Improve warnings on incompatible options involving -Zinstrument-coverage

Adds checks for:

* `no_core` attribute
* explicitly-enabled `legacy` symbol mangling
* mir_opt_level > 1 (which enables inlining)

I removed code from the `Inline` MIR pass that forcibly disabled
inlining if `-Zinstrument-coverage` was set. The default `mir_opt_level`
does not enable inlining anyway. But if the level is explicitly set and
is greater than 1, I issue a warning.

The new warnings show up in tests, which is much better for diagnosing
potential option conflicts in these cases.

2 files changed, 42 insertions, 8 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 54abb65dc38..47e494b78c7 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -174,6 +174,8 @@ pub enum MirSpanview {
     Block,
 }
 
+pub const MIR_OPT_LEVEL_DEFAULT: usize = 1;
+
 #[derive(Clone, PartialEq, Hash)]
 pub enum LinkerPluginLto {
     LinkerPlugin(PathBuf),
@@ -212,6 +214,12 @@ pub enum SymbolManglingVersion {
     V0,
 }
 
+impl SymbolManglingVersion {
+    pub fn default() -> Self {
+        SymbolManglingVersion::Legacy
+    }
+}
+
 impl_stable_hash_via_hash!(SymbolManglingVersion);
 
 #[derive(Clone, Copy, Debug, PartialEq, Hash)]
@@ -1757,7 +1765,33 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         // and reversible name mangling. Note, LLVM coverage tools can analyze coverage over
         // multiple runs, including some changes to source code; so mangled names must be consistent
         // across compilations.
-        debugging_opts.symbol_mangling_version = SymbolManglingVersion::V0;
+        match debugging_opts.symbol_mangling_version {
+            None => {
+                debugging_opts.symbol_mangling_version = Some(SymbolManglingVersion::V0);
+            }
+            Some(SymbolManglingVersion::Legacy) => {
+                early_warn(
+                    error_format,
+                    "-Z instrument-coverage requires symbol mangling version `v0`, \
+                    but `-Z symbol-mangling-version=legacy` was specified",
+                );
+            }
+            Some(SymbolManglingVersion::V0) => {}
+        }
+
+        match debugging_opts.mir_opt_level {
+            Some(level) if level > 1 => {
+                early_warn(
+                    error_format,
+                    &format!(
+                        "`-Z mir-opt-level={}` (any level > 1) enables function inlining, which \
+                        limits the effectiveness of `-Z instrument-coverage`.",
+                        level,
+                    ),
+                );
+            }
+            _ => {}
+        }
     }
 
     if let Ok(graphviz_font) = std::env::var("RUSTC_GRAPHVIZ_FONT") {
@@ -2162,7 +2196,7 @@ crate mod dep_tracking {
     impl_dep_tracking_hash_via_hash!(Edition);
     impl_dep_tracking_hash_via_hash!(LinkerPluginLto);
     impl_dep_tracking_hash_via_hash!(SwitchWithOptPath);
-    impl_dep_tracking_hash_via_hash!(SymbolManglingVersion);
+    impl_dep_tracking_hash_via_hash!(Option<SymbolManglingVersion>);
     impl_dep_tracking_hash_via_hash!(Option<SourceFileHashAlgorithm>);
     impl_dep_tracking_hash_via_hash!(TrimmedDefPaths);
 
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 74578f2dc17..1909550aca4 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -677,12 +677,12 @@ macro_rules! options {
         }
 
         fn parse_symbol_mangling_version(
-            slot: &mut SymbolManglingVersion,
+            slot: &mut Option<SymbolManglingVersion>,
             v: Option<&str>,
         ) -> bool {
             *slot = match v {
-                Some("legacy") => SymbolManglingVersion::Legacy,
-                Some("v0") => SymbolManglingVersion::V0,
+                Some("legacy") => Some(SymbolManglingVersion::Legacy),
+                Some("v0") => Some(SymbolManglingVersion::V0),
                 _ => return false,
             };
             true
@@ -970,7 +970,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
     mir_emit_retag: bool = (false, parse_bool, [TRACKED],
         "emit Retagging MIR statements, interpreted e.g., by miri; implies -Zmir-opt-level=0 \
         (default: no)"),
-    mir_opt_level: usize = (1, parse_uint, [TRACKED],
+    mir_opt_level: Option<usize> = (None, parse_opt_uint, [TRACKED],
         "MIR optimization level (0-3; default: 1)"),
     mutable_noalias: bool = (false, parse_bool, [TRACKED],
         "emit noalias metadata for mutable references (default: no)"),
@@ -1088,9 +1088,9 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "hash algorithm of source files in debug info (`md5`, `sha1`, or `sha256`)"),
     strip: Strip = (Strip::None, parse_strip, [UNTRACKED],
         "tell the linker which information to strip (`none` (default), `debuginfo` or `symbols`)"),
-    symbol_mangling_version: SymbolManglingVersion = (SymbolManglingVersion::Legacy,
+    symbol_mangling_version: Option<SymbolManglingVersion> = (None,
         parse_symbol_mangling_version, [TRACKED],
-        "which mangling version to use for symbol names"),
+        "which mangling version to use for symbol names ('legacy' (default) or 'v0')"),
     teach: bool = (false, parse_bool, [TRACKED],
         "show extended diagnostic help (default: no)"),
     terminal_width: Option<usize> = (None, parse_opt_uint, [UNTRACKED],