust/build/x86_64-unknown-linux-gnu
$ $BUILD/stage1/bin/rustc -Zinstrument-coverage \
$HOME/rust/src/test/run-make-fulldeps/instrument-coverage/main.rs
$ LLVM_PROFILE_FILE="main.profraw" ./main
called
$ $BUILD/llvm/bin/llvm-profdata merge -sparse main.profraw -o main.profdata
$ $BUILD/llvm/bin/llvm-cov show --instr-profile=main.profdata main
```
![rust coverage report only 20200706](https://user-images.githubusercontent.com/3827298/86697299-1cbe8f80-bfc3-11ea-8955-451b48626991.png)

r? @wesleywiser

Rust compiler MCP rust-lang/compiler-team#278
Relevant issue: #34701 - Implement support for LLVMs code coverage instrumentation

3 files changed, 8 insertions, 9 deletions

diff --git a/src/librustc_middle/mir/coverage/mod.rs b/src/librustc_middle/mir/coverage/mod.rs
index 327f321bd75..1e06dadfa24 100644
--- a/src/librustc_middle/mir/coverage/mod.rs
+++ b/src/librustc_middle/mir/coverage/mod.rs
@@ -2,9 +2,10 @@
 
 /// Positional arguments to `libcore::count_code_region()`
 pub mod count_code_region_args {
-    pub const COUNTER_INDEX: usize = 0;
-    pub const START_BYTE_POS: usize = 1;
-    pub const END_BYTE_POS: usize = 2;
+    pub const FUNCTION_SOURCE_HASH: usize = 0;
+    pub const COUNTER_INDEX: usize = 1;
+    pub const START_BYTE_POS: usize = 2;
+    pub const END_BYTE_POS: usize = 3;
 }
 
 /// Positional arguments to `libcore::coverage_counter_add()` and
diff --git a/src/librustc_middle/mir/mono.rs b/src/librustc_middle/mir/mono.rs
index d22bde2ff8b..1ad5008d28a 100644
--- a/src/librustc_middle/mir/mono.rs
+++ b/src/librustc_middle/mir/mono.rs
@@ -86,7 +86,7 @@ impl<'tcx> MonoItem<'tcx> {
             .debugging_opts
             .inline_in_all_cgus
             .unwrap_or_else(|| tcx.sess.opts.optimize != OptLevel::No)
-            && !tcx.sess.opts.cg.link_dead_code;
+            && tcx.sess.opts.cg.link_dead_code != Some(true);
 
         match *self {
             MonoItem::Fn(ref instance) => {
diff --git a/src/librustc_middle/mir/query.rs b/src/librustc_middle/mir/query.rs
index 402a5c421a7..6ce5d61fbed 100644
--- a/src/librustc_middle/mir/query.rs
+++ b/src/librustc_middle/mir/query.rs
@@ -400,13 +400,11 @@ pub struct DestructuredConst<'tcx> {
 /// `InstrumentCoverage` MIR pass and can be retrieved via the `coverageinfo` query.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub struct CoverageInfo {
-    /// A hash value that can be used by the consumer of the coverage profile data to detect
-    /// changes to the instrumented source of the associated MIR body (typically, for an
-    /// individual function).
-    pub hash: u64,
-
     /// The total number of coverage region counters added to the MIR `Body`.
     pub num_counters: u32,
+
+    /// The total number of coverage region counter expressions added to the MIR `Body`.
+    pub num_expressions: u32,
 }
 
 impl<'tcx> TyCtxt<'tcx> {